Welcome to Swift [605451]

​
​
Welcome to Swift

​
​
About Swift
Swift is a new programming language for iOS and OS X apps that builds on the best of C and Objective-C,
without the constraints of C compatibility. Swift adopts safe programming patterns and adds modern features to
make programming easier, more flexible, and more fun. Swift’s clean slate, backed by the mature and much-
loved Cocoa and Cocoa Touch frameworks, is an opportunity to reimagine how software development works.
Swift has been years in the making. Apple laid the foundation for Swift by advancing our existing compiler,
debugger, and framework infrastructure. We simplified memory management with Automatic Reference
Counting (ARC). Our framework stack, built on the solid base of Foundation and Cocoa, has been modernized
and standardized throughout. Objective-C itself has evolved to support blocks, collection literals, and modules,
enabling framework adoption of modern language technologies without disruption. Thanks to this groundwork,
we can now introduce a new language for the future of Apple software development.
Swift feels familiar to Objective-C developers. It adopts the readability of Objective-C’s named parameters and
the power of Objective-C’s dynamic object model. It provides seamless access to existing Cocoa frameworks
and mix-and-match interoperability with Objective-C code. Building from this common ground, Swift introduces
many new features and unifies the procedural and object-oriented portions of the language.
Swift is friendly to new programmers. It is the first industrial-quality systems programming language that is as
expressive and enjoyable as a scripting language. It supports playgrounds, an innovative feature that allows
programmers to experiment with Swift code and see the results immediately, without the overhead of building
and running an app.
Swift combines the best in modern language thinking with wisdom from the wider Apple engineering culture.
The compiler is optimized for performance, and the language is optimized for development, without
compromising on either. It’s designed to scale from “hello, world” to an entire operating system. All this makes
Swift a sound future investment for developers and for Apple.
Swift is a fantastic way to write iOS and OS X apps, and will continue to evolve with new features and
capabilities. Our goals for Swift are ambitious. We can’t wait to see what you create with it.

​
​
A Swift Tour
Tradition suggests that the first program in a new language should print the words “Hello, world” on the screen.
In Swift, this can be done in a single line:
1
println
(
"Hello, world"
)
If you have written code in C or Objective-C, this syntax looks familiar to you—in Swift, this line of code is a
complete program. You don’t need to import a separate library for functionality like input/output or string
handling. Code written at global scope is used as the entry point for the program, so you don’t need a
main
function. You also don’t need to write semicolons at the end of every statement.
This tour gives you enough information to start writing code in Swift by showing you how to accomplish a
variety of programming tasks. Don’t worry if you don’t understand something—everything introduced in this
tour is explained in detail in the rest of this book.
NOTE
For the best experience, open this chapter as a playground in Xcode. Playgrounds allow you to edit
the code listings and see the result immediately.
​
Simple Values
Use
let
to make a constant and
var
to make a variable. The value of a constant doesn’t need to be known at
compile time, but you must assign it a value exactly once. This means you can use constants to name a value
that you determine once but use in many places.

1
var

myVariable
=
42
2
myVariable
=
50
3
let

myConstant
=
42
A constant or variable must have the same type as the value you want to assign to it. However, you don’t
always have to write the type explicitly. Providing a value when you create a constant or variable lets the
compiler infer its type. In the example above, the compiler infers that
myVariable
is an integer because its
initial value is a integer.
If the initial value doesn’t provide enough information (or if there is no initial value), specify the type by writing it
after the variable, separated by a colon.
1
let

implicitInteger
=
70
2
let

implicitDouble
=
70.0
3
let

explicitDouble
:
Double
=
70
EXPERIMENT
Create a constant with an explicit type of
Float
and a value of
4
.
Values are never implicitly converted to another type. If you need to convert a value to a different type, explicitly
make an instance of the desired type.
1
let

label
=
"The width is "
2
let

width
=
94
3
let

widthLabel
=
label
+
String
(
width
)
EXPERIMENT

Try removing the conversion to
String
from the last line. What error do you get?
There’s an even simpler way to include values in strings: Write the value in parentheses, and write a backslash
(
\
) before the parentheses. For example:
1
let

apples
=
3
2
let

oranges
=
5
3
let

appleSummary
=
"I have
\(
apples
)
apples."
4
let

fruitSummary
=
"I have
\(
apples
+
oranges
)
pieces of fruit."
EXPERIMENT
Use
\()
to include a floating-point calculation in a string and to include someone’s name in a
greeting.
Create arrays and dictionaries using brackets (
[]
), and access their elements by writing the index or key in
brackets.
1
var

shoppingList
= [
"catfish"
,
"water"
,
"tulips"
,
"blue paint"
]
2
shoppingList
[
1
] =
"bottle of water"
3

4
var

occupations
= [
5

"Malcolm"
:
"Captain"
,
6

"Kaylee"
:
"Mechanic"
,
7
]
8
occupations
[
"Jayne"
] =
"Public Relations"
To create an empty array or dictionary, use the initializer syntax.

1
let

emptyArray
=
String
[]()
2
let

emptyDictionary
=
Dictionary
<
String
,
Float
>()
If type information can be inferred, you can write an empty array as
[]
and an empty dictionary as
[:]
—for
example, when you set a new value for a variable or pass an argument to a function.
1
shoppingList
= []
// Went shopping and bought everything.
​
Control Flow
Use
if
and
switch
to make conditionals, and use
for
–
in
,
for
,
while
, and
do
–
while
to make loops.
Parentheses around the condition or loop variable are optional. Braces around the body are required.
1
let

individualScores
= [
75
,
43
,
103
,
87
,
12
]
2
var

teamScore
=
0
3
for

score

in

individualScores
{
4

if

score
>
50
{
5

teamScore
+=
3
6
}
else
{
7

teamScore
+=
1
8
}
9
}
10
teamScore
In an
if
statement, the conditional must be a Boolean expression—this means that code such as
if score
{ … }
is an error, not an implicit comparison to zero.
You can use
if
and
let
together to work with values that might be missing. These values are represented as
optionals. An optional value either contains a value or contains
nil
to indicate that the value is missing. Write a
question mark (
?
) after the type of a value to mark the value as optional.
1
var

optionalString
:
String
? =
"Hello"

2
optionalString
==
nil
3

4
var

optionalName
:
String
? =
"John Appleseed"
5
var

greeting
=
"Hello!"
6
if

let

name
=
optionalName
{
7

greeting
=
"Hello,
\(
name
)
"
8
}
EXPERIMENT
Change
optionalName
to
nil
. What greeting do you get? Add an
else
clause that sets a
different greeting if
optionalName
is
nil
.
If the optional value is
nil
, the conditional is
false
and the code in braces is skipped. Otherwise, the optional
value is unwrapped and assigned to the constant after
let
, which makes the unwrapped value available inside
the block of code.
Switches support any kind of data and a wide variety of comparison operations—they aren’t limited to integers
and tests for equality.
1
let

vegetable
=
"red pepper"
2
switch

vegetable
{
3
case

"celery"
:
4

let

vegetableComment
=
"Add some raisins and make ants on a log."
5
case

"cucumber"
,
"watercress"
:
6

let

vegetableComment
=
"That would make a good tea sandwich."
7
case

let

x

where

x
.
hasSuffix
(
"pepper"
):
8

let

vegetableComment
=
"Is it a spicy
\(
x
)
?"
9
default
:
10

let

vegetableComment
=
"Everything tastes good in soup."
11
}

EXPERIMENT
Try removing the default case. What error do you get?
After executing the code inside the switch case that matched, the program exits from the switch statement.
Execution doesn’t continue to the next case, so there is no need to explicitly break out of the switch at the end of
each case’s code.
You use
for
–
in
to iterate over items in a dictionary by providing a pair of names to use for each key-value
pair.
1
let

interestingNumbers
= [
2

"Prime"
: [
2
,
3
,
5
,
7
,
11
,
13
],
3

"Fibonacci"
: [
1
,
1
,
2
,
3
,
5
,
8
],
4

"Square"
: [
1
,
4
,
9
,
16
,
25
],
5
]
6
var

largest
=
0
7
for
(
kind
,
numbers
)
in

interestingNumbers
{
8

for

number

in

numbers
{
9

if

number
>
largest
{
10

largest
=
number
11
}
12
}
13
}
14
largest
EXPERIMENT
Add another variable to keep track of which kind of number was the largest, as well as what that
largest number was.

Use
while
to repeat a block of code until a condition changes. The condition of a loop can be at the end instead,
ensuring that the loop is run at least once.
1
var

n
=
2
2
while

n
<
100
{
3

n
=
n
*
2
4
}
5
n
6

7
var

m
=
2
8
do
{
9

m
=
m
*
2
10
}
while

m
<
100
11
m
You can keep an index in a loop—either by using
..
to make a range of indexes or by writing an explicit
initialization, condition, and increment. These two loops do the same thing:
1
var

firstForLoop
=
0
2
for

i

in

0
..
3
{
3

firstForLoop
+=
i
4
}
5
firstForLoop
6

7
var

secondForLoop
=
0
8
for

var

i
=
0
;
i
<
3
; ++
i
{
9

secondForLoop
+=
1
10
}
11
secondForLoop
Use
..
to make a range that omits its upper value, and use
…
to make a range that includes both values.
​
Functions and Closures

Use
func
to declare a function. Call a function by following its name with a list of arguments in parentheses.
Use
->
to separate the parameter names and types from the function’s return type.
1
func

greet
(
name
:
String
,
day
:
String
) ->
String
{
2

return

"Hello
\(
name
)
, today is
\(
day
)
."
3
}
4
greet
(
"Bob"
,
"Tuesday"
)
EXPERIMENT
Remove the
day
parameter. Add a parameter to include today’s lunch special in the greeting.
Use a tuple to return multiple values from a function.
1
func

getGasPrices
() -> (
Double
,
Double
,
Double
) {
2

return
(
3.59
,
3.69
,
3.79
)
3
}
4
getGasPrices
()
Functions can also take a variable number of arguments, collecting them into an array.
1
func

sumOf
(
numbers
:
Int
…) ->
Int
{
2

var

sum
=
0
3

for

number

in

numbers
{
4

sum
+=
number
5
}
6

return

sum
7
}
8
sumOf
()
9
sumOf
(
42
,
597
,
12
)

EXPERIMENT
Write a function that calculates the average of its arguments.
Functions can be nested. Nested functions have access to variables that were declared in the outer function.
You can use nested functions to organize the code in a function that is long or complex.
1
func

returnFifteen
() ->
Int
{
2

var

y
=
10
3

func

add
() {
4

y
+=
5
5
}
6

add
()
7

return

y
8
}
9
returnFifteen
()
Functions are a first-class type. This means that a function can return another function as its value.
1
func

makeIncrementer
() -> (
Int
->
Int
) {
2

func

addOne
(
number
:
Int
) ->
Int
{
3

return

1
+
number
4
}
5

return

addOne
6
}
7
var

increment
=
makeIncrementer
()
8
increment
(
7
)
A function can take another function as one of its arguments.
1
func

hasAnyMatches
(
list
:
Int
[],
condition
:
Int
->
Bool
) ->
Bool
{

2

for

item

in

list
{
3

if

condition
(
item
) {
4

return

true
5
}
6
}
7

return

false
8
}
9
func

lessThanTen
(
number
:
Int
) ->
Bool
{
10

return

number
<
10
11
}
12
var

numbers
= [
20
,
19
,
7
,
12
]
13
hasAnyMatches
(
numbers
,
lessThanTen
)
Functions are actually a special case of closures. You can write a closure without a name by surrounding code
with braces (
{}
). Use
in
to separate the arguments and return type from the body.
1
numbers
.
map
({
2
(
number
:
Int
) ->
Int

in
3

let

result
=
3
*
number
4

return

result
5
})
EXPERIMENT
Rewrite the closure to return zero for all odd numbers.
You have several options for writing closures more concisely. When a closure’s type is already known, such as
the callback for a delegate, you can omit the type of its parameters, its return type, or both. Single statement
closures implicitly return the value of their only statement.
1
numbers
.
map
({
number

in

3
*
number
})

You can refer to parameters by number instead of by name—this approach is especially useful in very short
closures. A closure passed as the last argument to a function can appear immediately after the parentheses.
1
sort
([
1
,
5
,
3
,
12
,
2
]) {
$0
>
$1
}
​
Objects and Classes
Use
class
followed by the class’s name to create a class. A property declaration in a class is written the
same way as a constant or variable declaration, except that it is in the context of a class. Likewise, method and
function declarations are written the same way.
1
class

Shape
{
2

var

numberOfSides
=
0
3

func

simpleDescription
() ->
String
{
4

return

"A shape with
\(
numberOfSides
)
sides."
5
}
6
}
EXPERIMENT
Add a constant property with
let
, and add another method that takes an argument.
Create an instance of a class by putting parentheses after the class name. Use dot syntax to access the
properties and methods of the instance.
1
var

shape
=
Shape
()
2
shape
.
numberOfSides
=
7
3
var

shapeDescription
=
shape
.
simpleDescription
()

This version of the
Shape
class is missing something important: an initializer to set up the class when an
instance is created. Use
init
to create one.
1
class

NamedShape
{
2

var

numberOfSides
:
Int
=
0
3

var

name
:
String
4

5

init
(
name
:
String
) {
6

self
.
name
=
name
7
}
8

9

func

simpleDescription
() ->
String
{
10

return

"A shape with
\(
numberOfSides
)
sides."
11
}
12
}
Notice how
self
is used to distinguish the
name
property from the
name
argument to the initializer. The
arguments to the initializer are passed like a function call when you create an instance of the class. Every
property needs a value assigned—either in its declaration (as with
numberOfSides
) or in the initializer (as
with
name
).
Use
deinit
to create a deinitializer if you need to perform some cleanup before the object is deallocated.
Subclasses include their superclass name after their class name, separated by a colon. There is no
requirement for classes to subclass any standard root class, so you can include or omit a superclass as
needed.
Methods on a subclass that override the superclass’s implementation are marked with
override
—overriding
a method by accident, without
override
, is detected by the compiler as an error. The compiler also detects
methods with
override
that don’t actually override any method in the superclass.
1
class

Square
:
NamedShape
{
2

var

sideLength
:
Double
3

4

init
(
sideLength
:
Double
,
name
:
String
) {

5

self
.
sideLength
=
sideLength
6

super
.
init
(
name
:
name
)
7

numberOfSides
=
4
8
}
9

10

func

area
() ->
Double
{
11

return

sideLength
*
sideLength
12
}
13

14

override

func

simpleDescription
() ->
String
{
15

return

"A square with sides of length
\(
sideLength
)
."
16
}
17
}
18
let

test
=
Square
(
sideLength
:
5.2
,
name
:
"my test square"
)
19
test
.
area
()
20
test
.
simpleDescription
()
EXPERIMENT
Make another subclass of
NamedShape
called
Circle
that takes a radius and a name as
arguments to its initializer. Implement an
area
and a
describe
method on the
Circle
class.
In addition to simple properties that are stored, properties can have a getter and a setter.
1
class

EquilateralTriangle
:
NamedShape
{
2

var

sideLength
:
Double
=
0.0
3

4

init
(
sideLength
:
Double
,
name
:
String
) {
5

self
.
sideLength
=
sideLength
6

super
.
init
(
name
:
name
)
7

numberOfSides
=
3
8
}

9

10

var

perimeter
:
Double
{
11

get
{
12

return

3.0
*
sideLength
13
}
14

set
{
15

sideLength
=
newValue
/
3.0
16
}
17
}
18

19

override

func

simpleDescription
() ->
String
{
20

return

"An equilateral triagle with sides of length
\
(
sideLength
)
."
21
}
22
}
23
var

triangle
=
EquilateralTriangle
(
sideLength
:
3.1
,
name
:
"a

triangle"
)
24
triangle
.
perimeter
25
triangle
.
perimeter
=
9.9
26
triangle
.
sideLength
In the setter for
perimeter
, the new value has the implicit name
newValue
. You can provide an explicit
name in parentheses after
set
.
Notice that the initializer for the
EquilateralTriangle
class has three different steps:
1
.
Setting the value of properties that the subclass declares.
2
.
Calling the superclass’s initializer.
3
.
Changing the value of properties defined by the superclass. Any additional setup work that uses
methods, getters, or setters can also be done at this point.
If you don’t need to compute the property but still need to provide code that is run before and after setting a new
value, use
willSet
and
didSet
. For example, the class below ensures that the side length of its triangle is
always the same as the side length of its square.

1
class

TriangleAndSquare
{
2

var

triangle
:
EquilateralTriangle
{
3

willSet
{
4

square
.
sideLength
=
newValue
.
sideLength
5
}
6
}
7

var

square
:
Square
{
8

willSet
{
9

triangle
.
sideLength
=
newValue
.
sideLength
10
}
11
}
12

init
(
size
:
Double
,
name
:
String
) {
13

square
=
Square
(
sideLength
:
size
,
name
:
name
)
14

triangle
=
EquilateralTriangle
(
sideLength
:
size
,
name
:

name
)
15
}
16
}
17
var

triangleAndSquare
=
TriangleAndSquare
(
size
:
10
,
name
:

"another test shape"
)
18
triangleAndSquare
.
square
.
sideLength
19
triangleAndSquare
.
triangle
.
sideLength
20
triangleAndSquare
.
square
=
Square
(
sideLength
:
50
,
name
:
"larger

square"
)
21
triangleAndSquare
.
triangle
.
sideLength
Methods on classes have one important difference from functions. Parameter names in functions are used only
within the function, but parameters names in methods are also used when you call the method (except for the
first parameter). By default, a method has the same name for its parameters when you call it and within the
method itself. You can specify a second name, which is used inside the method.
1
class

Counter
{
2

var

count
:
Int
=
0
3

func

incrementBy
(
amount
:
Int
,
numberOfTimes

times
:
Int
) {
4

count
+=
amount
*
times
5
}
6
}

7
var

counter
=
Counter
()
8
counter
.
incrementBy
(
2
,
numberOfTimes
:
7
)
When working with optional values, you can write
?
before operations like methods, properties, and
subscripting. If the value before the
?
is
nil
, everything after the
?
is ignored and the value of the whole
expression is
nil
. Otherwise, the optional value is unwrapped, and everything after the
?
acts on the
unwrapped value. In both cases, the value of the whole expression is an optional value.
1
let

optionalSquare
:
Square
? =
Square
(
sideLength
:
2.5
,
name
:
"optional

square"
)
2
let

sideLength
=
optionalSquare
?.
sideLength
​
Enumerations and Structures
Use
enum
to create an enumeration. Like classes and all other named types, enumerations can have methods
associated with them.
1
enum

Rank
:
Int
{
2

case

Ace
=
1
3

case

Two
,
Three
,
Four
,
Five
,
Six
,
Seven
,
Eight
,
Nine
,
Ten
4

case

Jack
,
Queen
,
King
5

func

simpleDescription
() ->
String
{
6

switch

self
{
7

case
.
Ace
:
8

return

"ace"
9

case
.
Jack
:
10

return

"jack"
11

case
.
Queen
:
12

return

"queen"
13

case
.
King
:
14

return

"king"
15

default
:
16

return

String
(
self
.
toRaw
())

17
}
18
}
19
}
20
let

ace
=
Rank
.
Ace
21
let

aceRawValue
=
ace
.
toRaw
()
EXPERIMENT
Write a function that compares two
Rank
values by comparing their raw values.
In the example above, the raw value type of the enumeration is
Int
, so you only have to specify the first raw
value. The rest of the raw values are assigned in order. You can also use strings or floating-point numbers as
the raw type of an enumeration.
Use the
toRaw
and
fromRaw
functions to convert between the raw value and the enumeration value.
1
if

let

convertedRank
=
Rank
.
fromRaw
(
3
) {
2

let

threeDescription
=
convertedRank
.
simpleDescription
()
3
}
The member values of an enumeration are actual values, not just another way of writing their raw values. In
fact, in cases where there isn’t a meaningful raw value, you don’t have to provide one.
1
enum

Suit
{
2

case

Spades
,
Hearts
,
Diamonds
,
Clubs
3

func

simpleDescription
() ->
String
{
4

switch

self
{
5

case
.
Spades
:
6

return

"spades"
7

case
.
Hearts
:
8

return

"hearts"

9

case
.
Diamonds
:
10

return

"diamonds"
11

case
.
Clubs
:
12

return

"clubs"
13
}
14
}
15
}
16
let

hearts
=
Suit
.
Hearts
17
let

heartsDescription
=
hearts
.
simpleDescription
()
EXPERIMENT
Add a
color
method to
Suit
that returns “black” for spades and clubs, and returns “red” for hearts
and diamonds.
Notice the two ways that the
Hearts
member of the enumeration is referred to above: When assigning a value
to the
hearts
constant, the enumeration member
Suit.Hearts
is referred to by its full name because the
constant doesn’t have an explicit type specified. Inside the switch, the enumeration is referred to by the
abbreviated form
.Hearts
because the value of
self
is already known to be a suit. You can use the
abbreviated form anytime the value’s type is already known.
Use
struct
to create a structure. Structures support many of the same behaviors as classes, including
methods and initializers. One of the most important differences between structures and classes is that
structures are always copied when they are passed around in your code, but classes are passed by reference.
1
struct

Card
{
2

var

rank
:
Rank
3

var

suit
:
Suit
4

func

simpleDescription
() ->
String
{
5

return

"The
\(
rank
.
simpleDescription
())
of
\
(
suit
.
simpleDescription
())
"
6
}

7
}
8
let

threeOfSpades
=
Card
(
rank
: .
Three
,
suit
: .
Spades
)
9
let

threeOfSpadesDescription
=
threeOfSpades
.
simpleDescription
()
EXPERIMENT
Add a method to
Card
that creates a full deck of cards, with one card of each combination of rank
and suit.
An instance of an enumeration member can have values associated with the instance. Instances of the same
enumeration member can have different values associated with them. You provide the associated values when
you create the instance. Associated values and raw values are different: The raw value of an enumeration
member is the same for all of its instances, and you provide the raw value when you define the enumeration.
For example, consider the case of requesting the sunrise and sunset time from a server. The server either
responds with the information or it responds with some error information.
1
enum

ServerResponse
{
2

case

Result
(
String
,
String
)
3

case

Error
(
String
)
4
}
5

6
let

success
=
ServerResponse
.
Result
(
"6:00 am"
,
"8:09 pm"
)
7
let

failure
=
ServerResponse
.
Error
(
"Out of cheese."
)
8

9
switch

success
{
10
case

let
.
Result
(
sunrise
,
sunset
):
11

let

serverResponse
=
"Sunrise is at
\(
sunrise
)
and sunset

is at
\(
sunset
)
."
12
case

let
.
Error
(
error
):
13

let

serverResponse
=
"Failure…
\(
error
)
"
14
}

EXPERIMENT
Add a third case to
ServerResponse
and to the switch.
Notice how the sunrise and sunset times are extracted from the
ServerResponse
value as part of matching
the value against the switch cases.
​
Protocols and Extensions
Use
protocol
to declare a protocol.
1
protocol

ExampleProtocol
{
2

var

simpleDescription
:
String
{
get
}
3

mutating

func

adjust
()
4
}
Classes, enumerations, and structs can all adopt protocols.
1
class

SimpleClass
:
ExampleProtocol
{
2

var

simpleDescription
:
String
=
"A very simple class."
3

var

anotherProperty
:
Int
=
69105
4

func

adjust
() {
5

simpleDescription
+=
" Now 100% adjusted."
6
}
7
}
8
var

a
=
SimpleClass
()
9
a
.
adjust
()
10
let

aDescription
=
a
.
simpleDescription
11

12
struct

SimpleStructure
:
ExampleProtocol
{

13

var

simpleDescription
:
String
=
"A simple structure"
14

mutating

func

adjust
() {
15

simpleDescription
+=
" (adjusted)"
16
}
17
}
18
var

b
=
SimpleStructure
()
19
b
.
adjust
()
20
let

bDescription
=
b
.
simpleDescription
EXPERIMENT
Write an enumeration that conforms to this protocol.
Notice the use of the
mutating
keyword in the declaration of
SimpleStructure
to mark a method that
modifies the structure. The declaration of
SimpleClass
doesn’t need any of its methods marked as mutating
because methods on a class can always modify the class.
Use
extension
to add functionality to an existing type, such as new methods and computed properties. You
can use an extension to add protocol conformance to a type that is declared elsewhere, or even to a type that
you imported from a library or framework.
1
extension

Int
:
ExampleProtocol
{
2

var

simpleDescription
:
String
{
3

return

"The number
\(
self
)
"
4
}
5

mutating

func

adjust
() {
6

self
+=
42
7
}
8
}
9
7
.
simpleDescription

EXPERIMENT
Write an extension for the
Double
type that adds an
absoluteValue
property.
You can use a protocol name just like any other named type—for example, to create a collection of objects that
have different types but that all conform to a single protocol. When you work with values whose type is a
protocol type, methods outside the protocol definition are not available.
1
let

protocolValue
:
ExampleProtocol
=
a
2
protocolValue
.
simpleDescription
3
// protocolValue.anotherProperty // Uncomment to see the error
Even though the variable
protocolValue
has a runtime type of
SimpleClass
, the compiler treats it as the
given type of
ExampleProtocol
. This means that you can’t accidentally access methods or properties that
the class implements in addition to its protocol conformance.
​
Generics
Write a name inside angle brackets to make a generic function or type.
1
func

repeat
<
ItemType
>(
item
:
ItemType
,
times
:
Int
) ->
ItemType
[] {
2

var

result
=
ItemType
[]()
3

for

i

in

0
..
times
{
4

result
+=
item
5
}
6

return

result
7
}
8
repeat
(
"knock"
,
4
)
You can make generic forms of functions and methods, as well as classes, enumerations, and structures.

1
// Reimplement the Swift standard library's optional type
2
enum

OptionalValue
<
T
> {
3

case

None
4

case

Some
(
T
)
5
}
6
var

possibleInteger
:
OptionalValue
<
Int
> = .
None
7
possibleInteger
= .
Some
(
100
)
Use
where
after the type name to specify a list of requirements—for example, to require the type to implement
a protocol, to require two types to be the same, or to require a class to have a particular superclass.
1
func

anyCommonElements
<
T
,
U

where

T
:
Sequence
,
U
:
Sequence
,

T
.
GeneratorType
.
Element
:
Equatable
,

T
.
GeneratorType
.
Element
==
U
.
GeneratorType
.
Element
>

(
lhs
:
T
,
rhs
:
U
) ->
Bool
{
2

for

lhsItem

in

lhs
{
3

for

rhsItem

in

rhs
{
4

if

lhsItem
==
rhsItem
{
5

return

true
6
}
7
}
8
}
9

return

false
10
}
11
anyCommonElements
([
1
,
2
,
3
], [
3
])
EXPERIMENT
Modify the
anyCommonElements
function to make a function that returns an array of the
elements that any two sequences have in common.
In the simple cases, you can omit
where
and simply write the protocol or class name after a colon. Writing

<T: Equatable>
is the same as writing
<T where T: Equatable>
.

​
​
Language Guide

​
​
The Basics
Swift is a new programming language for iOS and OS X app development. Nonetheless, many parts of Swift
will be familiar from your experience of developing in C and Objective-C.
Swift provides its own versions of all fundamental C and Objective-C types, including
Int
for integers;
Double
and
Float
for floating-point values;
Bool
for Boolean values; and
String
for textual data. Swift
also provides powerful versions of the two primary collection types,
Array
and
Dictionary
, as described
in
Collection Types
.
Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use
of variables whose values cannot be changed. These are known as constants, and are much more powerful
than constants in C. Constants are used throughout Swift to make code safer and clearer in intent when you
work with values that do not need to change.
In addition to familiar types, Swift introduces advanced types not found in Objective-C. These include tuples,
which enable you to create and pass around groupings of values. Tuples can return multiple values from a
function as a single compound value.
Swift also introduces optional types, which handle the absence of a value. Optionals say either “there
is
a value,
and it equals
x
” or “there
isn’t
a value at all”. Optionals are similar to using
nil
with pointers in Objective-C,
but they work for any type, not just classes. Optionals are safer and more expressive than
nil
pointers in
Objective-C and are at the heart of many of Swift’s most powerful features.
Optionals are an example of the fact that Swift is a
type safe
language. Swift helps you to be clear about the
types of values your code can work with. If part of your code expects a
String
, type safety prevents you from
passing it an
Int
by mistake. This enables you to catch and fix errors as early as possible in the development
process.
​
Constants and Variables
Constants and variables associate a name (such as
maximumNumberOfLoginAttempts
or
welcomeMessage
) with a value of a particular type (such as the number
10
or the string
"Hello"
). The

value of a
constant
cannot be changed once it is set, whereas a
variable
can be set to a different value in the
future.
​
Declaring Constants and Variables
Constants and variables must be declared before they are used. You declare constants with the
let
keyword
and variables with the
var
keyword. Here’s an example of how constants and variables can be used to track
the number of login attempts a user has made:
1
let

maximumNumberOfLoginAttempts
=
10
2
var

currentLoginAttempt
=
0
This code can be read as:
“Declare a new constant called
maximumNumberOfLoginAttempts
, and give it a value of
10
. Then,
declare a new variable called
currentLoginAttempt
, and give it an initial value of
0
.”
In this example, the maximum number of allowed login attempts is declared as a constant, because the
maximum value never changes. The current login attempt counter is declared as a variable, because this value
must be incremented after each failed login attempt.
You can declare multiple constants or multiple variables on a single line, separated by commas:
1
var

x
=
0.0
,
y
=
0.0
,
z
=
0.0
NOTE
If a stored value in your code is not going to change, always declare it as a constant with the
let
keyword. Use variables only for storing values that need to be able to change.

​
Type Annotations
You can provide a
type annotation
when you declare a constant or variable, to be clear about the kind of values
the constant or variable can store. Write a type annotation by placing a colon after the constant or variable
name, followed by a space, followed by the name of the type to use.
This example provides a type annotation for a variable called
welcomeMessage
, to indicate that the variable
can store
String
values:
1
var

welcomeMessage
:
String
The colon in the declaration means
“…of type…,”
so the code above can be read as:
“Declare a variable called
welcomeMessage
that is of type
String
.”
The phrase “of type
String
” means “can store any
String
value.” Think of it as meaning “the type of thing”
(or “the kind of thing”) that can be stored.
The
welcomeMessage
variable can now be set to any string value without error:
1
welcomeMessage
=
"Hello"
NOTE
It is rare that you need to write type annotations in practice. If you provide an initial value for a
constant or variable at the point that it is defined, Swift can almost always infer the type to be used
for that constant or variable, as described in
Type Safety and Type Inference
. In the
welcomeMessage
example above, no initial value is provided, and so the type of the
welcomeMessage
variable is specified with a type annotation rather than being inferred from an
initial value.

​
Naming Constants and Variables
You can use almost any character you like for constant and variable names, including Unicode characters:
1
let

π
=
3.14159
2
let

你好
=
"
你好世界
"
3
let

=
"dogcow"
Constant and variable names cannot contain mathematical symbols, arrows, private-use (or invalid) Unicode
code points, or line- and box-drawing characters. Nor can they begin with a number, although numbers may be
included elsewhere within the name.
Once you’ve declared a constant or variable of a certain type, you can’t redeclare it again with the same name,
or change it to store values of a different type. Nor can you change a constant into a variable or a variable into a
constant.
NOTE
If you need to give a constant or variable the same name as a reserved Swift keyword, you can do
so by surrounding the keyword with back ticks (
`
) when using it as a name. However, you should
avoid using keywords as names unless you have absolutely no choice.
You can change the value of an existing variable to another value of a compatible type. In this example, the value
of
friendlyWelcome
is changed from
"Hello!"
to
"Bonjour!"
:
1
var

friendlyWelcome
=
"Hello!"
2
friendlyWelcome
=
"Bonjour!"
3
// friendlyWelcome is now "Bonjour!"
Unlike a variable, the value of a constant cannot be changed once it is set. Attempting to do so is reported as an

error when your code is compiled:
1
let

languageName
=
"Swift"
2
languageName
=
"Swift++"
3
// this is a compile-time error – languageName cannot be changed
​
Printing Constants and Variables
You can print the current value of a constant or variable with the
println
function:
1
println
(
friendlyWelcome
)
2
// prints "Bonjour!"
println
is a global function that prints a value, followed by a line break, to an appropriate output. If you are
working in Xcode, for example,
println
prints its output in Xcode’s “console” pane. (A second function,
print
, performs the same task without appending a line break to the end of the value to be printed.)
The
println
function prints any
String
value you pass to it:
1
println
(
"This is a string"
)
2
// prints "This is a string"
The
println
function can print more complex logging messages, in a similar manner to Cocoa’s
NSLog
function. These messages can include the current values of constants and variables.
Swift uses
string interpolation
to include the name of a constant or variable as a placeholder in a longer string,
and to prompt Swift to replace it with the current value of that constant or variable. Wrap the name in
parentheses and escape it with a backslash before the opening parenthesis:
1
println
(
"The current value of friendlyWelcome is
\(
friendlyWelcome
)
"
)
2
// prints "The current value of friendlyWelcome is Bonjour!"

NOTE
All options you can use with string interpolation are described in
String Interpolation
.
​
Comments
Use comments to include non-executable text in your code, as a note or reminder to yourself. Comments are
ignored by the Swift compiler when your code is compiled.
Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes
(
//
):
1
// this is a comment
You can also write multiline comments, which start with a forward-slash followed by an asterisk (
/*
) and end
with an asterisk followed by a forward-slash (
*/
):
1
/* this is also a comment,
2
but written over multiple lines */
Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments.
You write nested comments by starting a multiline comment block and then starting a second multiline
comment within the first block. The second block is then closed, followed by the first block:
1
/* this is the start of the first multiline comment
2
/* this is the second, nested multiline comment */
3
this is the end of the first multiline comment */
Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code

already contains multiline comments.
​
Semicolons
Unlike many other languages, Swift does not require you to write a semicolon (
;
) after each statement in your
code, although you can do so if you wish. Semicolons
are
required, however, if you want to write multiple
separate statements on a single line:
1
let

cat
=
""
;
println
(
cat
)
2
// prints ""
​
Integers
Integers
are whole numbers with no fractional component, such as
42
and
-23
. Integers are either
signed
(positive, zero, or negative) or
unsigned
(positive or zero).
Swift provides signed and unsigned integers in 8, 16, 32, and 64 bit forms. These integers follow a naming
convention similar to C, in that an 8-bit unsigned integer is of type
UInt8
, and a 32-bit signed integer is of type
Int32
. Like all types in Swift, these integer types have capitalized names.
​
Integer Bounds
You can access the minimum and maximum values of each integer type with its
min
and
max
properties:
1
let

minValue
=
UInt8
.
min

// minValue is equal to 0, and is of type

UInt8
2
let

maxValue
=
UInt8
.
max

// maxValue is equal to 255, and is of type

UInt8
The values of these properties are of the appropriate-sized number type (such as
UInt8
in the example above)

and can therefore be used in expressions alongside other values of the same type.
​
Int
In most cases, you don’t need to pick a specific size of integer to use in your code. Swift provides an additional
integer type,
Int
, which has the same size as the current platform’s native word size:
Unless you need to work with a specific size of integer, always use
Int
for integer values in your code. This
aids code consistency and interoperability. Even on 32-bit platforms,
Int
can store any value between
–
2,147,483,648
and
2,147,483,647
, and is large enough for many integer ranges.
​
UInt
Swift also provides an unsigned integer type,
UInt
, which has the same size as the current platform’s native
word size:
NOTE
Use
UInt
only when you specifically need an unsigned integer type with the same size as the
platform’s native word size. If this is not the case,
Int
is preferred, even when the values to be
stored are known to be non-negative. A consistent use of
Int
for integer values aids code
interoperability, avoids the need to convert between different number types, and matches integer
type inference, as described in
Type Safety and Type Inference
.
On a 32-bit platform,
Int
is the same size as
Int32
.
On a 64-bit platform,
Int
is the same size as
Int64
.
On a 32-bit platform,
UInt
is the same size as
UInt32
.
On a 64-bit platform,
UInt
is the same size as
UInt64
.

​
Floating-Point Numbers
Floating-point numbers
are numbers with a fractional component, such as
3.14159
,
0.1
, and
-273.15
.
Floating-point types can represent a much wider range of values than integer types, and can store numbers that
are much larger or smaller than can be stored in an
Int
. Swift provides two signed floating-point number
types:
NOTE
Double
has a precision of at least 15 decimal digits, whereas the precision of
Float
can be as
little as 6 decimal digits. The appropriate floating-point type to use depends on the nature and range
of values you need to work with in your code.
​
Type Safety and Type Inference
Swift is a
type safe
language. A type safe language encourages you to be clear about the types of values your
code can work with. If part of your code expects a
String
, you can’t pass it an
Int
by mistake.
Because Swift is type safe, it performs
type checks
when compiling your code and flags any mismatched types
as errors. This enables you to catch and fix errors as early as possible in the development process.
Type-checking helps you avoid errors when you’re working with different types of values. However, this doesn’t
mean that you have to specify the type of every constant and variable that you declare. If you don’t specify the
Double
represents a 64-bit floating-point number. Use it when floating-point values must be
very large or particularly precise.
Float
represents a 32-bit floating-point number. Use it when floating-point values do not
require 64-bit precision.

type of value you need, Swift uses
type inference
to work out the appropriate type. Type inference enables a
compiler to deduce the type of a particular expression automatically when it compiles your code, simply by
examining the values you provide.
Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C.
Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.
Type inference is particularly useful when you declare a constant or variable with an initial value. This is often
done by assigning a
literal value
(or
literal
) to the constant or variable at the point that you declare it. (A literal
value is a value that appears directly in your source code, such as
42
and
3.14159
in the examples below.)
For example, if you assign a literal value of
42
to a new constant without saying what type it is, Swift infers that
you want the constant to be an
Int
, because you have initialized it with a number that looks like an integer:
1
let

meaningOfLife
=
42
2
// meaningOfLife is inferred to be of type Int
Likewise, if you don’t specify a type for a floating-point literal, Swift infers that you want to create a
Double
:
1
let

pi
=
3.14159
2
// pi is inferred to be of type Double
Swift always chooses
Double
(rather than
Float
) when inferring the type of floating-point numbers.
If you combine integer and floating-point literals in an expression, a type of
Double
will be inferred from the
context:
1
let

anotherPi
=
3
+
0.14159
2
// anotherPi is also inferred to be of type Double
The literal value of
3
has no explicit type in and of itself, and so an appropriate output type of
Double
is inferred
from the presence of a floating-point literal as part of the addition.
​

Numeric Literals
Integer literals can be written as:
All of these integer literals have a decimal value of
17
:
1
let

decimalInteger
=
17
2
let

binaryInteger
=
0b10001

// 17 in binary notation
3
let

octalInteger
=
0o21

// 17 in octal notation
4
let

hexadecimalInteger
=
0x11

// 17 in hexadecimal notation
Floating-point literals can be decimal (with no prefix), or hexadecimal (with a
0x
prefix). They must always
have a number (or hexadecimal number) on both sides of the decimal point. They can also have an optional
exponent
, indicated by an uppercase or lowercase
e
for decimal floats, or an uppercase or lowercase
p
for
hexadecimal floats.
For decimal numbers with an exponent of
exp
, the base number is multiplied by 10
exp
:
For hexadecimal numbers with an exponent of
exp
, the base number is multiplied by 2
exp
:
All of these floating-point literals have a decimal value of
12.1875
:
A
decimal
number, with no prefix
A
binary
number, with a
0b
prefix
An
octal
number, with a
0o
prefix
A
hexadecimal
number, with a
0x
prefix
1.25e2
means 1.25 × 102
, or
125.0
.
1.25e-2
means 1.25 × 10-2
, or
0.0125
.
0xFp2
means 15 × 22
, or
60.0
.
0xFp-2
means 15 × 2-2
, or
3.75
.

1
let

decimalDouble
=
12.1875
2
let

exponentDouble
=
1.21875e1
3
let

hexadecimalDouble
=
0xC.3p0
Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be
padded with extra zeroes and can contain underscores to help with readability. Neither type of formatting affects
the underlying value of the literal:
1
let

paddedDouble
=
000123.456
2
let

oneMillion
=
1_000_000
3
let

justOverOneMillion
=
1_000_000.000_000_1
​
Numeric Type Conversion
Use the
Int
type for all general-purpose integer constants and variables in your code, even if they are known to
be non-negative. Using the default integer type in everyday situations means that integer constants and
variables are immediately interoperable in your code and will match the inferred type for integer literal values.
Use other integer types only when they are are specifically needed for the task at hand, because of explicitly-
sized data from an external source, or for performance, memory usage, or other necessary optimization. Using
explicitly-sized types in these situations helps to catch any accidental value overflows and implicitly documents
the nature of the data being used.
​
Integer Conversion
The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An
Int8
constant or variable can store numbers between
-128
and
127
, whereas a
UInt8
constant or variable
can store numbers between
0
and
255
. A number that will not fit into a constant or variable of a sized integer
type is reported as an error when your code is compiled:
1
let

cannotBeNegative
:
UInt8
= –
1
2
// UInt8 cannot store negative numbers, and so this will report an

error
3
let

tooBig
:
Int8
=
Int8
.
max
+
1
4
// Int8 cannot store a number larger than its maximum value,
5
// and so this will also report an error
Because each numeric type can store a different range of values, you must opt in to numeric type conversion on
a case-by-case basis. This opt-in approach prevents hidden conversion errors and helps make type conversion
intentions explicit in your code.
To convert one specific number type to another, you initialize a new number of the desired type with the existing
value. In the example below, the constant
twoThousand
is of type
UInt16
, whereas the constant
one
is of
type
UInt8
. They cannot be added together directly, because they are not of the same type. Instead, this
example calls
UInt16(one)
to create a new
UInt16
initialized with the value of
one
, and uses this value in
place of the original:
1
let

twoThousand
:
UInt16
=
2_000
2
let

one
:
UInt8
=
1
3
let

twoThousandAndOne
=
twoThousand
+
UInt16
(
one
)
Because both sides of the addition are now of type
UInt16
, the addition is allowed. The output constant
(
twoThousandAndOne
) is inferred to be of type
UInt16
, because it is the sum of two
UInt16
values.
SomeType(ofInitialValue)
is the default way to call the initializer of a Swift type and pass in an initial
value. Behind the scenes,
UInt16
has an initializer that accepts a
UInt8
value, and so this initializer is used
to make a new
UInt16
from an existing
UInt8
. You can’t pass in
any
type here, however—it has to be a type
for which
UInt16
provides an initializer. Extending existing types to provide initializers that accept new types
(including your own type definitions) is covered in
Extensions
.
​
Integer and Floating-Point Conversion
Conversions between integer and floating-point numeric types must be made explicit:
1
let

three
=
3

2
let

pointOneFourOneFiveNine
=
0.14159
3
let

pi
=
Double
(
three
) +
pointOneFourOneFiveNine
4
// pi equals 3.14159, and is inferred to be of type Double
Here, the value of the constant
three
is used to create a new value of type
Double
, so that both sides of the
addition are of the same type. Without this conversion in place, the addition would not be allowed.
The reverse is also true for floating-point to integer conversion, in that an integer type can be initialized with a
Double
or
Float
value:
1
let

integerPi
=
Int
(
pi
)
2
// integerPi equals 3, and is inferred to be of type Int
Floating-point values are always truncated when used to initialize a new integer value in this way. This means
that
4.75
becomes
4
, and
-3.9
becomes
-3
.
NOTE
The rules for combining numeric constants and variables are different from the rules for numeric
literals. The literal value
3
can be added directly to the literal value
0.14159
, because number
literals do not have an explicit type in and of themselves. Their type is inferred only at the point that
they are evaluated by the compiler.
​
Type Aliases
Type aliases
define an alternative name for an existing type. You define type aliases with the
typealias
keyword.
Type aliases are useful when you want to refer to an existing type by a name that is contextually more
appropriate, such as when working with data of a specific size from an external source:

1
typealias

AudioSample
=
UInt16
Once you define a type alias, you can use the alias anywhere you might use the original name:
1
var

maxAmplitudeFound
=
AudioSample
.
min
2
// maxAmplitudeFound is now 0
Here,
AudioSample
is defined as an alias for
UInt16
. Because it is an alias, the call to
AudioSample.min
actually calls
UInt16.min
, which provides an initial value of
0
for the
maxAmplitudeFound
variable.
​
Booleans
Swift has a basic
Boolean
type, called
Bool
. Boolean values are referred to as
logical
, because they can only
ever be true or false. Swift provides two Boolean constant values,
true
and
false
:
1
let

orangesAreOrange
=
true
2
let

turnipsAreDelicious
=
false
The types of
orangesAreOrange
and
turnipsAreDelicious
have been inferred as
Bool
from the
fact that they were initialized with Boolean literal values. As with
Int
and
Double
above, you don’t need to
declare constants or variables as
Bool
if you set them to
true
or
false
as soon as you create them. Type
inference helps make Swift code more concise and readable when it initializes constants or variables with other
values whose type is already known.
Boolean values are particularly useful when you work with conditional statements such as the
if
statement:
1
if

turnipsAreDelicious
{
2

println
(
"Mmm, tasty turnips!"
)
3
}
else
{
4

println
(
"Eww, turnips are horrible."
)
5
}

6
// prints "Eww, turnips are horrible."
Conditional statements such as the
if
statement are covered in more detail in
Control Flow
.
Swift’s type safety prevents non-Boolean values from being be substituted for
Bool
. The following example
reports a compile-time error:
1
let

i
=
1
2
if

i
{
3

// this example will not compile, and will report an error
4
}
However, the alternative example below is valid:
1
let

i
=
1
2
if

i
==
1
{
3

// this example will compile successfully
4
}
The result of the
i == 1
comparison is of type
Bool
, and so this second example passes the type-check.
Comparisons like
i == 1
are discussed in
Basic Operators
.
As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the
intention of a particular section of code is always clear.
​
Tuples
Tuples
group multiple values into a single compound value. The values within a tuple can be of any type and do
not have to be of the same type as each other.
In this example,
(404, "Not Found")
is a tuple that describes an
HTTP status code
. An HTTP status
code is a special value returned by a web server whenever you request a web page. A status code of
404 Not
Found
is returned if you request a webpage that doesn’t exist.

1
let

http404Error
= (
404
,
"Not Found"
)
2
// http404Error is of type (Int, String), and equals (404, "Not

Found")
The
(404, "Not Found")
tuple groups together an
Int
and a
String
to give the HTTP status code two
separate values: a number and a human-readable description. It can be described as “a tuple of type
(Int,
String)
”.
You can create tuples from any permutation of types, and they can contain as many different types as you like.
There’s nothing stopping you from having a tuple of type
(Int, Int, Int)
, or
(String, Bool)
, or
indeed any other permutation you require.
You can
decompose
a tuple’s contents into separate constants or variables, which you then access as usual:
1
let
(
statusCode
,
statusMessage
) =
http404Error
2
println
(
"The status code is
\(
statusCode
)
"
)
3
// prints "The status code is 404"
4
println
(
"The status message is
\(
statusMessage
)
"
)
5
// prints "The status message is Not Found"
If you only need some of the tuple’s values, ignore parts of the tuple with an underscore (
_
) when you
decompose the tuple:
1
let
(
justTheStatusCode
,
_
) =
http404Error
2
println
(
"The status code is
\(
justTheStatusCode
)
"
)
3
// prints "The status code is 404"
Alternatively, access the individual element values in a tuple using index numbers starting at zero:
1
println
(
"The status code is
\(
http404Error
.
0
)
"
)
2
// prints "The status code is 404"
3
println
(
"The status message is
\(
http404Error
.
1
)
"
)
4
// prints "The status message is Not Found"

You can name the individual elements in a tuple when the tuple is defined:
1
let

http200Status
= (
statusCode
:
200
,
description
:
"OK"
)
If you name the elements in a tuple, you can use the element names to access the values of those elements:
1
println
(
"The status code is
\(
http200Status
.
statusCode
)
"
)
2
// prints "The status code is 200"
3
println
(
"The status message is
\(
http200Status
.
description
)
"
)
4
// prints "The status message is OK"
Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might
return the
(Int, String)
tuple type to describe the success or failure of the page retrieval. By returning a
tuple with two distinct values, each of a different type, the function provides more useful information about its
outcome than if it could only return a single value of a single type. For more information, see
Functions with
Multiple Return Values
.
NOTE
Tuples are useful for temporary groups of related values. They are not suited to the creation of
complex data structures. If your data structure is likely to persist beyond a temporary scope, model
it as a class or structure, rather than as a tuple. For more information, see
Classes and Structures
.
​
Optionals
You use
optionals
in situations where a value may be absent. An optional says:
or
There
is
a value, and it equals
x

NOTE
The concept of optionals doesn’t exist in C or Objective-C. The nearest thing in Objective-C is the
ability to return
nil
from a method that would otherwise return an object, with
nil
meaning “the
absence of a valid object.” However, this only works for objects—it doesn’t work for structs, basic C
types, or enumeration values. For these types, Objective-C methods typically return a special value
(such as
NSNotFound
) to indicate the absence of a value. This approach assumes that the
method’s caller knows there is a special value to test against and remembers to check for it. Swift’s
optionals let you indicate the absence of a value for
any type at all
, without the need for special
constants.
Here’s an example. Swift’s
String
type has a method called
toInt
, which tries to convert a
String
value
into an
Int
value. However, not every string can be converted into an integer. The string
"123"
can be
converted into the numeric value
123
, but the string
"hello, world"
does not have an obvious numeric
value to convert to.
The example below uses the
toInt
method to try to convert a
String
into an
Int
:
1
let

possibleNumber
=
"123"
2
let

convertedNumber
=
possibleNumber
.
toInt
()
3
// convertedNumber is inferred to be of type "Int?", or "optional Int"
Because the
toInt
method might fail, it returns an
optional

Int
, rather than an
Int
. An optional
Int
is
written as
Int?
, not
Int
. The question mark indicates that the value it contains is optional, meaning that it
might contain
some

Int
value, or it might contain
no value at all
. (It can’t contain anything else, such as a
Bool
value or a
String
value. It’s either an
Int
, or it’s nothing at all.)
​
If Statements and Forced Unwrapping
There
isn’t
a value at all

You can use an
if
statement to find out whether an optional contains a value. If an optional does have a value, it
evaluates to
true
; if it has no value at all, it evaluates to
false
.
Once you’re sure that the optional
does
contain a value, you can access its underlying value by adding an
exclamation mark (
!
) to the end of the optional’s name. The exclamation mark effectively says, “I know that
this optional definitely has a value; please use it.” This is known as
forced unwrapping
of the optional’s value:
1
if

convertedNumber
{
2

println
(
"
\(
possibleNumber
)
has an integer value of
\
(
convertedNumber
!)
"
)
3
}
else
{
4

println
(
"
\(
possibleNumber
)
could not be converted to an integer"
)
5
}
6
// prints "123 has an integer value of 123"
For more on the
if
statement, see
Control Flow
.
NOTE
Trying to use
!
to access a non-existent optional value triggers a runtime error. Always make sure
that an optional contains a non-
nil
value before using
!
to force-unwrap its value.
​
Optional Binding
You use
optional binding
to find out whether an optional contains a value, and if so, to make that value available
as a temporary constant or variable. Optional binding can be used with
if
and
while
statements to check for
a value inside an optional, and to extract that value into a constant or variable, as part of a single action.
if
and
while
statements are described in more detail in
Control Flow
.

Write optional bindings for the
if
statement as follows:
if

let

constantName
=
someOptional
{

statements
}
You can rewrite the
possibleNumber
example from above to use optional binding rather than forced
unwrapping:
1
if

let

actualNumber
=
possibleNumber
.
toInt
() {
2

println
(
"
\(
possibleNumber
)
has an integer value of
\
(
actualNumber
)
"
)
3
}
else
{
4

println
(
"
\(
possibleNumber
)
could not be converted to an integer"
)
5
}
6
// prints "123 has an integer value of 123"
This can be read as:
“If the optional
Int
returned by
possibleNumber.toInt
contains a value, set a new constant called
actualNumber
to the value contained in the optional.”
If the conversion is successful, the
actualNumber
constant becomes available for use within the first branch
of the
if
statement. It has already been initialized with the value contained
within
the optional, and so there is no
need to use the
!
suffix to access its value. In this example,
actualNumber
is simply used to print the result
of the conversion.
You can use both constants and variables with optional binding. If you wanted to manipulate the value of
actualNumber
within the first branch of the
if
statement, you could write
if var actualNumber
instead, and the value contained within the optional would be made available as a variable rather than a
constant.
​

nil
You set an optional variable to a valueless state by assigning it the special value
nil
:
1
var

serverResponseCode
:
Int
? =
404
2
// serverResponseCode contains an actual Int value of 404
3
serverResponseCode
=
nil
4
// serverResponseCode now contains no value
NOTE
nil
cannot be used with non-optional constants and variables. If a constant or variable in your code
needs to be able to cope with the absence of a value under certain conditions, always declare it as an
optional value of the appropriate type.
If you define an optional constant or variable without providing a default value, the constant or variable is
automatically set to
nil
for you:
1
var

surveyAnswer
:
String
?
2
// surveyAnswer is automatically set to nil
NOTE
Swift’s
nil
is not the same as
nil
in Objective-C. In Objective-C,
nil
is a pointer to a non-
existent object. In Swift,
nil
is not a pointer—it is the absence of a value of a certain type. Optionals
of
any
type can be set to
nil
, not just object types.
​

Implicitly Unwrapped Optionals
As described above, optionals indicate that a constant or variable is allowed to have “no value”. Optionals can
be checked with an
if
statement to see if a value exists, and can be conditionally unwrapped with optional
binding to access the optional’s value if it does exist.
Sometimes it is clear from a program’s structure that an optional will
always
have a value, after that value is
first set. In these cases, it is useful to remove the need to check and unwrap the optional’s value every time it is
accessed, because it can be safely assumed to have a value all of the time.
These kinds of optionals are defined as
implicitly unwrapped optionals
. You write an implicitly unwrapped
optional by placing an exclamation mark (
String!
) rather than a question mark (
String?
) after the type
that you want to make optional.
Implicitly unwrapped optionals are useful when an optional’s value is confirmed to exist immediately after the
optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of
implicitly unwrapped optionals in Swift is during class initialization, as described in
Unowned References and
Implicitly Unwrapped Optional Properties
.
An implicitly unwrapped optional is a normal optional behind the scenes, but can also be used like a nonoptional
value, without the need to unwrap the optional value each time it is accessed. The following example shows the
difference in behavior between an optional
String
and an implicitly unwrapped optional
String
:
1
let

possibleString
:
String
? =
"An optional string."
2
println
(
possibleString
!)
// requires an exclamation mark to access its
value
3
// prints "An optional string."
4

5
let

assumedString
:
String
! =
"An implicitly unwrapped optional

string."
6
println
(
assumedString
)
// no exclamation mark is needed to access its
value
7
// prints "An implicitly unwrapped optional string."
You can think of an implicitly unwrapped optional as giving permission for the optional to be unwrapped
automatically whenever it is used. Rather than placing an exclamation mark after the optional’s name each time

you use it, you place an exclamation mark after the optional’s type when you declare it.
NOTE
If you try to access an implicitly unwrapped optional when it does not contain a value, you will trigger
a runtime error. The result is exactly the same as if you place an exclamation mark after a normal
optional that does not contain a value.
You can still treat an implicitly unwrapped optional like a normal optional, to check if it contains a value:
1
if

assumedString
{
2

println
(
assumedString
)
3
}
4
// prints "An implicitly unwrapped optional string."
You can also use an implicitly unwrapped optional with optional binding, to check and unwrap its value in a
single statement:
1
if

let

definiteString
=
assumedString
{
2

println
(
definiteString
)
3
}
4
// prints "An implicitly unwrapped optional string."
NOTE
Implicitly unwrapped optionals should not be used when there is a possibility of a variable becoming
nil
at a later point. Always use a normal optional type if you need to check for a
nil
value during
the lifetime of a variable.

​
Assertions
Optionals enable you to check for values that may or may not exist, and to write code that copes gracefully with
the absence of a value. In some cases, however, it is simply not possible for your code to continue execution if a
value does not exist, or if a provided value does not satisfy certain conditions. In these situations, you can trigger
an
assertion
in your code to end code execution and to provide an opportunity to debug the cause of the absent or
invalid value.
​
Debugging with Assertions
An assertion is a runtime check that a logical condition definitely evaluates to
true
. Literally put, an assertion
“asserts” that a condition is true. You use an assertion to make sure that an essential condition is satisfied
before executing any further code. If the condition evaluates to
true
, code execution continues as usual; if the
condition evaluates to
false
, code execution ends, and your app is terminated.
If your code triggers an assertion while running in a debug environment, such as when you build and run an app
in Xcode, you can see exactly where the invalid state occurred and query the state of your app at the time that
the assertion was triggered. An assertion also lets you provide a suitable debug message as to the nature of the
assert.
You write an assertion by calling the global
assert
function. You pass the
assert
function an expression that
evaluates to
true
or
false
and a message that should be displayed if the result of the condition is
false
:
1
let

age
= –
3
2
assert
(
age
>=
0
,
"A person's age cannot be less than zero"
)
3
// this causes the assertion to trigger, because age is not >= 0
In this example, code execution will continue only if
age >= 0
evaluates to
true
, that is, if the value of
age
is non-negative. If the value of
age

is
negative, as in the code above, then
age >= 0
evaluates to
false
, and
the assertion is triggered, terminating the application.
Assertion messages cannot use string interpolation. The assertion message can be omitted if desired, as in the
following example:

1
assert
(
age
>=
0
)
​
When to Use Assertions
Use an assertion whenever a condition has the potential to be false, but must
definitely
be true in order for your
code to continue execution. Suitable scenarios for an assertion check include:
See also
Subscripts
and
Functions
.
NOTE
Assertions cause your app to terminate and are not a substitute for designing your code in such a
way that invalid conditions are unlikely to arise. Nonetheless, in situations where invalid conditions
are possible, an assertion is an effective way to ensure that such conditions are highlighted and
noticed during development, before your app is published.
An integer subscript index is passed to a custom subscript implementation, but the subscript
index value could be too low or too high.
A value is passed to a function, but an invalid value means that the function cannot fulfill its task.
An optional value is currently
nil
, but a non-
nil
value is essential for subsequent code to
execute successfully.

​
​
Basic Operators
An
operator
is a special symbol or phrase that you use to check, change, or combine values. For example, the
addition operator (
+
) adds two numbers together (as in
let i = 1 + 2
). More complex examples include
the logical AND operator
&&
(as in
if enteredDoorCode && passedRetinaScan
) and the increment
operator
++i
, which is a shortcut to increase the value of
i
by
1
.
Swift supports most standard C operators and improves several capabilities to eliminate common coding
errors. The assignment operator (
=
) does not return a value, to prevent it from being mistakenly used when the
equal to operator (
==
) is intended. Arithmetic operators (
+
,
–
,
*
,
/
,
%
and so forth) detect and disallow value
overflow, to avoid unexpected results when working with numbers that become larger or smaller than the
allowed value range of the type that stores them. You can opt in to value overflow behavior by using Swift’s
overflow operators, as described in
Overflow Operators
.
Unlike C, Swift lets you perform remainder (
%
) calculations on floating-point numbers. Swift also provides two
range operators (
a..b
and
a…b
) not found in C, as a shortcut for expressing a range of values.
This chapter describes the common operators in Swift.
Advanced Operators
covers Swift’s advanced
operators, and describes how to define your own custom operators and implement the standard operators for
your own custom types.
​
Terminology
Operators are unary, binary, or ternary:
Unary
operators operate on a single target (such as
-a
). Unary
prefix
operators appear
immediately before their target (such as
!b
), and unary
postfix
operators appear immediately
after their target (such as
i++
).
Binary
operators operate on two targets (such as
2 + 3
) and are
infix
because they appear in
between their two targets.
Ternary
operators operate on three targets. Like C, Swift has only one ternary operator, the
ternary conditional operator (
a ? b : c
).

The values that operators affect are
operands
. In the expression
1 + 2
, the
+
symbol is a binary operator and
its two operands are the values
1
and
2
.
​
Assignment Operator
The
assignment operator
(
a = b
) initializes or updates the value of
a
with the value of
b
:
1
let

b
=
10
2
var

a
=
5
3
a
=
b
4
// a is now equal to 10
If the right side of the assignment is a tuple with multiple values, its elements can be decomposed into multiple
constants or variables at once:
1
let
(
x
,
y
) = (
1
,
2
)
2
// x is equal to 1, and y is equal to 2
Unlike the assignment operator in C and Objective-C, the assignment operator in Swift does not itself return a
value. The following statement is not valid:
1
if

x
=
y
{
2

// this is not valid, because x = y does not return a value
3
}
This feature prevents the assignment operator (
=
) from being used by accident when the equal to operator (
==
)
is actually intended. By making
if x = y
invalid, Swift helps you to avoid these kinds of errors in your code.
​
Arithmetic Operators

Swift supports the four standard
arithmetic operators
for all number types:
1
1
+
2

// equals 3
2
5
–
3

// equals 2
3
2
*
3

// equals 6
4
10.0
/
2.5

// equals 4.0
Unlike the arithmetic operators in C and Objective-C, the Swift arithmetic operators do not allow values to
overflow by default. You can opt in to value overflow behavior by using Swift’s overflow operators (such as
a
&+ b
). See
Overflow Operators
.
The addition operator is also supported for
String
concatenation:
1
"hello, "
+
"world"

// equals "hello, world"
Two
Character
values, or one
Character
value and one
String
value, can be added together to make a
new
String
value:
1
let

dog
:
Character
=
""
2
let

cow
:
Character
=
""
3
let

dogCow
=
dog
+
cow
4
// dogCow is equal to ""
See also
Concatenating Strings and Characters
.
​
Addition (
+
)
Subtraction (
–
)
Multiplication (
*
)
Division (
/
)

Remainder Operator
The
remainder operator
(
a % b
) works out how many multiples of
b
will fit inside
a
and returns the value that
is left over (known as the
remainder
).
NOTE
The remainder operator (
%
) is also known as a
modulo operator
in other languages. However, its
behavior in Swift for negative numbers means that it is, strictly speaking, a remainder rather than a
modulo operation.
Here’s how the remainder operator works. To calculate
9 % 4
, you first work out how many
4
s will fit inside
9
:
You can fit two
4
s inside
9
, and the remainder is
1
(shown in orange).
In Swift, this would be written as:
1
9
%
4

// equals 1
To determine the answer for
a % b
, the
%
operator calculates the following equation and returns
remainder
as its output:
a
= (
b
×
some multiplier
) +
remainder

where
some multiplier
is the largest number of multiples of
b
that will fit inside
a
.
Inserting
9
and
4
into this equation yields:
9
= (
4
×
2
) +
1
The same method is applied when calculating the remainder for a negative value of
a
:
1
–
9
%
4

// equals -1
Inserting
-9
and
4
into the equation yields:
-9
= (
4
×
-2
) +
-1
giving a remainder value of
-1
.
The sign of
b
is ignored for negative values of
b
. This means that
a % b
and
a % -b
always give the same
answer.
​
Floating-Point Remainder Calculations
Unlike the remainder operator in C and Objective-C, Swift’s remainder operator can also operate on floating-
point numbers:
1
8
%
2.5

// equals 0.5
In this example,
8
divided by
2.5
equals
3
, with a remainder of
0.5
, so the remainder operator returns a
Double
value of
0.5
.

​
Increment and Decrement Operators
Like C, Swift provides an
increment operator
(
++
) and a
decrement operator
(
–
) as a shortcut to increase or
decrease the value of a numeric variable by
1
. You can use these operators with variables of any integer or
floating-point type.
1
var

i
=
0
2
++
i

// i now equals 1
Each time you call
++i
, the value of
i
is increased by
1
. Essentially,
++i
is shorthand for saying
i = i +
1
. Likewise,
–i
can be used as shorthand for
i = i – 1
.
The
++
and
–
symbols can be used as prefix operators or as postfix operators.
++i
and
i++
are both valid
ways to increase the value of
i
by
1
. Similarly,
–i
and
i–
are both valid ways to decrease the value of
i
by
1
.
Note that these operators modify
i
and also return a value. If you only want to increment or decrement the value
stored in
i
, you can ignore the returned value. However, if you
do
use the returned value, it will be different
based on whether you used the prefix or postfix version of the operator, according to the following rules:
For example:
1
var

a
=
0
If the operator is written
before
the variable, it increments the variable
before
returning its value.
If the operator is written
after
the variable, it increments the variable
after
returning its value.

2
let

b
= ++
a
3
// a and b are now both equal to 1
4
let

c
=
a
++
5
// a is now equal to 2, but c has been set to the pre-increment value

of 1
In the example above,
let b = ++a
increments
a

before
returning its value. This is why both
a
and
b
are
equal to to the new value of
1
.
However,
let c = a++
increments
a

after
returning its value. This means that
c
gets the old value of
1
, and
a
is then updated to equal
2
.
Unless you need the specific behavior of
i++
, it is recommended that you use
++i
and
–i
in all cases,
because they have the typical expected behavior of modifying
i
and returning the result.
​
Unary Minus Operator
The sign of a numeric value can be toggled using a prefixed
–
, known as the
unary minus operator
:
1
let

three
=
3
2
let

minusThree
= –
three

// minusThree equals -3
3
let

plusThree
= –
minusThree

// plusThree equals 3, or "minus minus

three"
The unary minus operator (
–
) is prepended directly before the value it operates on, without any white space.
​
Unary Plus Operator
The
unary plus operator
(
+
) simply returns the value it operates on, without any change:
1
let

minusSix
= –
6

2
let

alsoMinusSix
= +
minusSix

// alsoMinusSix equals -6
Although the unary plus operator doesn’t actually do anything, you can use it to provide symmetry in your code
for positive numbers when also using the unary minus operator for negative numbers.
​
Compound Assignment Operators
Like C, Swift provides
compound assignment operators
that combine assignment (
=
) with another operation.
One example is the
addition assignment operator
(
+=
):
1
var

a
=
1
2
a
+=
2
3
// a is now equal to 3
The expression
a += 2
is shorthand for
a = a + 2
. Effectively, the addition and the assignment are
combined into one operator that performs both tasks at the same time.
NOTE
The compound assignment operators do not return a value. You cannot write
let b = a += 2
,
for example. This behavior is different from the increment and decrement operators mentioned
above.
A complete list of compound assignment operators can be found in
Expressions
.
​
Comparison Operators
Swift supports all standard C
comparison operators
:

NOTE
Swift also provides two
identity operators
(
===
and
!==
), which you use to test whether two object
references both refer to the same object instance. For more information, see
Classes and
Structures
.
Each of the comparison operators returns a
Bool
value to indicate whether or not the statement is true:
1
1
==
1

// true, because 1 is equal to 1
2
2
!=
1

// true, because 2 is not equal to 1
3
2
>
1

// true, because 2 is greater than 1
4
1
<
2

// true, because 1 is less than 2
5
1
>=
1

// true, because 1 is greater than or equal to 1
6
2
<=
1

// false, because 2 is not less than or equal to 1
Comparison operators are often used in conditional statements, such as the
if
statement:
1
let

name
=
"world"
2
if

name
==
"world"
{
3

println
(
"hello, world"
)
4
}
else
{
5

println
(
"I'm sorry
\(
name
)
, but I don't recognize you"
)
Equal to (
a == b
)
Not equal to (
a != b
)
Greater than (
a > b
)
Less than (
a < b
)
Greater than or equal to (
a >= b
)
Less than or equal to (
a <= b
)

6
}
7
// prints "hello, world", because name is indeed equal to "world"
For more on the
if
statement, see
Control Flow
.
​
Ternary Conditional Operator
The
ternary conditional operator
is a special operator with three parts, which takes the form
question ?
answer1 : answer2
. It is a shortcut for evaluating one of two expressions based on whether
question
is
true or false. If
question
is true, it evaluates
answer1
and returns its value; otherwise, it evaluates
answer2
and returns its value.
The ternary conditional operator is shorthand for the code below:
1
if

question
{
2

answer1
3
}
else
{
4

answer2
5
}
Here’s an example, which calculates the pixel height for a table row. The row height should be 50 pixels taller
than the content height if the row has a header, and 20 pixels taller if the row doesn’t have a header:
1
let

contentHeight
=
40
2
let

hasHeader
=
true
3
let

rowHeight
=
contentHeight
+ (
hasHeader
?
50
:
20
)
4
// rowHeight is equal to 90
The preceding example is shorthand for the code below:
1
let

contentHeight
=
40
2
let

hasHeader
=
true

3
var

rowHeight
=
contentHeight
4
if

hasHeader
{
5

rowHeight
=
rowHeight
+
50
6
}
else
{
7

rowHeight
=
rowHeight
+
20
8
}
9
// rowHeight is equal to 90
The first example’s use of the ternary conditional operator means that
rowHeight
can be set to the correct
value on a single line of code. This is more concise than the second example, and removes the need for
rowHeight
to be a variable, because its value does not need to be modified within an
if
statement.
The ternary conditional operator provides an efficient shorthand for deciding which of two expressions to
consider. Use the ternary conditional operator with care, however. Its conciseness can lead to hard-to-read
code if overused. Avoid combining multiple instances of the ternary conditional operator into one compound
statement.
​
Range Operators
Swift includes two
range operators
, which are shortcuts for expressing a range of values.
​
Closed Range Operator
The
closed range operator
(
a…b
) defines a range that runs from
a
to
b
, and includes the values
a
and
b
.
The closed range operator is useful when iterating over a range in which you want all of the values to be used,
such as with a
for
–
in
loop:
1
for

index

in

1
…
5
{
2

println
(
"
\(
index
)
times 5 is
\(
index
*
5
)
"
)
3
}
4
// 1 times 5 is 5

5
// 2 times 5 is 10
6
// 3 times 5 is 15
7
// 4 times 5 is 20
8
// 5 times 5 is 25
For more on
for
–
in
loops, see
Control Flow
.
​
Half-Closed Range Operator
The
half-closed range operator
(
a..b
) defines a range that runs from
a
to
b
, but does not include
b
. It is said to
be
half-closed
because it contains its first value, but not its final value.
Half-closed ranges are particularly useful when you work with zero-based lists such as arrays, where it is
useful to count up to (but not including) the length of the list:
1
let

names
= [
"Anna"
,
"Alex"
,
"Brian"
,
"Jack"
]
2
let

count
=
names
.
count
3
for

i

in

0
..
count
{
4

println
(
"Person
\(
i
+
1
)
is called
\(
names
[
i
])
"
)
5
}
6
// Person 1 is called Anna
7
// Person 2 is called Alex
8
// Person 3 is called Brian
9
// Person 4 is called Jack
Note that the array contains four items, but
0..count
only counts as far as
3
(the index of the last item in the
array), because it is a half-closed range. For more on arrays, see
Arrays
.
​
Logical Operators
Logical operators
modify or combine the Boolean logic values
true
and
false
. Swift supports the three
standard logical operators found in C-based languages:

​
Logical NOT Operator
The
logical NOT operator
(
!a
) inverts a Boolean value so that
true
becomes
false
, and
false
becomes
true
.
The logical NOT operator is a prefix operator, and appears immediately before the value it operates on, without
any white space. It can be read as “not
a
”, as seen in the following example:
1
let

allowedEntry
=
false
2
if
!
allowedEntry
{
3

println
(
"ACCESS DENIED"
)
4
}
5
// prints "ACCESS DENIED"
The phrase
if !allowedEntry
can be read as “if not allowed entry.” The subsequent line is only executed
if “not allowed entry” is true; that is, if
allowedEntry
is
false
.
As in this example, careful choice of Boolean constant and variable names can help to keep code readable and
concise, while avoiding double negatives or confusing logic statements.
​
Logical AND Operator
The
logical AND operator
(
a && b
) creates logical expressions where both values must be
true
for the
overall expression to also be
true
.
If either value is
false
, the overall expression will also be
false
. In fact, if the
first
value is
false
, the
second value won’t even be evaluated, because it can’t possibly make the overall expression equate to
true
.
Logical NOT (
!a
)
Logical AND (
a && b
)
Logical OR (
a || b
)

This is known as
short-circuit evaluation
.
This example considers two
Bool
values and only allows access if both values are
true
:
1
let

enteredDoorCode
=
true
2
let

passedRetinaScan
=
false
3
if

enteredDoorCode
&&
passedRetinaScan
{
4

println
(
"Welcome!"
)
5
}
else
{
6

println
(
"ACCESS DENIED"
)
7
}
8
// prints "ACCESS DENIED"
​
Logical OR Operator
The
logical OR operator
(
a || b
) is an infix operator made from two adjacent pipe characters. You use it to
create logical expressions in which only
one
of the two values has to be
true
for the overall expression to be
true
.
Like the Logical AND operator above, the Logical OR operator uses short-circuit evaluation to consider its
expressions. If the left side of a Logical OR expression is
true
, the right side is not evaluated, because it
cannot change the outcome of the overall expression.
In the example below, the first
Bool
value (
hasDoorKey
) is
false
, but the second value
(
knowsOverridePassword
) is
true
. Because one value is
true
, the overall expression also evaluates
to
true
, and access is allowed:
1
let

hasDoorKey
=
false
2
let

knowsOverridePassword
=
true
3
if

hasDoorKey
||
knowsOverridePassword
{
4

println
(
"Welcome!"
)
5
}
else
{
6

println
(
"ACCESS DENIED"
)
7
}

8
// prints "Welcome!"
​
Combining Logical Operators
You can combine multiple logical operators to create longer compound expressions:
1
if

enteredDoorCode
&&
passedRetinaScan
||
hasDoorKey
||

knowsOverridePassword
{
2

println
(
"Welcome!"
)
3
}
else
{
4

println
(
"ACCESS DENIED"
)
5
}
6
// prints "Welcome!"
This example uses multiple
&&
and
||
operators to create a longer compound expression. However, the
&&
and
||
operators still operate on only two values, so this is actually three smaller expressions chained
together. It can be read as:
If we’ve entered the correct door code and passed the retina scan; or if we have a valid door key; or if we know
the emergency override password, then allow access.
Based on the values of
enteredDoorCode
,
passedRetinaScan
, and
hasDoorKey
, the first two mini-
expressions are
false
. However, the emergency override password is known, so the overall compound
expression still evaluates to
true
.
​
Explicit Parentheses
It is sometimes useful to include parentheses when they are not strictly needed, to make the intention of a
complex expression easier to read. In the door access example above, it is useful to add parentheses around
the first part of the compound expression to make its intent explicit:

1
if
(
enteredDoorCode
&&
passedRetinaScan
) ||
hasDoorKey
||

knowsOverridePassword
{
2

println
(
"Welcome!"
)
3
}
else
{
4

println
(
"ACCESS DENIED"
)
5
}
6
// prints "Welcome!"
The parentheses make it clear that the first two values are considered as part of a separate possible state in the
overall logic. The output of the compound expression doesn’t change, but the overall intention is clearer to the
reader. Readability is always preferred over brevity; use parentheses where they help to make your intentions
clear.

​
​
Strings and Characters
A
string
is an ordered collection of characters, such as
"hello, world"
or
"albatross"
. Swift strings
are represented by the
String
type, which in turn represents a collection of values of
Character
type.
Swift’s
String
and
Character
types provide a fast, Unicode-compliant way to work with text in your code.
The syntax for string creation and manipulation is lightweight and readable, with a similar syntax to C strings.
String concatenation is as simple as adding together two strings with the
+
operator, and string mutability is
managed by choosing between a constant or a variable, just like any other value in Swift.
Despite this simplicity of syntax, Swift’s
String
type is a fast, modern string implementation. Every string is
composed of encoding-independent Unicode characters, and provides support for accessing those characters in
various Unicode representations.
Strings can also be used to insert constants, variables, literals, and expressions into longer strings, in a process
known as string interpolation. This makes it easy to create custom string values for display, storage, and
printing.
NOTE
Swift’s
String
type is bridged seamlessly to Foundation’s
NSString
class. If you are working
with the Foundation framework in Cocoa or Cocoa Touch, the entire
NSString
API is available to
call on any
String
value you create, in addition to the
String
features described in this chapter.
You can also use a
String
value with any API that requires an
NSString
instance.
For more information about using
String
with Foundation and Cocoa, see
Using Swift with Cocoa
and Objective-C
.
​
String Literals

You can include predefined
String
values within your code as
string literals
. A string literal is a fixed
sequence of textual characters surrounded by a pair of double quotes (
""
).
A string literal can be used to provide an initial value for a constant or variable:
1
let

someString
=
"Some string literal value"
Note that Swift infers a type of
String
for the
someString
constant, because it is initialized with a string
literal value.
String literals can include the following special characters:
The code below shows an example of each kind of special character. The
wiseWords
constant contains two
escaped double quote characters. The
dollarSign
,
blackHeart
, and
sparklingHeart
constants
demonstrate the three different Unicode scalar character formats:
1
let

wiseWords
=
"\"Imagination is more important than knowledge\" –

Einstein"
2
// "Imagination is more important than knowledge" – Einstein
3
let

dollarSign
=
"\x24"

// $, Unicode scalar U+0024
4
let

blackHeart
=
"\u2665"

// ♥, Unicode scalar U+2665
5
let

sparklingHeart
=
"\U0001F496"

// , Unicode scalar U+1F496
​
Initializing an Empty String
The escaped special characters
\0
(null character),
\\
(backslash),
\t
(horizontal tab),
\n
(line feed),
\r
(carriage return),
\"
(double quote) and
\'
(single quote)
Single-byte Unicode scalars, written as
\xnn
, where
nn
is two hexadecimal digits
Two-byte Unicode scalars, written as
\unnnn
, where
nnnn
is four hexadecimal digits
Four-byte Unicode scalars, written as
\Unnnnnnnn
, where
nnnnnnnn
is eight hexadecimal
digits

To create an empty
String
value as the starting point for building a longer string, either assign an empty
string literal to a variable, or initialize a new
String
instance with initializer syntax:
1
var

emptyString
=
""

// empty string literal
2
var

anotherEmptyString
=
String
()
// initializer syntax
3
// these two strings are both empty, and are equivalent to each other
You can find out whether a
String
value is empty by checking its Boolean
isEmpty
property:
1
if

emptyString
.
isEmpty
{
2

println
(
"Nothing to see here"
)
3
}
4
// prints "Nothing to see here"
​
String Mutability
You indicate whether a particular
String
can be modified (or
mutated
) by assigning it to a variable (in which
case it can be modified), or to a constant (in which case it cannot be modified):
1
var

variableString
=
"Horse"
2
variableString
+=
" and carriage"
3
// variableString is now "Horse and carriage"
4

5
let

constantString
=
"Highlander"
6
constantString
+=
" and another Highlander"
7
// this reports a compile-time error – a constant string cannot be

modified
NOTE
This approach is different from string mutation in Objective-C and Cocoa, where you choose

between two classes (
NSString
and
NSMutableString
) to indicate whether a string can be
mutated.
​
Strings Are Value Types
Swift’s
String
type is a
value type
. If you create a new
String
value, that
String
value is
copied
when it
is passed to a function or method, or when it is assigned to a constant or variable. In each case, a new copy of
the existing
String
value is created, and the new copy is passed or assigned, not the original version. Value
types are described in
Structures and Enumerations Are Value Types
.
NOTE
This behavior differs from that of
NSString
in Cocoa. When you create an
NSString
instance in
Cocoa, and pass it to a function or method or assign it to a variable, you are always passing or
assigning a
reference
to the same single
NSString
. No copying of the string takes place, unless
you specifically request it.
Swift’s copy-by-default
String
behavior ensures that when a function or method passes you a
String
value,
it is clear that you own that exact
String
value, regardless of where it came from. You can be confident that
the string you are passed will not be modified unless you modify it yourself.
Behind the scenes, Swift’s compiler optimizes string usage so that actual copying takes place only when
absolutely necessary. This means you always get great performance when working with strings as value types.
​
Working with Characters
Swift’s
String
type represents a collection of
Character
values in a specified order. Each
Character
value represents a single Unicode character. You can access the individual
Character
values in a string by

iterating over that string with a
for
–
in
loop:
1
for

character

in

"Dog!"
{
2

println
(
character
)
3
}
4
// D
5
// o
6
// g
7
// !
8
//
The
for
–
in
loop is described in
For Loops
.
Alternatively, create a stand-alone
Character
constant or variable from a single-character string literal by
providing a
Character
type annotation:
1
let

yenSign
:
Character
=
"¥"
​
Counting Characters
To retrieve a count of the characters in a string, call the global
countElements
function and pass in a string
as the function’s sole parameter:
1
let

unusualMenagerie
=
"Koala , Snail , Penguin , Dromedary

"
2
println
(
"unusualMenagerie has
\(
countElements
(
unusualMenagerie
))

characters"
)
3
// prints "unusualMenagerie has 40 characters"
NOTE

Different Unicode characters and different representations of the same Unicode character can
require different amounts of memory to store. Because of this, characters in Swift do not each take
up the same amount of memory within a string’s representation. As a result, the length of a string
cannot be calculated without iterating through the string to consider each of its characters in turn. If
you are working with particularly long string values, be aware that the
countElements
function
must iterate over the characters within a string in order to calculate an accurate character count for
that string.
Note also that the character count returned by
countElements
is not always the same as the
length
property of an
NSString
that contains the same characters. The length of an
NSString
is based on the number of 16-bit code units within the string’s UTF-16 representation and not the
number of Unicode characters within the string. To reflect this fact, the
length
property from
NSString
is called
utf16count
when it is accessed on a Swift
String
value.
​
Concatenating Strings and Characters
String
and
Character
values can be added together (or
concatenated
) with the addition operator (
+
) to
create a new
String
value:
1
let

string1
=
"hello"
2
let

string2
=
" there"
3
let

character1
:
Character
=
"!"
4
let

character2
:
Character
=
"?"
5

6
let

stringPlusCharacter
=
string1
+
character1

// equals

"hello!"
7
let

stringPlusString
=
string1
+
string2

// equals "hello
there"
8
let

characterPlusString
=
character1
+
string1

// equals

"!hello"
9
let

characterPlusCharacter
=
character1
+
character2

// equals "!?"
You can also append a
String
or
Character
value to an existing
String
variable with the addition
assignment operator (
+=
):

1
var

instruction
=
"look over"
2
instruction
+=
string2
3
// instruction now equals "look over there"
4

5
var

welcome
=
"good morning"
6
welcome
+=
character1
7
// welcome now equals "good morning!"
NOTE
You can’t append a
String
or
Character
to an existing
Character
variable, because a
Character
value must contain a single character only.
​
String Interpolation
String interpolation
is a way to construct a new
String
value from a mix of constants, variables, literals, and
expressions by including their values inside a string literal. Each item that you insert into the string literal is
wrapped in a pair of parentheses, prefixed by a backslash:
1
let

multiplier
=
3
2
let

message
=
"
\(
multiplier
)
times 2.5 is
\(
Double
(
multiplier
) *
2.5
)
"
3
// message is "3 times 2.5 is 7.5"
In the example above, the value of
multiplier
is inserted into a string literal as
\(multiplier)
. This
placeholder is replaced with the actual value of
multiplier
when the string interpolation is evaluated to
create an actual string.
The value of
multiplier
is also part of a larger expression later in the string. This expression calculates the
value of
Double(multiplier) * 2.5
and inserts the result (
7.5
) into the string. In this case, the
expression is written as
\(Double(multiplier) * 2.5)
when it is included inside the string literal.

NOTE
The expressions you write inside parentheses within an interpolated string cannot contain an
unescaped double quote (
"
) or backslash (
\
), and cannot contain a carriage return or line feed.
​
Comparing Strings
Swift provides three ways to compare
String
values: string equality, prefix equality, and suffix equality.
​
String Equality
Two
String
values are considered equal if they contain exactly the same characters in the same order:
1
let

quotation
=
"We're a lot alike, you and I."
2
let

sameQuotation
=
"We're a lot alike, you and I."
3
if

quotation
==
sameQuotation
{
4

println
(
"These two strings are considered equal"
)
5
}
6
// prints "These two strings are considered equal"
​
Prefix and Suffix Equality
To check whether a string has a particular string prefix or suffix, call the string’s
hasPrefix
and
hasSuffix
methods, both of which take a single argument of type
String
and return a Boolean value. Both
methods perform a character-by-character comparison between the base string and the prefix or suffix string.
The examples below consider an array of strings representing the scene locations from the first two acts of
Shakespeare’s
Romeo and Juliet
:

1
let

romeoAndJuliet
= [
2

"Act 1 Scene 1: Verona, A public place"
,
3

"Act 1 Scene 2: Capulet's mansion"
,
4

"Act 1 Scene 3: A room in Capulet's mansion"
,
5

"Act 1 Scene 4: A street outside Capulet's mansion"
,
6

"Act 1 Scene 5: The Great Hall in Capulet's mansion"
,
7

"Act 2 Scene 1: Outside Capulet's mansion"
,
8

"Act 2 Scene 2: Capulet's orchard"
,
9

"Act 2 Scene 3: Outside Friar Lawrence's cell"
,
10

"Act 2 Scene 4: A street in Verona"
,
11

"Act 2 Scene 5: Capulet's mansion"
,
12

"Act 2 Scene 6: Friar Lawrence's cell"
13
]
You can use the
hasPrefix
method with the
romeoAndJuliet
array to count the number of scenes in Act
1 of the play:
1
var

act1SceneCount
=
0
2
for

scene

in

romeoAndJuliet
{
3

if

scene
.
hasPrefix
(
"Act 1 "
) {
4
++
act1SceneCount
5
}
6
}
7
println
(
"There are
\(
act1SceneCount
)
scenes in Act 1"
)
8
// prints "There are 5 scenes in Act 1"
Similarly, use the
hasSuffix
method to count the number of scenes that take place in or around Capulet’s
mansion and Friar Lawrence’s cell:
1
var

mansionCount
=
0
2
var

cellCount
=
0
3
for

scene

in

romeoAndJuliet
{
4

if

scene
.
hasSuffix
(
"Capulet's mansion"
) {

5
++
mansionCount
6
}
else

if

scene
.
hasSuffix
(
"Friar Lawrence's cell"
) {
7
++
cellCount
8
}
9
}
10
println
(
"
\(
mansionCount
)
mansion scenes;
\(
cellCount
)
cell

scenes"
)
11
// prints "6 mansion scenes; 2 cell scenes"
​
Uppercase and Lowercase Strings
You can access an uppercase or lowercase version of a string with its
uppercaseString
and
lowercaseString
properties:
1
let

normal
=
"Could you help me, please?"
2
let

shouty
=
normal
.
uppercaseString
3
// shouty is equal to "COULD YOU HELP ME, PLEASE?"
4
let

whispered
=
normal
.
lowercaseString
5
// whispered is equal to "could you help me, please?"
​
Unicode
Unicode
is an international standard for encoding and representing text. It enables you to represent almost any
character from any language in a standardized form, and to read and write those characters to and from an
external source such as a text file or web page.
Swift’s
String
and
Character
types are fully Unicode-compliant. They support a number of different
Unicode encodings, as described below.
​
Unicode Terminology

Every character in Unicode can be represented by one or more
unicode scalars
. A unicode scalar is a unique
21-bit number (and name) for a character or modifier, such as
U+0061
for
LOWERCASE LATIN LETTER A
(
"a"
), or
U+1F425
for
FRONT-FACING BABY CHICK
(
""
).
When a Unicode string is written to a text file or some other storage, these unicode scalars are encoded in one
of several Unicode-defined formats. Each format encodes the string in small chunks known as
code units
.
These include the UTF-8 format (which encodes a string as 8-bit code units) and the UTF-16 format (which
encodes a string as 16-bit code units).
​
Unicode Representations of Strings
Swift provides several different ways to access Unicode representations of strings.
You can iterate over the string with a
for
–
in
statement, to access its individual
Character
values as
Unicode characters. This process is described in
Working with Characters
.
Alternatively, access a
String
value in one of three other Unicode-compliant representations:
Each example below shows a different representation of the following string, which is made up of the characters
D
,
o
,
g
,
!
, and the character (
DOG FACE
, or Unicode scalar
U+1F436
):
1
let

dogString
=
"Dog!"
​
UTF-8
You can access a UTF-8 representation of a
String
by iterating over its
utf8
property. This property is of
A collection of UTF-8 code units (accessed with the string’s
utf8
property)
A collection of UTF-16 code units (accessed with the string’s
utf16
property)
A collection of 21-bit Unicode scalar values (accessed with the string’s
unicodeScalars
property)

type
UTF8View
, which is a collection of unsigned 8-bit (
UInt8
) values, one for each byte in the string’s UTF-
8 representation:
1
for

codeUnit

in

dogString
.
utf8
{
2

print
(
"
\(
codeUnit
)
"
)
3
}
4
print
(
"\n"
)
5
// 68 111 103 33 240 159 144 182
In the example above, the first four decimal
codeUnit
values (
68
,
111
,
103
,
33
) represent the characters
D
,
o
,
g
, and
!
, whose UTF-8 representation is the same as their ASCII representation. The last four
codeUnit
values (
240
,
159
,
144
,
182
) are a four-byte UTF-8 representation of the
DOG FACE
character.
​
UTF-16
You can access a UTF-16 representation of a
String
by iterating over its
utf16
property. This property is of
type
UTF16View
, which is a collection of unsigned 16-bit (
UInt16
) values, one for each 16-bit code unit in
the string’s UTF-16 representation:
1
for

codeUnit

in

dogString
.
utf16
{
2

print
(
"
\(
codeUnit
)
"
)
3
}
4
print
(
"\n"
)
5
// 68 111 103 33 55357 56374
Again, the first four
codeUnit
values (
68
,
111
,
103
,
33
) represent the characters
D
,
o
,
g
, and
!
, whose
UTF-16 code units have the same values as in the string’s UTF-8 representation.
The fifth and sixth
codeUnit
values (
55357
and
56374
) are a UTF-16 surrogate pair representation of the
DOG FACE
character. These values are a lead surrogate value of
U+D83D
(decimal value
55357
) and a trail
surrogate value of
U+DC36
(decimal value
56374
).
​

Unicode Scalars
You can access a Unicode scalar representation of a
String
value by iterating over its
unicodeScalars
property. This property is of type
UnicodeScalarView
, which is a collection of values of type
UnicodeScalar
. A Unicode scalar is any 21-bit Unicode code point that is not a lead surrogate or trail
surrogate code point.
Each
UnicodeScalar
has a
value
property that returns the scalar’s 21-bit value, represented within a
UInt32
value:
1
for

scalar

in

dogString
.
unicodeScalars
{
2

print
(
"
\(
scalar
.
value
)
"
)
3
}
4
print
(
"\n"
)
5
// 68 111 103 33 128054
The
value
properties for the first four
UnicodeScalar
values (
68
,
111
,
103
,
33
) once again represent
the characters
D
,
o
,
g
, and
!
. The
value
property of the fifth and final
UnicodeScalar
,
128054
, is a
decimal equivalent of the hexadecimal value
1F436
, which is equivalent to the Unicode scalar
U+1F436
for
the
DOG FACE
character.
As an alternative to querying their
value
properties, each
UnicodeScalar
value can also be used to
construct a new
String
value, such as with string interpolation:
1
for

scalar

in

dogString
.
unicodeScalars
{
2

println
(
"
\(
scalar
)
"
)
3
}
4
// D
5
// o
6
// g
7
// !
8
//

​
​
Collection Types
Swift provides two
collection types
, known as arrays and dictionaries, for storing collections of values. Arrays
store ordered lists of values of the same type. Dictionaries store unordered collections of values of the same
type, which can be referenced and looked up through a unique identifier (also known as a key).
Arrays and dictionaries in Swift are always clear about the types of values and keys that they can store. This
means that you cannot insert a value of the wrong type into an array or dictionary by mistake. It also means you
can be confident about the types of values you will retrieve from an array or dictionary. Swift’s use of explicitly
typed collections ensures that your code is always clear about the types of values it can work with and enables
you to catch any type mismatches early in your code’s development.
NOTE
Swift’s
Array
type exhibits different behavior to other types when assigned to a constant or
variable, or when passed to a function or method. For more information, see
Mutability of
Collections
and
Assignment and Copy Behavior for Collection Types
.
​
Arrays
An
array
stores multiple values of the same type in an ordered list. The same value can appear in an array
multiple times at different positions.
Swift arrays are specific about the kinds of values they can store. They differ from Objective-C’s
NSArray
and
NSMutableArray
classes, which can store any kind of object and do not provide any information about the
nature of the objects they return. In Swift, the type of values that a particular array can store is always made
clear, either through an explicit type annotation, or through type inference, and does not have to be a class type.
If you create an array of
Int
values, for example, you can’t insert any value other than
Int
values into that
array. Swift arrays are type safe, and are always clear about what they may contain.

​
Array Type Shorthand Syntax
The type of a Swift array is written in full as
Array<SomeType>
, where
SomeType
is the type that the array
is allowed to store. You can also write the type of an array in shorthand form as
SomeType[]
. Although the
two forms are functionally identical, the shorthand form is preferred, and is used throughout this guide when
referring to the type of an array.
​
Array Literals
You can initialize an array with an
array literal
, which is a shorthand way to write one or more values as an
array collection. An array literal is written as a list of values, separated by commas, surrounded by a pair of
square brackets:
[
value 1
,
value 2
,
value 3
]
The example below creates an array called
shoppingList
to store
String
values:
1
var

shoppingList
:
String
[] = [
"Eggs"
,
"Milk"
]
2
// shoppingList has been initialized with two initial items
The
shoppingList
variable is declared as “an array of
String
values”, written as
String[]
. Because
this particular array has specified a value type of
String
, it is
only
allowed to store
String
values. Here, the
shoppingList
array is initialized with two
String
values (
"Eggs"
and
"Milk"
), written within an array
literal.
NOTE
The
shoppingList
array is declared as a variable (with the
var
introducer) and not a constant
(with the
let
introducer) because more items are added to the shopping list in the examples below.

In this case, the array literal contains two
String
values and nothing else. This matches the type of the
shoppingList
variable’s declaration (an array that can only contain
String
values), and so the
assignment of the array literal is permitted as a way to initialize
shoppingList
with two initial items.
Thanks to Swift’s type inference, you don’t have to write the type of the array if you’re initializing it with an array
literal containing values of the same type. The initialization of
shoppingList
could have been written in a
shorter form instead:
1
var

shoppingList
= [
"Eggs"
,
"Milk"
]
Because all values in the array literal are of the same type, Swift can infer that
String[]
is the correct type to
use for the
shoppingList
variable.
​
Accessing and Modifying an Array
You access and modify an array through its methods and properties, or by using subscript syntax.
To find out the number of items in an array, check its read-only
count
property:
1
println
(
"The shopping list contains
\(
shoppingList
.
count
)
items."
)
2
// prints "The shopping list contains 2 items."
Use the Boolean
isEmpty
property as a shortcut for checking whether the
count
property is equal to
0
:
1
if

shoppingList
.
isEmpty
{
2

println
(
"The shopping list is empty."
)
3
}
else
{
4

println
(
"The shopping list is not empty."
)
5
}
6
// prints "The shopping list is not empty."

You can add a new item to the end of an array by calling the array’s
append
method:
1
shoppingList
.
append
(
"Flour"
)
2
// shoppingList now contains 3 items, and someone is making pancakes
Alternatively, add a new item to the end of an array with the addition assignment operator (
+=
):
1
shoppingList
+=
"Baking Powder"
2
// shoppingList now contains 4 items
You can also append an array of compatible items with the addition assignment operator (
+=
):
1
shoppingList
+= [
"Chocolate Spread"
,
"Cheese"
,
"Butter"
]
2
// shoppingList now contains 7 items
Retrieve a value from the array by using
subscript syntax
, passing the index of the value you want to retrieve
within square brackets immediately after the name of the array:
1
var

firstItem
=
shoppingList
[
0
]
2
// firstItem is equal to "Eggs"
Note that the first item in the array has an index of
0
, not
1
. Arrays in Swift are always zero-indexed.
You can use subscript syntax to change an existing value at a given index:
1
shoppingList
[
0
] =
"Six eggs"
2
// the first item in the list is now equal to "Six eggs" rather than

"Eggs"
You can also use subscript syntax to change a range of values at once, even if the replacement set of values has
a different length than the range you are replacing. The following example replaces
"Chocolate Spread"
,
"Cheese"
, and
"Butter"
with
"Bananas"
and
"Apples"
:

1
shoppingList
[
4
…
6
] = [
"Bananas"
,
"Apples"
]
2
// shoppingList now contains 6 items
NOTE
You can’t use subscript syntax to append a new item to the end of an array. If you try to use subscript
syntax to retrieve or set a value for an index that is outside of an array’s existing bounds, you will
trigger a runtime error. However, you can check that an index is valid before using it, by comparing
it to the array’s
count
property. Except when
count
is
0
(meaning the array is empty), the largest
valid index in an array will always be
count – 1
, because arrays are indexed from zero.
To insert an item into the array at a specified index, call the array’s
insert(atIndex:)
method:
1
shoppingList
.
insert
(
"Maple Syrup"
,
atIndex
:
0
)
2
// shoppingList now contains 7 items
3
// "Maple Syrup" is now the first item in the list
This call to the
insert
method inserts a new item with a value of
"Maple Syrup"
at the very beginning of
the shopping list, indicated by an index of
0
.
Similarly, you remove an item from the array with the
removeAtIndex
method. This method removes the
item at the specified index and returns the removed item (although you can ignore the returned value if you do
not need it):
1
let

mapleSyrup
=
shoppingList
.
removeAtIndex
(
0
)
2
// the item that was at index 0 has just been removed
3
// shoppingList now contains 6 items, and no Maple Syrup
4
// the mapleSyrup constant is now equal to the removed "Maple Syrup"

string
Any gaps in an array are closed when an item is removed, and so the value at index
0
is once again equal to
"Six eggs"
:

1
firstItem
=
shoppingList
[
0
]
2
// firstItem is now equal to "Six eggs"
If you want to remove the final item from an array, use the
removeLast
method rather than the
removeAtIndex
method to avoid the need to query the array’s
count
property. Like the
removeAtIndex
method,
removeLast
returns the removed item:
1
let

apples
=
shoppingList
.
removeLast
()
2
// the last item in the array has just been removed
3
// shoppingList now contains 5 items, and no cheese
4
// the apples constant is now equal to the removed "Apples" string
​
Iterating Over an Array
You can iterate over the entire set of values in an array with the
for
–
in
loop:
1
for

item

in

shoppingList
{
2

println
(
item
)
3
}
4
// Six eggs
5
// Milk
6
// Flour
7
// Baking Powder
8
// Bananas
If you need the integer index of each item as well as its value, use the global
enumerate
function to iterate
over the array instead. The
enumerate
function returns a tuple for each item in the array composed of the
index and the value for that item. You can decompose the tuple into temporary constants or variables as part of
the iteration:
1
for
(
index
,
value
)
in

enumerate
(
shoppingList
) {

2

println
(
"Item
\(
index
+
1
)
:
\(
value
)
"
)
3
}
4
// Item 1: Six eggs
5
// Item 2: Milk
6
// Item 3: Flour
7
// Item 4: Baking Powder
8
// Item 5: Bananas
For more about the
for
–
in
loop, see
For Loops
.
​
Creating and Initializing an Array
You can create an empty array of a certain type (without setting any initial values) using initializer syntax:
1
var

someInts
=
Int
[]()
2
println
(
"someInts is of type Int[] with
\(
someInts
.
count
)
items."
)
3
// prints "someInts is of type Int[] with 0 items."
Note that the type of the
someInts
variable is inferred to be
Int[]
, because it is set to the output of an
Int[]
initializer.
Alternatively, if the context already provides type information, such as a function argument or an already-typed
variable or constant, you can create an empty array with an empty array literal, which is written as
[]
(an
empty pair of square brackets):
1
someInts
.
append
(
3
)
2
// someInts now contains 1 value of type Int
3
someInts
= []
4
// someInts is now an empty array, but is still of type Int[]
Swift’s
Array
type also provides an initializer for creating an array of a certain size with all of its values set to
a provided default value. You pass this initializer the number of items to be added to the new array (called
count
) and a default value of the appropriate type (called
repeatedValue
):

1
var

threeDoubles
=
Double
[](
count
:
3
,
repeatedValue
:
0.0
)
2
// threeDoubles is of type Double[], and equals [0.0, 0.0, 0.0]
Thanks to type inference, you don’t need to specify the type to be stored in the array when using this initializer,
because it can be inferred from the default value:
1
var

anotherThreeDoubles
=
Array
(
count
:
3
,
repeatedValue
:
2.5
)
2
// anotherThreeDoubles is inferred as Double[], and equals [2.5, 2.5,

2.5]
Finally, you can create a new array by adding together two existing arrays of compatible type with the addition
operator (
+
). The new array’s type is inferred from the type of the two arrays you add together:
1
var

sixDoubles
=
threeDoubles
+
anotherThreeDoubles
2
// sixDoubles is inferred as Double[], and equals [0.0, 0.0, 0.0, 2.5,
2.5, 2.5]
​
Dictionaries
A
dictionary
is a container that stores multiple values of the same type. Each value is associated with a unique
key
, which acts as an identifier for that value within the dictionary. Unlike items in an array, items in a
dictionary do not have a specified order. You use a dictionary when you need to look up values based on their
identifier, in much the same way that a real-world dictionary is used to look up the definition for a particular
word.
Swift dictionaries are specific about the types of keys and values they can store. They differ from Objective-C’s
NSDictionary
and
NSMutableDictionary
classes, which can use any kind of object as their keys and
values and do not provide any information about the nature of these objects. In Swift, the type of keys and values
that a particular dictionary can store is always made clear, either through an explicit type annotation or through
type inference.
Swift’s dictionary type is written as
Dictionary<KeyType, ValueType>
, where
KeyType
is the type

of value that can be used as a dictionary key, and
ValueType
is the type of value that the dictionary stores for
those keys.
The only restriction is that
KeyType
must be
hashable
—that is, it must provide a way to make itself uniquely
representable. All of Swift’s basic types (such as
String
,
Int
,
Double
, and
Bool
) are hashable by default,
and all of these types can be used as the keys of a dictionary. Enumeration member values without associated
values (as described in
Enumerations
) are also hashable by default.
​
Dictionary Literals
You can initialize a dictionary with with a
dictionary literal
, which has a similar syntax to the array literal seen
earlier. A dictionary literal is a shorthand way to write one or more key-value pairs as a
Dictionary
collection.
A
key-value pair
is a combination of a key and a value. In a dictionary literal, the key and value in each key-
value pair are separated by a colon. The key-value pairs are written as a list, separated by commas,
surrounded by a pair of square brackets:
[
key 1
:
value 1
,
key 2
:
value 2
,
key 3
:
value 3
]
The example below creates a dictionary to store the names of international airports. In this dictionary, the keys
are three-letter International Air Transport Association codes, and the values are airport names:
1
var

airports
:
Dictionary
<
String
,
String
> = [
"TYO"
:
"Tokyo"
,
"DUB"
:

"Dublin"
]
The
airports
dictionary is declared as having a type of
Dictionary<String, String>
, which
means “a
Dictionary
whose keys are of type
String
, and whose values are also of type
String
”.
NOTE

The
airports
dictionary is declared as a variable (with the
var
introducer), and not a constant
(with the
let
introducer), because more airports will be added to the dictionary in the examples
below.
The
airports
dictionary is initialized with a dictionary literal containing two key-value pairs. The first pair
has a key of
"TYO"
and a value of
"Tokyo"
. The second pair has a key of
"DUB"
and a value of
"Dublin"
.
This dictionary literal contains two
String: String
pairs. This matches the type of the
airports
variable declaration (a dictionary with only
String
keys, and only
String
values) and so the assignment of
the dictionary literal is permitted as a way to initialize the
airports
dictionary with two initial items.
As with arrays, you don’t have to write the type of the dictionary if you’re initializing it with a dictionary literal
whose keys and values have consistent types. The initialization of
airports
could have been be written in a
shorter form instead:
1
var

airports
= [
"TYO"
:
"Tokyo"
,
"DUB"
:
"Dublin"
]
Because all keys in the literal are of the same type as each other, and likewise all values are of the same type
as each other, Swift can infer that
Dictionary<String, String>
is the correct type to use for the
airports
dictionary.
​
Accessing and Modifying a Dictionary
You access and modify a dictionary through its methods and properties, or by using subscript syntax. As with
an array, you can find out the number of items in a
Dictionary
by checking its read-only
count
property:
1
println
(
"The dictionary of airports contains
\(
airports
.
count
)

items."
)
2
// prints "The dictionary of airports contains 2 items."
You can add a new item to a dictionary with subscript syntax. Use a new key of the appropriate type as the

subscript index, and assign a new value of the appropriate type:
1
airports
[
"LHR"
] =
"London"
2
// the airports dictionary now contains 3 items
You can also use subscript syntax to change the value associated with a particular key:
1
airports
[
"LHR"
] =
"London Heathrow"
2
// the value for "LHR" has been changed to "London Heathrow"
As an alternative to subscripting, use a dictionary’s
updateValue(forKey:)
method to set or update the
value for a particular key. Like the subscript examples above, the
updateValue(forKey:)
method sets a
value for a key if none exists, or updates the value if that key already exists. Unlike a subscript, however, the
updateValue(forKey:)
method returns the
old
value after performing an update. This enables you to
check whether or not an update took place.
The
updateValue(forKey:)
method returns an optional value of the dictionary’s value type. For a
dictionary that stores
String
values, for example, the method returns a value of type
String?
, or “optional
String
”. This optional value contains the old value for that key if one existed before the update, or
nil
if no
value existed:
1
if

let

oldValue
=
airports
.
updateValue
(
"Dublin International"
,
forKey
:
"DUB"
) {
2

println
(
"The old value for DUB was
\(
oldValue
)
."
)
3
}
4
// prints "The old value for DUB was Dublin."
You can also use subscript syntax to retrieve a value from the dictionary for a particular key. Because it is
possible to request a key for which no value exists, a dictionary’s subscript returns an optional value of the
dictionary’s value type. If the dictionary contains a value for the requested key, the subscript returns an optional
value containing the existing value for that key. Otherwise, the subscript returns
nil
:
1
if

let

airportName
=
airports
[
"DUB"
] {
2

println
(
"The name of the airport is
\(
airportName
)
."
)

3
}
else
{
4

println
(
"That airport is not in the airports dictionary."
)
5
}
6
// prints "The name of the airport is Dublin International."
You can use subscript syntax to remove a key-value pair from a dictionary by assigning a value of
nil
for that
key:
1
airports
[
"APL"
] =
"Apple International"
2
// "Apple International" is not the real airport for APL, so delete it
3
airports
[
"APL"
] =
nil
4
// APL has now been removed from the dictionary
Alternatively, remove a key-value pair from a dictionary with the
removeValueForKey
method. This
method removes the key-value pair if it exists and returns the removed value, or returns
nil
if no value
existed:
1
if

let

removedValue
=
airports
.
removeValueForKey
(
"DUB"
) {
2

println
(
"The removed airport's name is
\(
removedValue
)
."
)
3
}
else
{
4

println
(
"The airports dictionary does not contain a value for

DUB."
)
5
}
6
// prints "The removed airport's name is Dublin International."
​
Iterating Over a Dictionary
You can iterate over the key-value pairs in a dictionary with a
for
–
in
loop. Each item in the dictionary is
returned as a
(key, value)
tuple, and you can decompose the tuple’s members into temporary constants
or variables as part of the iteration:
1
for
(
airportCode
,
airportName
)
in

airports
{

2

println
(
"
\(
airportCode
)
:
\(
airportName
)
"
)
3
}
4
// TYO: Tokyo
5
// LHR: London Heathrow
For more about the
for
–
in
loop, see
For Loops
.
You can also retrieve an iteratable collection of a dictionary’s keys or values by accessing its
keys
and
values
properties:
1
for

airportCode

in

airports
.
keys
{
2

println
(
"Airport code:
\(
airportCode
)
"
)
3
}
4
// Airport code: TYO
5
// Airport code: LHR
6

7
for

airportName

in

airports
.
values
{
8

println
(
"Airport name:
\(
airportName
)
"
)
9
}
10
// Airport name: Tokyo
11
// Airport name: London Heathrow
If you need to use a dictionary’s keys or values with an API that takes an
Array
instance, initialize a new array
with the
keys
or
values
property:
1
let

airportCodes
=
Array
(
airports
.
keys
)
2
// airportCodes is ["TYO", "LHR"]
3

4
let

airportNames
=
Array
(
airports
.
values
)
5
// airportNames is ["Tokyo", "London Heathrow"]
NOTE

Swift’s
Dictionary
type is an unordered collection. The order in which keys, values, and key-
value pairs are retrieved when iterating over a dictionary is not specified.
​
Creating an Empty Dictionary
As with arrays, you can create an empty
Dictionary
of a certain type by using initializer syntax:
1
var

namesOfIntegers
=
Dictionary
<
Int
,
String
>()
2
// namesOfIntegers is an empty Dictionary<Int, String>
This example creates an empty dictionary of type
Int
,
String
to store human-readable names of integer
values. Its keys are of type
Int
, and its values are of type
String
.
If the context already provides type information, create an empty dictionary with an empty dictionary literal,
which is written as
[:]
(a colon inside a pair of square brackets):
1
namesOfIntegers
[
16
] =
"sixteen"
2
// namesOfIntegers now contains 1 key-value pair
3
namesOfIntegers
= [:]
4
// namesOfIntegers is once again an empty dictionary of type Int,

String
NOTE
Behind the scenes, Swift’s array and dictionary types are implemented as
generic collections
. For
more on generic types and collections, see
Generics
.

​
Mutability of Collections
Arrays and dictionaries store multiple values together in a single collection. If you create an array or a
dictionary and assign it to a variable, the collection that is created will be
mutable
. This means that you can
change (or
mutate
) the size of the collection after it is created by adding more items to the collection, or by
removing existing items from the ones it already contains. Conversely, if you assign an array or a dictionary to
a constant, that array or dictionary is
immutable
, and its size cannot be changed.
For dictionaries, immutability also means that you cannot replace the value for an existing key in the dictionary.
An immutable dictionary’s contents cannot be changed once they are set.
Immutability has a slightly different meaning for arrays, however. You are still not allowed to perform any action
that has the potential to change the size of an immutable array, but you
are
allowed to set a new value for an
existing index in the array. This enables Swift’s
Array
type to provide optimal performance for array
operations when the size of an array is fixed.
The mutability behavior of Swift’s
Array
type also affects how array instances are assigned and modified. For
more information, see
Assignment and Copy Behavior for Collection Types
.
NOTE
It is good practice to create immutable collections in all cases where the collection’s size does not
need to change. Doing so enables the Swift compiler to optimize the performance of the collections
you create.

​
​
Control Flow
Swift provides all the familiar control flow constructs of C-like languages. These include
for
and
while
loops
to perform a task multiple times;
if
and
switch
statements to execute different branches of code based on
certain conditions; and statements such as
break
and
continue
to transfer the flow of execution to another
point in your code.
In addition to the traditional
for
–
condition
–
increment
loop found in C, Swift adds a
for
–
in
loop that
makes it easy to iterate over arrays, dictionaries, ranges, strings, and other sequences.
Swift’s
switch
statement is also considerably more powerful than its counterpart in C. The cases of a
switch
statement do not “fall through” to the next case in Swift, avoiding common C errors caused by missing
break
statements. Cases can match many different types of pattern, including range matches, tuples, and
casts to a specific type. Matched values in a
switch
case can be bound to temporary constants or variables
for use within the case’s body, and complex matching conditions can be expressed with a
where
clause for
each case.
​
For Loops
A
for
loop performs a set of statements a certain number of times. Swift provides two kinds of
for
loop:
​
For-In
You use the
for
–
in
loop to iterate over collections of items, such as ranges of numbers, items in an array, or
characters in a string.
for
–
in
performs a set of statements for each item in a range, sequence, collection, or
progression.
for
–
condition
–
increment
performs a set of statements until a specific condition is met,
typically by incrementing a counter each time the loop ends.

This example prints the first few entries in the five-times-table:
1
for

index

in

1
…
5
{
2

println
(
"
\(
index
)
times 5 is
\(
index
*
5
)
"
)
3
}
4
// 1 times 5 is 5
5
// 2 times 5 is 10
6
// 3 times 5 is 15
7
// 4 times 5 is 20
8
// 5 times 5 is 25
The collection of items being iterated is a closed range of numbers from
1
to
5
inclusive, as indicated by the
use of the closed range operator (
…
). The value of
index
is set to the first number in the range (
1
), and the
statements inside the loop are executed. In this case, the loop contains only one statement, which prints an entry
from the five-times-table for the current value of
index
. After the statement is executed, the value of
index
is
updated to contain the second value in the range (
2
), and the
println
function is called again. This process
continues until the end of the range is reached.
In the example above,
index
is a constant whose value is automatically set at the start of each iteration of the
loop. As such, it does not have to be declared before it is used. It is implicitly declared simply by its inclusion in
the loop declaration, without the need for a
let
declaration keyword.
NOTE
The
index
constant exists only within the scope of the loop. If you want to check the value of
index
after the loop completes, or if you want to work with its value as a variable rather than a
constant, you must declare it yourself before its use in the loop.
If you don’t need each value from the range, you can ignore the values by using an underscore in place of a
variable name:
1
let

base
=
3

2
let

power
=
10
3
var

answer
=
1
4
for

_

in

1
…
power
{
5

answer
*=
base
6
}
7
println
(
"
\(
base
)
to the power of
\(
power
)
is
\(
answer
)
"
)
8
// prints "3 to the power of 10 is 59049"
This example calculates the value of one number to the power of another (in this case,
3
to the power of
10
). It
multiplies a starting value of
1
(that is,
3
to the power of
0
) by
3
, ten times, using a half-closed loop that starts
with
0
and ends with
9
. This calculation doesn’t need to know the individual counter values each time through
the loop—it simply needs to execute the loop the correct number of times. The underscore character
_
(used in
place of a loop variable) causes the individual values to be ignored and does not provide access to the current
value during each iteration of the loop.
Use the
for
–
in
loop with an array to iterate over its items:
1
let

names
= [
"Anna"
,
"Alex"
,
"Brian"
,
"Jack"
]
2
for

name

in

names
{
3

println
(
"Hello,
\(
name
)
!"
)
4
}
5
// Hello, Anna!
6
// Hello, Alex!
7
// Hello, Brian!
8
// Hello, Jack!
You can also iterate over a dictionary to access its key-value pairs. Each item in the dictionary is returned as a
(key, value)
tuple when the dictionary is iterated, and you can decompose the
(key, value)
tuple’s
members as explicitly named constants for use within in the body of the
for
–
in
loop. Here, the dictionary’s
keys are decomposed into a constant called
animalName
, and the dictionary’s values are decomposed into a
constant called
legCount
:
1
let

numberOfLegs
= [
"spider"
:
8
,
"ant"
:
6
,
"cat"
:
4
]
2
for
(
animalName
,
legCount
)
in

numberOfLegs
{
3

println
(
"
\(
animalName
)
s have
\(
legCount
)
legs"
)

4
}
5
// spiders have 8 legs
6
// ants have 6 legs
7
// cats have 4 legs
Items in a
Dictionary
may not necessarily be iterated in the same order as they were inserted. The
contents of a
Dictionary
are inherently unordered, and iterating over them does not guarantee the order in
which they will be retrieved. For more on arrays and dictionaries, see
Collection Types
.)
In addition to arrays and dictionaries, you can also use the
for
–
in
loop to iterate over the
Character
values
in a string:
1
for

character

in

"Hello"
{
2

println
(
character
)
3
}
4
// H
5
// e
6
// l
7
// l
8
// o
​
For-Condition-Increment
In addition to
for
–
in
loops, Swift supports traditional C-style
for
loops with a condition and an incrementer:
1
for

var

index
=
0
;
index
<
3
; ++
index
{
2

println
(
"index is
\(
index
)
"
)
3
}
4
// index is 0
5
// index is 1
6
// index is 2
Here’s the general form of this loop format:

for

initialization
;
condition
;
increment
{

statements
}
Semicolons separate the three parts of the loop’s definition, as in C. However, unlike C, Swift doesn’t need
parentheses around the entire “initialization; condition; increment” block.
The loop is executed as follows:
1
.
When the loop is first entered, the
initialization expression
is evaluated once, to set up any constants
or variables that are needed for the loop.
2
.
The
condition expression
is evaluated. If it evaluates to
false
, the loop ends, and code execution
continues after the
for
loop’s closing brace (
}
). If the expression evaluates to
true
, code
execution continues by executing the statements inside the braces.
3
.
After all statements are executed, the
increment expression
is evaluated. It might increase or
decrease the value of a counter, or set one of the initialized variables to a new value based on the
outcome of the statements. After the increment expression has been evaluated, execution returns to
step 2, and the condition expression is evaluated again.
The loop format and execution process described above is shorthand for (and equivalent to) the outline below:
initialization
while

condition
{

statements

increment
}
Constants and variables declared within the initialization expression (such as
var index = 0
) are only
valid within the scope of the
for
loop itself. To retrieve the final value of
index
after the loop ends, you must
declare
index
before the loop’s scope begins:

1
var

index
:
Int
2
for

index
=
0
;
index
<
3
; ++
index
{
3

println
(
"index is
\(
index
)
"
)
4
}
5
// index is 0
6
// index is 1
7
// index is 2
8
println
(
"The loop statements were executed
\(
index
)
times"
)
9
// prints "The loop statements were executed 3 times"
Note that the final value of
index
after this loop is completed is
3
, not
2
. The last time the increment statement
++index
is called, it sets
index
to
3
, which causes
index < 3
to equate to
false
, ending the loop.
​
While Loops
A
while
loop performs a set of statements until a condition becomes
false
. These kinds of loops are best
used when the number of iterations is not known before the first iteration begins. Swift provides two kinds of
while
loop:
​
While
A
while
loop starts by evaluating a single condition. If the condition is
true
, a set of statements is repeated
until the condition becomes
false
.
Here’s the general form of a
while
loop:
while

condition
{
while
evaluates its condition at the start of each pass through the loop.
do
–
while
evaluates its condition at the end of each pass through the loop.

statements
}
This example plays a simple game of
Snakes and Ladders
(also known as
Chutes and Ladders
):
The rules of the game are as follows:
The board has 25 squares, and the aim is to land on or beyond square 25.
Each turn, you roll a six-sided dice and move by that number of squares, following the horizontal
path indicated by the dotted arrow above.
If your turn ends at the bottom of a ladder, you move up that ladder.

The game board is represented by an array of
Int
values. Its size is based on a constant called
finalSquare
, which is used to initialize the array and also to check for a win condition later in the example.
The board is initialized with 26 zero
Int
values, not 25 (one each at indices
0
through
25
inclusive):
1
let

finalSquare
=
25
2
var

board
=
Int
[](
count
:
finalSquare
+
1
,
repeatedValue
:
0
)
Some squares are then set to have more specific values for the snakes and ladders. Squares with a ladder base
have a positive number to move you up the board, whereas squares with a snake head have a negative number
to move you back down the board:
1
board
[
03
] = +
08
;
board
[
06
] = +
11
;
board
[
09
] = +
09
;
board
[
10
] = +
02
2
board
[
14
] = –
10
;
board
[
19
] = –
11
;
board
[
22
] = –
02
;
board
[
24
] = –
08
Square 3 contains the bottom of a ladder that moves you up to square 11. To represent this,
board[03]
is
equal to
+08
, which is equivalent to an integer value of
8
(the difference between
3
and
11
). The unary plus
operator (
+i
) balances with the unary minus operator (
-i
), and numbers lower than
10
are padded with zeros
so that all board definitions align. (Neither stylistic tweak is strictly necessary, but they lead to neater code.)
The player’s starting square is “square zero”, which is just off the bottom left corner of the board. The first dice
roll always moves the player on to the board:
1
var

square
=
0
2
var

diceRoll
=
0
3
while

square
<
finalSquare
{
4

// roll the dice
5

if
++
diceRoll
==
7
{
diceRoll
=
1
}
6

// move by the rolled amount
7

square
+=
diceRoll
8

if

square
<
board
.
count
{
9

// if we're still on the board, move up or down for a snake or
a ladder
If your turn ends at the head of a snake, you move down that snake.

10

square
+=
board
[
square
]
11
}
12
}
13
println
(
"Game over!"
)
This example uses a very simple approach to dice rolling. Instead of a random number generator, it starts with
a
diceRoll
value of
0
. Each time through the
while
loop,
diceRoll
is incremented with the prefix
increment operator (
++i
), and is then checked to see if it has become too large. The return value of
++diceRoll
is equal to the value of
diceRoll

after
it is incremented. Whenever this return value equals
7
,
the dice roll has become too large, and is reset to a value of
1
. This gives a sequence of
diceRoll
values that
is always
1
,
2
,
3
,
4
,
5
,
6
,
1
,
2
and so on.
After rolling the dice, the player moves forward by
diceRoll
squares. It’s possible that the dice roll may have
moved the player beyond square 25, in which case the game is over. To cope with this scenario, the code
checks that
square
is less than the
board
array’s
count
property before adding the value stored in
board[square]
onto the current
square
value to move the player up or down any ladders or snakes.
Had this check not been performed,
board[square]
might try to access a value outside the bounds of the
board
array, which would trigger an error. If
square
is now equal to
26
, the code would try to check the
value of
board[26]
, which is larger than the size of the array.
The current
while
loop execution then ends, and the loop’s condition is checked to see if the loop should be
executed again. If the player has moved on or beyond square number
25
, the loop’s condition evaluates to
false
, and the game ends.
A
while
loop is appropriate in this case because the length of the game is not clear at the start of the
while
loop. Instead, the loop is executed until a particular condition is satisfied.
​
Do-While
The other variation of the
while
loop, known as the
do
–
while
loop, performs a single pass through the loop
block first,
before
considering the loop’s condition. It then continues to repeat the loop until the condition is
false
.

Here’s the general form of a
do
–
while
loop:
do
{

statements
}
while

condition
Here’s the
Snakes and Ladders
example again, written as a
do
–
while
loop rather than a
while
loop. The
values of
finalSquare
,
board
,
square
, and
diceRoll
are initialized in exactly the same way as with a
while
loop:
1
let

finalSquare
=
25
2
var

board
=
Int
[](
count
:
finalSquare
+
1
,
repeatedValue
:
0
)
3
board
[
03
] = +
08
;
board
[
06
] = +
11
;
board
[
09
] = +
09
;
board
[
10
] = +
02
4
board
[
14
] = –
10
;
board
[
19
] = –
11
;
board
[
22
] = –
02
;
board
[
24
] = –
08
5
var

square
=
0
6
var

diceRoll
=
0
In this version of the game, the
first
action in the loop is to check for a ladder or a snake. No ladder on the board
takes the player straight to square 25, and so it is not possible to win the game by moving up a ladder.
Therefore, it is safe to check for a snake or a ladder as the first action in the loop.
At the start of the game, the player is on “square zero”.
board[0]
always equals
0
, and has no effect:
1
do
{
2

// move up or down for a snake or ladder
3

square
+=
board
[
square
]
4

// roll the dice
5

if
++
diceRoll
==
7
{
diceRoll
=
1
}
6

// move by the rolled amount
7

square
+=
diceRoll
8
}
while

square
<
finalSquare
9
println
(
"Game over!"
)

After the code checks for snakes and ladders, the dice is rolled, and the player is moved forward by
diceRoll
squares. The current loop execution then ends.
The loop’s condition (
while square < finalSquare
) is the same as before, but this time it is not
evaluated until the
end
of the first run through the loop. The structure of the
do
–
while
loop is better suited to
this game than the
while
loop in the previous example. In the
do
–
while
loop above,
square +=
board[square]
is always executed
immediately after
the loop’s
while
condition confirms that
square
is
still on the board. This behavior removes the need for the array bounds check seen in the earlier version of the
game.
​
Conditional Statements
It is often useful to execute different pieces of code based on certain conditions. You might want to run an extra
piece of code when an error occurs, or to display a message when a value becomes too high or too low. To do
this, you make parts of your code
conditional
.
Swift provides two ways to add conditional branches to your code, known as the
if
statement and the
switch
statement. Typically, you use the
if
statement to evaluate simple conditions with only a few possible
outcomes. The
switch
statement is better suited to more complex conditions with multiple possible
permutations, and is useful in situations where pattern-matching can help select an appropriate code branch to
execute.
​
If
In its simplest form, the
if
statement has a single
if
condition. It executes a set of statements only if that
condition is
true
:
1
var

temperatureInFahrenheit
=
30
2
if

temperatureInFahrenheit
<=
32
{
3

println
(
"It's very cold. Consider wearing a scarf."
)
4
}
5
// prints "It's very cold. Consider wearing a scarf."

The preceding example checks whether the temperature is less than or equal to 32 degrees Fahrenheit (the
freezing point of water). If it is, a message is printed. Otherwise, no message is printed, and code execution
continues after the
if
statement’s closing brace.
The
if
statement can provide an alternative set of statements, known as an
else clause
, for when the
if
condition is
false
. These statements are indicated by the
else
keyword:
1
temperatureInFahrenheit
=
40
2
if

temperatureInFahrenheit
<=
32
{
3

println
(
"It's very cold. Consider wearing a scarf."
)
4
}
else
{
5

println
(
"It's not that cold. Wear a t-shirt."
)
6
}
7
// prints "It's not that cold. Wear a t-shirt."
One of these two branches is always executed. Because the temperature has increased to
40
degrees
Fahrenheit, it is no longer cold enough to advise wearing a scarf, and so the
else
branch is triggered instead.
You can chain multiple
if
statements together, to consider additional clauses:
1
temperatureInFahrenheit
=
90
2
if

temperatureInFahrenheit
<=
32
{
3

println
(
"It's very cold. Consider wearing a scarf."
)
4
}
else

if

temperatureInFahrenheit
>=
86
{
5

println
(
"It's really warm. Don't forget to wear sunscreen."
)
6
}
else
{
7

println
(
"It's not that cold. Wear a t-shirt."
)
8
}
9
// prints "It's really warm. Don't forget to wear sunscreen."
Here, an additional
if
statement is added to respond to particularly warm temperatures. The final
else
clause
remains, and prints a response for any temperatures that are neither too warm nor too cold.
The final
else
clause is optional, however, and can be excluded if the set of conditions does not need to be
complete:

1
temperatureInFahrenheit
=
72
2
if

temperatureInFahrenheit
<=
32
{
3

println
(
"It's very cold. Consider wearing a scarf."
)
4
}
else

if

temperatureInFahrenheit
>=
86
{
5

println
(
"It's really warm. Don't forget to wear sunscreen."
)
6
}
In this example, the temperature is neither too cold nor too warm to trigger the
if
or
else if
conditions, and
so no message is printed.
​
Switch
A
switch
statement considers a value and compares it against several possible matching patterns. It then
executes an appropriate block of code, based on the first pattern that matches successfully. A
switch
statement provides an alternative to the
if
statement for responding to multiple potential states.
In its simplest form, a
switch
statement compares a value against one or more values of the same type:
switch

some value to consider
{
case

value 1
:

respond to value 1
case

value 2
,
value 3
:

respond to value 2 or 3
default
:

otherwise, do something else
}
Every
switch
statement consists of multiple possible
cases
, each of which begins with the
case
keyword. In
addition to comparing against specific values, Swift provides several ways for each case to specify more

complex matching patterns. These options are described later in this section.
The body of each
switch
case is a separate branch of code execution, in a similar manner to the branches of
an
if
statement. The
switch
statement determines which branch should be selected. This is known as
switching
on the value that is being considered.
Every
switch
statement must be
exhaustive
. That is, every possible value of the type being considered must
be matched by one of the
switch
cases. If it is not appropriate to provide a
switch
case for every possible
value, you can define a default catch-all case to cover any values that are not addressed explicitly. This catch-
all case is indicated by the keyword
default
, and must always appear last.
This example uses a
switch
statement to consider a single lowercase character called
someCharacter
:
1
let

someCharacter
:
Character
=
"e"
2
switch

someCharacter
{
3
case

"a"
,
"e"
,
"i"
,
"o"
,
"u"
:
4

println
(
"
\(
someCharacter
)
is a vowel"
)
5
case

"b"
,
"c"
,
"d"
,
"f"
,
"g"
,
"h"
,
"j"
,
"k"
,
"l"
,
"m"
,
6
"n"
,
"p"
,
"q"
,
"r"
,
"s"
,
"t"
,
"v"
,
"w"
,
"x"
,
"y"
,
"z"
:
7

println
(
"
\(
someCharacter
)
is a consonant"
)
8
default
:
9

println
(
"
\(
someCharacter
)
is not a vowel or a consonant"
)
10
}
11
// prints "e is a vowel"
The
switch
statement’s first case matches all five lowercase vowels in the English language. Similarly, its
second case matches all lowercase English consonants.
It is not practical to write all other possible characters as part of a
switch
case, and so this
switch
statement provides a
default
case to match all other characters that are not vowels or consonants. This
provision ensures that the
switch
statement is exhaustive.
​
No Implicit Fallthrough

In contrast with
switch
statements in C and Objective-C,
switch
statements in Swift do not fall through the
bottom of each case and into the next one by default. Instead, the entire
switch
statement finishes its execution
as soon as the first matching
switch
case is completed, without requiring an explicit
break
statement. This
makes the
switch
statement safer and easier to use than in C, and avoids executing more than one
switch
case by mistake.
NOTE
You can still break out of a matched
switch
case before that case has completed its execution if
you need to. See
Break in a Switch Statement
for details.
The body of each case
must
contain at least one executable statement. It is not valid to write the following code,
because the first case is empty:
1
let

anotherCharacter
:
Character
=
"a"
2
switch

anotherCharacter
{
3
case

"a"
:
4
case

"A"
:
5

println
(
"The letter A"
)
6
default
:
7

println
(
"Not the letter A"
)
8
}
9
// this will report a compile-time error
Unlike a
switch
statement in C, this
switch
statement does not match both
"a"
and
"A"
. Rather, it reports
a compile-time error that
case "a":
does not contain any executable statements. This approach avoids
accidental fallthrough from one case to another, and makes for safer code that is clearer in its intent.
Multiple matches for a single
switch
case can be separated by commas, and can be written over multiple
lines if the list is long:

switch

some value to consider
{
case

value 1
,
value 2
:

statements
}
NOTE
To opt in to fallthrough behavior for a particular
switch
case, use the
fallthrough
keyword, as
described in
Fallthrough
.
​
Range Matching
Values in
switch
cases can be checked for their inclusion in a range. This example uses number ranges to
provide a natural-language count for numbers of any size:
1
let

count
=
3_000_000_000_000
2
let

countedThings
=
"stars in the Milky Way"
3
var

naturalCount
:
String
4
switch

count
{
5
case

0
:
6

naturalCount
=
"no"
7
case

1
…
3
:
8

naturalCount
=
"a few"
9
case

4
…
9
:
10

naturalCount
=
"several"
11
case

10
…
99
:
12

naturalCount
=
"tens of"

13
case

100
…
999
:
14

naturalCount
=
"hundreds of"
15
case

1000
…
999_999
:
16

naturalCount
=
"thousands of"
17
default
:
18

naturalCount
=
"millions and millions of"
19
}
20
println
(
"There are
\(
naturalCount
)

\(
countedThings
)
."
)
21
// prints "There are millions and millions of stars in the

Milky Way."
​
Tuples
You can use tuples to test multiple values in the same
switch
statement. Each element of the tuple can be
tested against a different value or range of values. Alternatively, use the underscore (
_
) identifier to match any
possible value.
The example below takes an (x, y) point, expressed as a simple tuple of type
(Int, Int)
, and categorizes it
on the graph that follows the example:
1
let

somePoint
= (
1
,
1
)
2
switch

somePoint
{
3
case
(
0
,
0
):
4

println
(
"(0, 0) is at the origin"
)
5
case
(
_
,
0
):
6

println
(
"(
\(
somePoint
.
0
)
, 0) is on the x-axis"
)
7
case
(
0
,
_
):
8

println
(
"(0,
\(
somePoint
.
1
)
) is on the y-axis"
)
9
case
(-
2
…
2
, –
2
…
2
):
10

println
(
"(
\(
somePoint
.
0
)
,
\(
somePoint
.
1
)
) is inside the

box"
)
11
default
:
12

println
(
"(
\(
somePoint
.
0
)
,
\(
somePoint
.
1
)
) is outside of the

box"
)

13
}
14
// prints "(1, 1) is inside the box"
The
switch
statement determines if the point is at the origin (0, 0); on the red x-axis; on the orange y-axis;
inside the blue 4-by-4 box centered on the origin; or outside of the box.
Unlike C, Swift allows multiple
switch
cases to consider the same value or values. In fact, the point (0, 0)
could match all
four
of the cases in this example. However, if multiple matches are possible, the first matching
case is always used. The point (0, 0) would match
case (0, 0)
first, and so all other matching cases would
be ignored.
​
Value Bindings

A
switch
case can bind the value or values it matches to temporary constants or variables, for use in the body
of the case. This is known as
value binding
, because the values are “bound” to temporary constants or variables
within the case’s body.
The example below takes an (x, y) point, expressed as a tuple of type
(Int, Int)
and categorizes it on the
graph that follows:
1
let

anotherPoint
= (
2
,
0
)
2
switch

anotherPoint
{
3
case
(
let

x
,
0
):
4

println
(
"on the x-axis with an x value of
\(
x
)
"
)
5
case
(
0
,
let

y
):
6

println
(
"on the y-axis with a y value of
\(
y
)
"
)
7
case

let
(
x
,
y
):
8

println
(
"somewhere else at (
\(
x
)
,
\(
y
)
)"
)
9
}
10
// prints "on the x-axis with an x value of 2"

The
switch
statement determines if the point is on the red x-axis, on the orange y-axis, or elsewhere, on
neither axis.
The three
switch
cases declare placeholder constants
x
and
y
, which temporarily take on one or both tuple
values from
anotherPoint
. The first case,
case (let x, 0)
, matches any point with a
y
value of
0
and assigns the point’s
x
value to the temporary constant
x
. Similarly, the second case,
case (0, let y)
,
matches any point with an
x
value of
0
and assigns the point’s
y
value to the temporary constant
y
.
Once the temporary constants are declared, they can be used within the case’s code block. Here, they are used
as shorthand for printing the values with the
println
function.
Note that this
switch
statement does not have a
default
case. The final case,
case let (x, y)
,
declares a tuple of two placeholder constants that can match any value. As a result, it matches all possible
remaining values, and a
default
case is not needed to make the
switch
statement exhaustive.

In the example above,
x
and
y
are declared as constants with the
let
keyword, because there is no need to
modify their values within the body of the case. However, they could have been declared as variables instead,
with the
var
keyword. If this had been done, a temporary variable would have been created and initialized with
the appropriate value. Any changes to that variable would only have an effect within the body of the case.
​
Where
A
switch
case can use a
where
clause to check for additional conditions.
The example below categorizes an (x, y) point on the following graph:
1
let

yetAnotherPoint
= (
1
, –
1
)
2
switch

yetAnotherPoint
{
3
case

let
(
x
,
y
)
where

x
==
y
:
4

println
(
"(
\(
x
)
,
\(
y
)
) is on the line x == y"
)
5
case

let
(
x
,
y
)
where

x
== –
y
:
6

println
(
"(
\(
x
)
,
\(
y
)
) is on the line x == -y"
)
7
case

let
(
x
,
y
):
8

println
(
"(
\(
x
)
,
\(
y
)
) is just some arbitrary point"
)
9
}
10
// prints "(1, -1) is on the line x == -y"

The
switch
statement determines if the point is on the green diagonal line where
x == y
, on the purple
diagonal line where
x == -y
, or neither.
The three
switch
cases declare placeholder constants
x
and
y
, which temporarily take on the two tuple
values from
point
. These constants are used as part of a
where
clause, to create a dynamic filter. The
switch
case matches the current value of
point
only if the
where
clause’s condition evaluates to
true
for
that value.
As in the previous example, the final case matches all possible remaining values, and so a
default
case is
not needed to make the
switch
statement exhaustive.
​
Control Transfer Statements

Control transfer statements
change the order in which your code is executed, by transferring control from one
piece of code to another. Swift has four control transfer statements:
The
control
,
break
and
fallthrough
statements are described below. The
return
statement is
described in
Functions
.
​
Continue
The
continue
statement tells a loop to stop what it is doing and start again at the beginning of the next
iteration through the loop. It says “I am done with the current loop iteration” without leaving the loop altogether.
NOTE
In a
for
–
condition
–
increment
loop, the incrementer is still evaluated after calling the
continue
statement. The loop itself continues to work as usual; only the code within the loop’s
body is skipped.
The following example removes all vowels and spaces from a lowercase string to create a cryptic puzzle
phrase:
1
let

puzzleInput
=
"great minds think alike"
2
var

puzzleOutput
=
""
3
for

character

in

puzzleInput
{
4

switch

character
{
continue
break
fallthrough
return

5

case

"a"
,
"e"
,
"i"
,
"o"
,
"u"
,
" "
:
6

continue
7

default
:
8

puzzleOutput
+=
character
9
}
10
}
11
println
(
puzzleOutput
)
12
// prints "grtmndsthnklk"
The code above calls the
continue
keyword whenever it matches a vowel or a space, causing the current
iteration of the loop to end immediately and to jump straight to the start of the next iteration. This behavior
enables the switch block to match (and ignore) only the vowel and space characters, rather than requiring the
block to match every character that should get printed.
​
Break
The
break
statement ends execution of an entire control flow statement immediately. The
break
statement
can be used inside a
switch
statement or loop statement when you want to terminate the execution of the
switch
or loop statement earlier than would otherwise be the case.
​
Break in a Loop Statement
When used inside a loop statement,
break
ends the loop’s execution immediately, and transfers control to the
first line of code after the loop’s closing brace (
}
). No further code from the current iteration of the loop is
executed, and no further iterations of the loop are started.
​
Break in a Switch Statement
When used inside a
switch
statement,
break
causes the
switch
statement to end its execution
immediately, and to transfer control to the first line of code after the
switch
statement’s closing brace (
}
).

This behavior can be used to match and ignore one or more cases in a
switch
statement. Because Swift’s
switch
statement is exhaustive and does not allow empty cases, it is sometimes necessary to deliberately
match and ignore a case in order to make your intentions explicit. You do this by writing the
break
statement
as the entire body of the case you want to ignore. When that case is matched by the
switch
statement, the
break
statement inside the case ends the
switch
statement’s execution immediately.
NOTE
A
switch
case that only contains a comment is reported as a compile-time error. Comments are
not statements and do not cause a
switch
case to be ignored. Always use a
break
statement to
ignore a
switch
case.
The following example switches on a
Character
value and determines whether it represents a number
symbol in one of four languages. Multiple values are covered in a single
switch
case for brevity:
1
let

numberSymbol
:
Character
=
"
三
"

// Simplified Chinese for the

number 3
2
var

possibleIntegerValue
:
Int
?
3
switch

numberSymbol
{
4
case

"1"
,
"
١
"
,
"
⼀
"
,
"
๑
"
:
5

possibleIntegerValue
=
1
6
case

"2"
,
"
٢
"
,
"
⼆
"
,
"
๒
"
:
7

possibleIntegerValue
=
2
8
case

"3"
,
"
٣
"
,
"
三
"
,
"
๓
"
:
9

possibleIntegerValue
=
3
10
case

"4"
,
"
٤
"
,
"
四
"
,
"
๔
"
:
11

possibleIntegerValue
=
4
12
default
:
13

break
14
}
15
if

let

integerValue
=
possibleIntegerValue
{
16

println
(
"The integer value of
\(
numberSymbol
)
is
\

(
integerValue
)
."
)
17
}
else
{
18

println
(
"An integer value could not be found for
\
(
numberSymbol
)
."
)
19
}
20
// prints "The integer value of
三
is 3."
This example checks
numberSymbol
to determine whether it is a Latin, Arabic, Chinese, or Thai symbol for
the numbers
1
to
4
. If a match is found, one of the
switch
statement’s cases sets an optional
Int?
variable
called
possibleIntegerValue
to an appropriate integer value.
After the switch statement completes its execution, the example uses optional binding to determine whether a
value was found. The
possibleIntegerValue
variable has an implicit initial value of
nil
by virtue of
being an optional type, and so the optional binding will succeed only if
possibleIntegerValue
was set to
an actual value by one of the
switch
statement’s first four cases.
It is not practical to list every possible
Character
value in the example above, so a
default
case provides
a catchall for any characters that are not matched. This
default
case does not need to perform any action,
and so it is written with a single
break
statement as its body. As soon as the
default
statement is matched,
the
break
statement ends the
switch
statement’s execution, and code execution continues from the
if let
statement.
​
Fallthrough
Switch statements in Swift do not fall through the bottom of each case and into the next one. Instead, the entire
switch statement completes its execution as soon as the first matching case is completed. By contrast, C
requires you to insert an explicit
break
statement at the end of every
switch
case to prevent fallthrough.
Avoiding default fallthrough means that Swift
switch
statements are much more concise and predictable than
their counterparts in C, and thus they avoid executing multiple
switch
cases by mistake.
If you really need C-style fallthrough behavior, you can opt in to this behavior on a case-by-case basis with the
fallthrough
keyword. The example below uses
fallthrough
to create a textual description of a
number:

1
let

integerToDescribe
=
5
2
var

description
=
"The number
\(
integerToDescribe
)
is"
3
switch

integerToDescribe
{
4
case

2
,
3
,
5
,
7
,
11
,
13
,
17
,
19
:
5

description
+=
" a prime number, and also"
6

fallthrough
7
default
:
8

description
+=
" an integer."
9
}
10
println
(
description
)
11
// prints "The number 5 is a prime number, and also an

integer."
This example declares a new
String
variable called
description
and assigns it an initial value. The
function then considers the value of
integerToDescribe
using a
switch
statement. If the value of
integerToDescribe
is one of the prime numbers in the list, the function appends text to the end of
description
, to note that the number is prime. It then uses the
fallthrough
keyword to “fall into” the
default
case as well. The
default
case adds some extra text to the end of the description, and the
switch
statement is complete.
If the value of
integerToDescribe
is
not
in the list of known prime numbers, it is not matched by the first
switch
case at all. There are no other specific cases, and so
integerToDescribe
is matched by the
catchall
default
case.
After the
switch
statement has finished executing, the number’s description is printed using the
println
function. In this example, the number
5
is correctly identified as a prime number.
NOTE
The
fallthrough
keyword does not check the case conditions for the
switch
case that it
causes execution to fall into. The
fallthrough
keyword simply causes code execution to move
directly to the statements inside the next case (or
default
case) block, as in C’s standard
switch
statement behavior.

​
Labeled Statements
You can nest loops and
switch
statements inside other loops and
switch
statements in Swift to create
complex control flow structures. However, loops and
switch
statements can both use the
break
statement
to end their execution prematurely. Therefore, it is sometimes useful to be explicit about which loop or
switch
statement you want a
break
statement to terminate. Similarly, if you have multiple nested loops, it can be
useful to be explicit about which loop the
continue
statement should affect.
To achieve these aims, you can mark a loop statement or
switch
statement with a
statement label
, and use
this label with the
break
statement or
continue
statement to end or continue the execution of the labeled
statement.
A labeled statement is indicated by placing a label on the same line as the statement’s introducer keyword,
followed by a colon. Here’s an example of this syntax for a
while
loop, although the principle is the same for
all loops and
switch
statements:
label name
:
while

condition
{

statements
}
The following example uses the
break
and
continue
statements with a labeled
while
loop for an adapted
version of the
Snakes and Ladders
game that you saw earlier in this chapter. This time around, the game has an
extra rule:
If a particular dice roll would take you beyond square 25, you must roll again until you roll the exact number
needed to land on square 25.
The game board is the same as before:
To win, you must land
exactly
on square 25.

The values of
finalSquare
,
board
,
square
, and
diceRoll
are initialized in the same way as before:
1
let

finalSquare
=
25
2
var

board
=
Int
[](
count
:
finalSquare
+
1
,
repeatedValue
:
0
)
3
board
[
03
] = +
08
;
board
[
06
] = +
11
;
board
[
09
] = +
09
;
board
[
10
] = +
02
4
board
[
14
] = –
10
;
board
[
19
] = –
11
;
board
[
22
] = –
02
;
board
[
24
] = –
08
5
var

square
=
0
6
var

diceRoll
=
0
This version of the game uses a
while
loop and a
switch
statement to implement the game’s logic. The
while
loop has a statement label called
gameLoop
, to indicate that it is the main game loop for the Snakes
and Ladders game.

The
while
loop’s condition is
while square != finalSquare
, to reflect that you must land exactly on
square 25:
1
gameLoop
:
while

square
!=
finalSquare
{
2

if
++
diceRoll
==
7
{
diceRoll
=
1
}
3

switch

square
+
diceRoll
{
4

case

finalSquare
:
5

// diceRoll will move us to the final square, so the game is

over
6

break

gameLoop
7

case

let

newSquare

where

newSquare
>
finalSquare
:
8

// diceRoll will move us beyond the final square, so roll

again
9

continue

gameLoop
10

default
:
11

// this is a valid move, so find out its effect
12

square
+=
diceRoll
13

square
+=
board
[
square
]
14
}
15
}
16
println
(
"Game over!"
)
The dice is rolled at the start of each loop. Rather than moving the player immediately, a
switch
statement is
used to consider the result of the move, and to work out if the move is allowed:
If the dice roll will move the player onto the final square, the game is over. The
break
gameLoop
statement transfers control to the first line of code outside of the
while
loop, which
ends the game.
If the dice roll will move the player
beyond
the final square, the move is invalid, and the player
needs to roll again. The
continue gameLoop
statement ends the current
while
loop
iteration and begins the next iteration of the loop.
In all other cases, the dice roll is a valid move. The player moves forward by
diceRoll
squares, and the game logic checks for any snakes and ladders. The loop then ends, and control
returns to the
while
condition to decide whether another turn is required.

NOTE
If the
break
statement above did not use the
gameLoop
label, it would break out of the
switch
statement, not the
while
statement. Using the
gameLoop
label makes it clear which control
statement should be terminated.
Note also that it is not strictly necessary to use the
gameLoop
label when calling
continue
gameLoop
to jump to the next iteration of the loop. There is only one loop in the game, and so there
is no ambiguity as to which loop the
continue
statement will affect. However, there is no harm in
using the
gameLoop
label with the
continue
statement. Doing so is consistent with the label’s
use alongside the
break
statement, and helps make the game’s logic clearer to read and
understand.

​
​
Functions
Functions
are self-contained chunks of code that perform a specific task. You give a function a name that
identifies what it does, and this name is used to “call” the function to perform its task when needed.
Swift’s unified function syntax is flexible enough to express anything from a simple C-style function with no
parameter names to a complex Objective-C-style method with local and external parameter names for each
parameter. Parameters can provide default values to simplify function calls and can be passed as in-out
parameters, which modify a passed variable once the function has completed its execution.
Every function in Swift has a type, consisting of the function’s parameter types and return type. You can use this
type like any other type in Swift, which makes it easy to pass functions as parameters to other functions, and to
return functions from functions. Functions can also be written within other functions to encapsulate useful
functionality within a nested function scope.
​
Defining and Calling Functions
When you define a function, you can optionally define one or more named, typed values that the function takes
as input (known as
parameters
), and/or a type of value that the function will pass back as output when it is done
(known as its
return type
).
Every function has a
function name
, which describes the task that the function performs. To use a function, you
“call” that function with its name and pass it input values (known as
arguments
) that match the types of the
function’s parameters. A function’s arguments must always be provided in the same order as the function’s
parameter list.
The function in the example below is called
greetingForPerson
, because that’s what it does—it takes a
person’s name as input and returns a greeting for that person. To accomplish this, you define one input
parameter—a
String
value called
personName
—and a return type of
String
, which will contain a
greeting for that person:
1
func

sayHello
(
personName
:
String
) ->
String
{
2

let

greeting
=
"Hello, "
+
personName
+
"!"

3

return

greeting
4
}
All of this information is rolled up into the function’s
definition
, which is prefixed with the
func
keyword. You
indicate the function’s return type with the
return arrow

->
(a hyphen followed by a right angle bracket), which
is followed by the name of the type to return.
The definition describes what the function does, what it expects to receive, and what it returns when it is done.
The definition makes it easy for the function to be called elsewhere in your code in a clear and unambiguous
way:
1
println
(
sayHello
(
"Anna"
))
2
// prints "Hello, Anna!"
3
println
(
sayHello
(
"Brian"
))
4
// prints "Hello, Brian!"
You call the
sayHello
function by passing it a
String
argument value in parentheses, such as
sayHello("Anna")
. Because the function returns a
String
value,
sayHello
can be wrapped in a call to
the
println
function to print that string and see its return value, as shown above.
The body of the
sayHello
function starts by defining a new
String
constant called
greeting
and setting it
to a simple greeting message for
personName
. This greeting is then passed back out of the function using the
return
keyword. As soon as
return greeting
is called, the function finishes its execution and returns
the current value of
greeting
.
You can call the
sayHello
function multiple times with different input values. The example above shows what
happens if it is called with an input value of
"Anna"
, and an input value of
"Brian"
. The function returns a
tailored greeting in each case.
To simplify the body of this function, combine the message creation and the return statement into one line:
1
func

sayHelloAgain
(
personName
:
String
) ->
String
{
2

return

"Hello again, "
+
personName
+
"!"
3
}
4
println
(
sayHelloAgain
(
"Anna"
))

5
// prints "Hello again, Anna!"
​
Function Parameters and Return Values
Function parameters and return values are extremely flexible in Swift. You can define anything from a simple
utility function with a single unnamed parameter to a complex function with expressive parameter names and
different parameter options.
​
Multiple Input Parameters
Functions can have multiple input parameters, which are written within the function’s parentheses, separated by
commas.
This function takes a start and an end index for a half-open range, and works out how many elements the range
contains:
1
func

halfOpenRangeLength
(
start
:
Int
,
end
:
Int
) ->
Int
{
2

return

end
–
start
3
}
4
println
(
halfOpenRangeLength
(
1
,
10
))
5
// prints "9"
​
Functions Without Parameters
Functions are not required to define input parameters. Here’s a function with no input parameters, which always
returns the same
String
message whenever it is called:
1
func

sayHelloWorld
() ->
String
{
2

return

"hello, world"
3
}

4
println
(
sayHelloWorld
())
5
// prints "hello, world"
The function definition still needs parentheses after the function’s name, even though it does not take any
parameters. The function name is also followed by an empty pair of parentheses when the function is called.
​
Functions Without Return Values
Functions are not required to define a return type. Here’s a version of the
sayHello
function, called
waveGoodbye
, which prints its own
String
value rather than returning it:
1
func

sayGoodbye
(
personName
:
String
) {
2

println
(
"Goodbye,
\(
personName
)
!"
)
3
}
4
sayGoodbye
(
"Dave"
)
5
// prints "Goodbye, Dave!"
Because it does not need to return a value, the function’s definition does not include the return arrow (
->
) or a
return type.
NOTE
Strictly speaking, the
sayGoodbye
function
does
still return a value, even though no return value is
defined. Functions without a defined return type return a special value of type
Void
. This is simply
an empty tuple, in effect a tuple with zero elements, which can be written as
()
.
The return value of a function can be ignored when it is called:
1
func

printAndCount
(
stringToPrint
:
String
) ->
Int
{

2

println
(
stringToPrint
)
3

return

countElements
(
stringToPrint
)
4
}
5
func

printWithoutCounting
(
stringToPrint
:
String
) {
6

printAndCount
(
stringToPrint
)
7
}
8
printAndCount
(
"hello, world"
)
9
// prints "hello, world" and returns a value of 12
10
printWithoutCounting
(
"hello, world"
)
11
// prints "hello, world" but does not return a value
The first function,
printAndCount
, prints a string, and then returns its character count as an
Int
. The
second function,
printWithoutCounting
, calls the first function, but ignores its return value. When the
second function is called, the message is still printed by the first function, but the returned value is not used.
NOTE
Return values can be ignored, but a function that says it will return a value must always do so. A
function with a defined return type cannot allow control to fall out of the bottom of the function without
returning a value, and attempting to do so will result in a compile-time error.
​
Functions with Multiple Return Values
You can use a tuple type as the return type for a function to return multiple values as part of one compound
return value.
The example below defines a function called
count
, which counts the number of vowels, consonants, and
other characters in a string, based on the standard set of vowels and consonants used in American English:
1
func

count
(
string
:
String
) -> (
vowels
:
Int
,
consonants
:
Int
,
others
:

Int
) {
2

var

vowels
=
0
,
consonants
=
0
,
others
=
0
3

for

character

in

string
{
4

switch

String
(
character
).
lowercaseString
{
5

case

"a"
,
"e"
,
"i"
,
"o"
,
"u"
:
6
++
vowels
7

case

"b"
,
"c"
,
"d"
,
"f"
,
"g"
,
"h"
,
"j"
,
"k"
,
"l"
,
"m"
,
8

"n"
,
"p"
,
"q"
,
"r"
,
"s"
,
"t"
,
"v"
,
"w"
,
"x"
,
"y"
,
"z"
:
9
++
consonants
10

default
:
11
++
others
12
}
13
}
14

return
(
vowels
,
consonants
,
others
)
15
}
You can use this
count
function to count the characters in an arbitrary string, and to retrieve the counted totals
as a tuple of three named
Int
values:
1
let

total
=
count
(
"some arbitrary string!"
)
2
println
(
"
\(
total
.
vowels
)
vowels and
\(
total
.
consonants
)
consonants"
)
3
// prints "6 vowels and 13 consonants"
Note that the tuple’s members do not need to be named at the point that the tuple is returned from the function,
because their names are already specified as part of the function’s return type.
​
Function Parameter Names
All of the above functions define
parameter names
for their parameters:
1
func

someFunction
(
parameterName
:
Int
) {
2

// function body goes here, and can use parameterName
3

// to refer to the argument value for that parameter

4
}
However, these parameter names are only used within the body of the function itself, and cannot be used when
calling the function. These kinds of parameter names are known as
local parameter names
, because they are
only available for use within the function’s body.
​
External Parameter Names
Sometimes it’s useful to name each parameter when you
call
a function, to indicate the purpose of each
argument you pass to the function.
If you want users of your function to provide parameter names when they call your function, define an
external
parameter name
for each parameter, in addition to the local parameter name. You write an external parameter
name before the local parameter name it supports, separated by a space:
1
func

someFunction
(
externalParameterName

localParameterName
:
Int
) {
2

// function body goes here, and can use localParameterName
3

// to refer to the argument value for that parameter
4
}
NOTE
If you provide an external parameter name for a parameter, that external name must
always
be used
when calling the function.
As an example, consider the following function, which joins two strings by inserting a third “joiner” string
between them:
1
func

join
(
s1
:
String
,
s2
:
String
,
joiner
:
String
) ->
String
{
2

return

s1
+
joiner
+
s2

3
}
When you call this function, the purpose of the three strings that you pass to the function is unclear:
1
join
(
"hello"
,
"world"
,
", "
)
2
// returns "hello, world"
To make the purpose of these
String
values clearer, define external parameter names for each
join
function parameter:
1
func

join
(
string

s1
:
String
,
toString

s2
:
String
,
withJoiner

joiner
:

String
)
2
->
String
{
3

return

s1
+
joiner
+
s2
4
}
In this version of the
join
function, the first parameter has an external name of
string
and a local name of
s1
; the second parameter has an external name of
toString
and a local name of
s2
; and the third parameter
has an external name of
withJoiner
and a local name of
joiner
.
You can now use these external parameter names to call the function in a clear and unambiguous way:
1
join
(
string
:
"hello"
,
toString
:
"world"
,
withJoiner
:
", "
)
2
// returns "hello, world"
The use of external parameter names enables this second version of the
join
function to be called in an
expressive, sentence-like manner by users of the function, while still providing a function body that is readable
and clear in intent.
NOTE
Consider using external parameter names whenever the purpose of a function’s arguments would

be unclear to someone reading your code for the first time. You do not need to specify external
parameter names if the purpose of each parameter is clear and unambiguous when the function is
called.
​
Shorthand External Parameter Names
If you want to provide an external parameter name for a function parameter, and the local parameter name is
already an appropriate name to use, you do not need to write the same name twice for that parameter. Instead,
write the name once, and prefix the name with a hash symbol (
#
). This tells Swift to use that name as both the
local parameter name and the external parameter name.
This example defines a function called
containsCharacter
, which defines external parameter names for
both of its parameters by placing a hash symbol before their local parameter names:
1
func

containsCharacter
(#
string
:
String
, #
characterToFind
:
Character
) –
>
Bool
{
2

for

character

in

string
{
3

if

character
==
characterToFind
{
4

return

true
5
}
6
}
7

return

false
8
}
This function’s choice of parameter names makes for a clear, readable function body, while also enabling the
function to be called without ambiguity:
1
let

containsAVee
=
containsCharacter
(
string
:
"aardvark"
,

characterToFind
:
"v"
)
2
// containsAVee equals true, because "aardvark" contains a "v"
​

Default Parameter Values
You can define a
default value
for any parameter as part of a function’s definition. If a default value is defined,
you can omit that parameter when calling the function.
NOTE
Place parameters with default values at the end of a function’s parameter list. This ensures that all
calls to the function use the same order for their non-default arguments, and makes it clear that the
same function is being called in each case.
Here’s a version of the
join
function from earlier, which provides a default value for its
joiner
parameter:
1
func

join
(
string

s1
:
String
,
toString

s2
:
String
,
2

withJoiner

joiner
:
String
=
" "
) ->
String
{
3

return

s1
+
joiner
+
s2
4
}
If a string value for
joiner
is provided when the
join
function is called, that string value is used to join the
two strings together, as before:
1
join
(
string
:
"hello"
,
toString
:
"world"
,
withJoiner
:
"-"
)
2
// returns "hello-world"
However, if no value of
joiner
is provided when the function is called, the default value of a single space (
"
"
) is used instead:
1
join
(
string
:
"hello"
,
toString
:
"world"
)
2
// returns "hello world"

​
External Names for Parameters with Default Values
In most cases, it is useful to provide (and therefore require) an external name for any parameter with a default
value. This ensures that the argument for that parameter is clear in purpose if a value is provided when the
function is called.
To make this process easier, Swift provides an automatic external name for any defaulted parameter you
define, if you do not provide an external name yourself. The automatic external name is the same as the local
name, as if you had written a hash symbol before the local name in your code.
Here’s a version of the
join
function from earlier, which does not provide external names for any of its
parameters, but still provides a default value for its
joiner
parameter:
1
func

join
(
s1
:
String
,
s2
:
String
,
joiner
:
String
=
" "
) ->
String
{
2

return

s1
+
joiner
+
s2
3
}
In this case, Swift automatically provides an external parameter name of
joiner
for the defaulted parameter.
The external name must therefore be provided when calling the function, making the parameter’s purpose clear
and unambiguous:
1
join
(
"hello"
,
"world"
,
joiner
:
"-"
)
2
// returns "hello-world"
NOTE
You can opt out of this behavior by writing an underscore (
_
) instead of an explicit external name
when you define the parameter. However, external names for defaulted parameters are always
preferred where appropriate.

​
Variadic Parameters
A
variadic parameter
accepts zero or more values of a specified type. You use a variadic parameter to specify
that the parameter can be passed a varying number of input values when the function is called. Write variadic
parameters by inserting three period characters (
…
) after the parameter’s type name.
The values passed to a variadic parameter are made available within the function’s body as an array of the
appropriate type. For example, a variadic parameter with a name of
numbers
and a type of
Double…
is
made available within the function’s body as a constant array called
numbers
of type
Double[]
.
The example below calculates the
arithmetic mean
(also known as the
average
) for a list of numbers of any
length:
1
func

arithmeticMean
(
numbers
:
Double
…) ->
Double
{
2

var

total
:
Double
=
0
3

for

number

in

numbers
{
4

total
+=
number
5
}
6

return

total
/
Double
(
numbers
.
count
)
7
}
8
arithmeticMean
(
1
,
2
,
3
,
4
,
5
)
9
// returns 3.0, which is the arithmetic mean of these five numbers
10
arithmeticMean
(
3
,
8
,
19
)
11
// returns 10.0, which is the arithmetic mean of these three

numbers
NOTE
A function may have at most one variadic parameter, and it must always appear last in the
parameter list, to avoid ambiguity when calling the function with multiple parameters.
If your function has one or more parameters with a default value, and also has a variadic parameter,
place the variadic parameter after all the defaulted parameters at the very end of the list.

​
Constant and Variable Parameters
Function parameters are constants by default. Trying to change the value of a function parameter from within
the body of that function results in a compile-time error. This means that you can’t change the value of a
parameter by mistake.
However, sometimes it is useful for a function to have a
variable
copy of a parameter’s value to work with. You
can avoid defining a new variable yourself within the function by specifying one or more parameters as
variable
parameters
instead. Variable parameters are available as variables rather than as constants, and give a new
modifiable copy of the parameter’s value for your function to work with.
Define variable parameters by prefixing the parameter name with the keyword
var
:
1
func

alignRight
(
var

string
:
String
,
count
:
Int
,
pad
:
Character
) ->

String
{
2

let

amountToPad
=
count
–
countElements
(
string
)
3

for

_

in

1
…
amountToPad
{
4

string
=
pad
+
string
5
}
6

return

string
7
}
8
let

originalString
=
"hello"
9
let

paddedString
=
alignRight
(
originalString
,
10
,
"-"
)
10
// paddedString is equal to "––hello"
11
// originalString is still equal to "hello"
This example defines a new function called
alignRight
, which aligns an input string to the right edge of a
longer output string. Any space on the left is filled with a specified padding character. In this example, the string
"hello"
is converted to the string
"––hello"
.
The
alignRight
function defines the input parameter
string
to be a variable parameter. This means that
string
is now available as a local variable, initialized with the passed-in string value, and can be manipulated
within the body of the function.

The function starts by working out how many characters need to be added to the left of
string
in order to
right-align it within the overall string. This value is stored in a local constant called
amountToPad
. The
function then adds
amountToPad
copies of the
pad
character to the left of the existing string and returns the
result. It uses the
string
variable parameter for all its string manipulation.
NOTE
The changes you make to a variable parameter do not persist beyond the end of each call to the
function, and are not visible outside the function’s body. The variable parameter only exists for the
lifetime of that function call.
​
In-Out Parameters
Variable parameters, as described above, can only be changed within the function itself. If you want a function to
modify a parameter’s value, and you want those changes to persist after the function call has ended, define that
parameter as an
in-out parameter
instead.
You write an in-out parameter by placing the
inout
keyword at the start of its parameter definition. An in-out
parameter has a value that is passed
in
to the function, is modified by the function, and is passed back
out
of the
function to replace the original value.
You can only pass a variable as the argument for an in-out parameter. You cannot pass a constant or a literal
value as the argument, because constants and literals cannot be modified. You place an ampersand (
&
) directly
before a variable’s name when you pass it as an argument to an inout parameter, to indicate that it can be
modified by the function.
NOTE
In-out parameters cannot have default values, and variadic parameters cannot be marked as
inout
. If you mark a parameter as
inout
, it cannot also be marked as
var
or
let
.

Here’s an example of a function called
swapTwoInts
, which has two in-out integer parameters called
a
and
b
:
1
func

swapTwoInts
(
inout

a
:
Int
,
inout

b
:
Int
) {
2

let

temporaryA
=
a
3

a
=
b
4

b
=
temporaryA
5
}
The
swapTwoInts
function simply swaps the value of
b
into
a
, and the value of
a
into
b
. The function
performs this swap by storing the value of
a
in a temporary constant called
temporaryA
, assigning the value
of
b
to
a
, and then assigning
temporaryA
to
b
.
You can call the
swapTwoInts
function with two variables of type
Int
to swap their values. Note that the
names of
someInt
and
anotherInt
are prefixed with an ampersand when they are passed to the
swapTwoInts
function:
1
var

someInt
=
3
2
var

anotherInt
=
107
3
swapTwoInts
(&
someInt
, &
anotherInt
)
4
println
(
"someInt is now
\(
someInt
)
, and anotherInt is now
\
(
anotherInt
)
"
)
5
// prints "someInt is now 107, and anotherInt is now 3"
The example above shows that the original values of
someInt
and
anotherInt
are modified by the
swapTwoInts
function, even though they were originally defined outside of the function.
NOTE

In-out parameters are not the same as returning a value from a function. The
swapTwoInts
example above does not define a return type or return a value, but it still modifies the values of
someInt
and
anotherInt
. In-out parameters are an alternative way for a function to have an
effect outside of the scope of its function body.
​
Function Types
Every function has a specific
function type
, made up of the parameter types and the return type of the function.
For example:
1
func

addTwoInts
(
a
:
Int
,
b
:
Int
) ->
Int
{
2

return

a
+
b
3
}
4
func

multiplyTwoInts
(
a
:
Int
,
b
:
Int
) ->
Int
{
5

return

a
*
b
6
}
This example defines two simple mathematical functions called
addTwoInts
and
multiplyTwoInts
.
These functions each take two
Int
values, and return an
Int
value, which is the result of performing an
appropriate mathematical operation.
The type of both of these functions is
(Int, Int) -> Int
. This can be read as:
“A function type that has two parameters, both of type
Int
, and that returns a value of type
Int
.”
Here’s another example, for a function with no parameters or return value:
1
func

printHelloWorld
() {
2

println
(
"hello, world"
)
3
}

The type of this function is
() -> ()
, or “a function that has no parameters, and returns
Void
.” Functions
that don’t specify a return value always return
Void
, which is equivalent to an empty tuple in Swift, shown as
()
.
​
Using Function Types
You use function types just like any other types in Swift. For example, you can define a constant or variable to
be of a function type and assign an appropriate function to that variable:
1
var

mathFunction
: (
Int
,
Int
) ->
Int
=
addTwoInts
This can be read as:
“Define a variable called
mathFunction
, which has a type of ‘a function that takes two
Int
values, and
returns an
Int
value.’ Set this new variable to refer to the function called
addTwoInts
.”
The
addTwoInts
function has the same type as the
mathFunction
variable, and so this assignment is
allowed by Swift’s type-checker.
You can now call the assigned function with the name
mathFunction
:
1
println
(
"Result:
\(
mathFunction
(
2
,
3
))
"
)
2
// prints "Result: 5"
A different function with the same matching type can be assigned to the same variable, in the same way as for
non-function types:
1
mathFunction
=
multiplyTwoInts
2
println
(
"Result:
\(
mathFunction
(
2
,
3
))
"
)
3
// prints "Result: 6"
As with any other type, you can leave it to Swift to infer the function type when you assign a function to a

constant or variable:
1
let

anotherMathFunction
=
addTwoInts
2
// anotherMathFunction is inferred to be of type (Int, Int) -> Int
​
Function Types as Parameter Types
You can use a function type such as
(Int, Int) -> Int
as a parameter type for another function. This
enables you to leave some aspects of a function’s implementation for the function’s caller to provide when the
function is called.
Here’s an example to print the results of the math functions from above:
1
func

printMathResult
(
mathFunction
: (
Int
,
Int
) ->
Int
,
a
:
Int
,
b
:
Int
)

{
2

println
(
"Result:
\(
mathFunction
(
a
,
b
))
"
)
3
}
4
printMathResult
(
addTwoInts
,
3
,
5
)
5
// prints "Result: 8"
This example defines a function called
printMathResult
, which has three parameters. The first parameter
is called
mathFunction
, and is of type
(Int, Int) -> Int
. You can pass any function of that type as
the argument for this first parameter. The second and third parameters are called
a
and
b
, and are both of type
Int
. These are used as the two input values for the provided math function.
When
printMathResult
is called, it is passed the
addTwoInts
function, and the integer values
3
and
5
.
It calls the provided function with the values
3
and
5
, and prints the result of
8
.
The role of
printMathResult
is to print the result of a call to a math function of an appropriate type. It
doesn’t matter what that function’s implementation actually does—it matters only that the function is of the
correct type. This enables
printMathResult
to hand off some of its functionality to the caller of the function
in a type-safe way.

​
Function Types as Return Types
You can use a function type as the return type of another function. You do this by writing a complete function type
immediately after the return arrow (
->
) of the returning function.
The next example defines two simple functions called
stepForward
and
stepBackward
. The
stepForward
function returns a value one more than its input value, and the
stepBackward
function
returns a value one less than its input value. Both functions have a type of
(Int) -> Int
:
1
func

stepForward
(
input
:
Int
) ->
Int
{
2

return

input
+
1
3
}
4
func

stepBackward
(
input
:
Int
) ->
Int
{
5

return

input
–
1
6
}
Here’s a function called
chooseStepFunction
, whose return type is “a function of type
(Int) -> Int
”.
chooseStepFunction
returns the
stepForward
function or the
stepBackward
function based on a
Boolean parameter called
backwards
:
1
func

chooseStepFunction
(
backwards
:
Bool
) -> (
Int
) ->
Int
{
2

return

backwards
?
stepBackward
:
stepForward
3
}
You can now use
chooseStepFunction
to obtain a function that will step in one direction or the other:
1
var

currentValue
=
3
2
let

moveNearerToZero
=
chooseStepFunction
(
currentValue
>
0
)
3
// moveNearerToZero now refers to the stepBackward() function
The preceding example works out whether a positive or negative step is needed to move a variable called
currentValue
progressively closer to zero.
currentValue
has an initial value of
3
, which means that

currentValue > 0
returns
true
, causing
chooseStepFunction
to return the
stepBackward
function. A reference to the returned function is stored in a constant called
moveNearerToZero
.
Now that
moveNearerToZero
refers to the correct function, it can be used to count to zero:
1
println
(
"Counting to zero:"
)
2
// Counting to zero:
3
while

currentValue
!=
0
{
4

println
(
"
\(
currentValue
)
… "
)
5

currentValue
=
moveNearerToZero
(
currentValue
)
6
}
7
println
(
"zero!"
)
8
// 3…
9
// 2…
10
// 1…
11
// zero!
​
Nested Functions
All of the functions you have encountered so far in this chapter have been examples of
global functions
, which
are defined at a global scope. You can also define functions inside the bodies of other functions, known as
nested
functions
.
Nested functions are hidden from the outside world by default, but can still be called and used by their enclosing
function. An enclosing function can also return one of its nested functions to allow the nested function to be used
in another scope.
You can rewrite the
chooseStepFunction
example above to use and return nested functions:
1
func

chooseStepFunction
(
backwards
:
Bool
) -> (
Int
) ->
Int
{
2

func

stepForward
(
input
:
Int
) ->
Int
{
return

input
+
1
}
3

func

stepBackward
(
input
:
Int
) ->
Int
{
return

input
–
1
}
4

return

backwards
?
stepBackward
:
stepForward
5
}

6
var

currentValue
= –
4
7
let

moveNearerToZero
=
chooseStepFunction
(
currentValue
>
0
)
8
// moveNearerToZero now refers to the nested stepForward() function
9
while

currentValue
!=
0
{
10

println
(
"
\(
currentValue
)
… "
)
11

currentValue
=
moveNearerToZero
(
currentValue
)
12
}
13
println
(
"zero!"
)
14
// -4…
15
// -3…
16
// -2…
17
// -1…
18
// zero!

​
​
Closures
Closures
are self-contained blocks of functionality that can be passed around and used in your code. Closures
in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.
Closures can capture and store references to any constants and variables from the context in which they are
defined. This is known as
closing
over those constants and variables, hence the name “closures”. Swift handles
all of the memory management of capturing for you.
NOTE
Don’t worry if you are not familiar with the concept of “capturing”. It is explained in detail below in
Capturing Values
.
Global and nested functions, as introduced in
Functions
, are actually special cases of closures. Closures take
one of three forms:
Swift’s closure expressions have a clean, clear style, with optimizations that encourage brief, clutter-free
syntax in common scenarios. These optimizations include:
Global functions are closures that have a name and do not capture any values.
Nested functions are closures that have a name and can capture values from their enclosing
function.
Closure expressions are unnamed closures written in a lightweight syntax that can capture
values from their surrounding context.
Inferring parameter and return value types from context
Implicit returns from single-expression closures
Shorthand argument names

​
Closure Expressions
Nested functions, as introduced in
Nested Functions
, are a convenient means of naming and defining self-
contained blocks of code as part of a larger function. However, it is sometimes useful to write shorter versions
of function-like constructs without a full declaration and name. This is particularly true when you work with
functions that take other functions as one or more of their arguments.
Closure expressions
are a way to write inline closures in a brief, focused syntax. Closure expressions provide
several syntax optimizations for writing closures in their simplest form without loss of clarity or intent. The
closure expression examples below illustrate these optimizations by refining a single example of the
sort
function over several iterations, each of which expresses the same functionality in a more succinct way.
​
The Sort Function
Swift’s standard library provides a function called
sort
, which sorts an array of values of a known type, based
on the output of a sorting closure that you provide. Once it completes the sorting process, the
sort
function
returns a new array of the same type and size as the old one, with its elements in the correct sorted order.
The closure expression examples below use the
sort
function to sort an array of
String
values in reverse
alphabetical order. Here’s the initial array to be sorted:
1
let

names
= [
"Chris"
,
"Alex"
,
"Ewa"
,
"Barry"
,
"Daniella"
]
The
sort
function takes two arguments:
Trailing closure syntax
An array of values of a known type.
A closure that takes two arguments of the same type as the array’s contents, and returns a
Bool
value to say whether the first value should appear before or after the second value once the
values are sorted. The sorting closure needs to return
true
if the first value should appear
before
the second value, and
false
otherwise.

This example is sorting an array of
String
values, and so the sorting closure needs to be a function of type
(String, String) -> Bool
.
One way to provide the sorting closure is to write a normal function of the correct type, and to pass it in as the
sort
function’s second parameter:
1
func

backwards
(
s1
:
String
,
s2
:
String
) ->
Bool
{
2

return

s1
>
s2
3
}
4
var

reversed
=
sort
(
names
,
backwards
)
5
// reversed is equal to ["Ewa", "Daniella", "Chris", "Barry", "Alex"]
If the first string (
s1
) is greater than the second string (
s2
), the
backwards
function will return
true
,
indicating that
s1
should appear before
s2
in the sorted array. For characters in strings, “greater than” means
“appears later in the alphabet than”. This means that the letter
"B"
is “greater than” the letter
"A"
, and the
string
"Tom"
is greater than the string
"Tim"
. This gives a reverse alphabetical sort, with
"Barry"
being
placed before
"Alex"
, and so on.
However, this is a rather long-winded way to write what is essentially a single-expression function (
a > b
). In
this example, it would be preferable to write the sorting closure inline, using closure expression syntax.
​
Closure Expression Syntax
Closure expression syntax has the following general form:
{ (
parameters
) ->
return type

in

statements
}
Closure expression syntax can use constant parameters, variable parameters, and
inout
parameters. Default

values cannot be provided. Variadic parameters can be used if you name the variadic parameter and place it
last in the parameter list. Tuples can also be used as parameter types and return types.
The example below shows a closure expression version of the
backwards
function from earlier:
1
reversed
=
sort
(
names
, { (
s1
:
String
,
s2
:
String
) ->
Bool

in
2

return

s1
>
s2
3
})
Note that the declaration of parameters and return type for this inline closure is identical to the declaration from
the
backwards
function. In both cases, it is written as
(s1: String, s2: String) -> Bool
.
However, for the inline closure expression, the parameters and return type are written
inside
the curly braces,
not outside of them.
The start of the closure’s body is introduced by the
in
keyword. This keyword indicates that the definition of the
closure’s parameters and return type has finished, and the body of the closure is about to begin.
Because the body of the closure is so short, it can even be written on a single line:
1
reversed
=
sort
(
names
, { (
s1
:
String
,
s2
:
String
) ->
Bool

in

return

s1
>
s2
} )
This illustrates that the overall call to the
sort
function has remained the same. A pair of parentheses still
wrap the entire set of arguments for the function. However, one of those arguments is now an inline closure.
​
Inferring Type From Context
Because the sorting closure is passed as an argument to a function, Swift can infer the types of its parameters
and the type of the value it returns from the type of the
sort
function’s second parameter. This parameter is
expecting a function of type
(String, String) -> Bool
. This means that the
String
,
String
, and
Bool
types do not need to be written as part of the closure expression’s definition. Because all of the types can
be inferred, the return arrow (
->
) and the parentheses around the names of the parameters can also be omitted:

1
reversed
=
sort
(
names
, {
s1
,
s2

in

return

s1
>
s2
} )
It is always possible to infer parameter types and return type when passing a closure to a function as an inline
closure expression. As a result, you rarely need to write an inline closure in its fullest form.
Nonetheless, you can make the types explicit if you wish, and doing so is encouraged if it avoids ambiguity for
readers of your code. In the case of the
sort
function, the purpose of the closure is clear from the fact that
sorting is taking place, and it is safe for a reader to assume that the closure is likely to be working with
String
values, because it is assisting with the sorting of an array of strings.
​
Implicit Returns from Single-Expression Closures
Single-expression closures can implicitly return the result of their single expression by omitting the
return
keyword from their declaration, as in this version of the previous example:
1
reversed
=
sort
(
names
, {
s1
,
s2

in

s1
>
s2
} )
Here, the function type of the
sort
function’s second argument makes it clear that a
Bool
value must be
returned by the closure. Because the closure’s body contains a single expression (
s1 > s2
) that returns a
Bool
value, there is no ambiguity, and the
return
keyword can be omitted.
​
Shorthand Argument Names
Swift automatically provides shorthand argument names to inline closures, which can be used to refer to the
values of the closure’s arguments by the names
$0
,
$1
,
$2
, and so on.
If you use these shorthand argument names within your closure expression, you can omit the closure’s
argument list from its definition, and the number and type of the shorthand argument names will be inferred
from the expected function type. The
in
keyword can also be omitted, because the closure expression is made
up entirely of its body:
1
reversed
=
sort
(
names
, {
$0
>
$1
} )

Here,
$0
and
$1
refer to the closure’s first and second
String
arguments.
​
Operator Functions
There’s actually an even
shorter
way to write the closure expression above. Swift’s
String
type defines its
string-specific implementation of the greater-than operator (
>
) as a function that has two parameters of type
String
, and returns a value of type
Bool
. This exactly matches the function type needed for the
sort
function’s second parameter. Therefore, you can simply pass in the greater-than operator, and Swift will infer
that you want to use its string-specific implementation:
1
reversed
=
sort
(
names
, >)
For more about operator functions, see
Operator Functions
.
​
Trailing Closures
If you need to pass a closure expression to a function as the function’s final argument and the closure
expression is long, it can be useful to write it as a
trailing closure
instead. A trailing closure is a closure
expression that is written outside of (and
after
) the parentheses of the function call it supports:
1
func

someFunctionThatTakesAClosure
(
closure
: () -> ()) {
2

// function body goes here
3
}
4

5
// here's how you call this function without using a trailing closure:
6

7
someFunctionThatTakesAClosure
({
8

// closure's body goes here
9
})
10

11
// here's how you call this function with a trailing closure

instead:
12

13
someFunctionThatTakesAClosure
() {
14

// trailing closure's body goes here
15
}
NOTE
If a closure expression is provided as the function’s only argument and you provide that expression
as a trailing closure, you do not need to write a pair of parentheses
()
after the function’s name
when you call the function.
The string-sorting closure from the
Closure Expression Syntax
section above can be written outside of the
sort
function’s parentheses as a trailing closure:
1
reversed
=
sort
(
names
) {
$0
>
$1
}
Trailing closures are most useful when the closure is sufficiently long that it is not possible to write it inline on a
single line. As an example, Swift’s
Array
type has a
map
method which takes a closure expression as its
single argument. The closure is called once for each item in the array, and returns an alternative mapped value
(possibly of some other type) for that item. The nature of the mapping and the type of the returned value is left up
to the closure to specify.
After applying the provided closure to each array element, the
map
method returns a new array containing all of
the new mapped values, in the same order as their corresponding values in the original array.
Here’s how you can use the
map
method with a trailing closure to convert an array of
Int
values into an array
of
String
values. The array
[16, 58, 510]
is used to create the new array
["OneSix",
"FiveEight", "FiveOneZero"]
:
1
let

digitNames
= [

2

0
:
"Zero"
,
1
:
"One"
,
2
:
"Two"
,
3
:
"Three"
,
4
:
"Four"
,
3

5
:
"Five"
,
6
:
"Six"
,
7
:
"Seven"
,
8
:
"Eight"
,
9
:
"Nine"
4
]
5
let

numbers
= [
16
,
58
,
510
]
The code above creates a dictionary of mappings between the integer digits and English-language versions of
their names. It also defines an array of integers, ready to be converted into strings.
You can now use the
numbers
array to create an array of
String
values, by passing a closure expression to
the array’s
map
method as a trailing closure. Note that the call to
numbers.map
does not need to include any
parentheses after
map
, because the
map
method has only one parameter, and that parameter is provided as a
trailing closure:
1
let

strings
=
numbers
.
map
{
2
(
var

number
) ->
String

in
3

var

output
=
""
4

while

number
>
0
{
5

output
=
digitNames
[
number
%
10
]! +
output
6

number
/=
10
7
}
8

return

output
9
}
10
// strings is inferred to be of type String[]
11
// its value is ["OneSix", "FiveEight", "FiveOneZero"]
The
map
function calls the closure expression once for each item in the array. You do not need to specify the
type of the closure’s input parameter,
number
, because the type can be inferred from the values in the array to
be mapped.
In this example, the closure’s
number
parameter is defined as a
variable parameter
, as described in
Constant
and Variable Parameters
, so that the parameter’s value can be modified within the closure body, rather than
declaring a new local variable and assigning the passed
number
value to it. The closure expression also
specifies a return type of
String
, to indicate the type that will be stored in the mapped output array.
The closure expression builds a string called
output
each time it is called. It calculates the last digit of

number
by using the remainder operator (
number % 10
), and uses this digit to look up an appropriate string
in the
digitNames
dictionary.
NOTE
The call to the
digitNames
dictionary’s subscript is followed by an exclamation mark (
!
),
because dictionary subscripts return an optional value to indicate that the dictionary lookup can fail if
the key does not exist. In the example above, it is guaranteed that
number % 10
will always be a
valid subscript key for the
digitNames
dictionary, and so an exclamation mark is used to force-
unwrap the
String
value stored in the subscript’s optional return value.
The string retrieved from the
digitNames
dictionary is added to the
front
of
output
, effectively building a
string version of the number in reverse. (The expression
number % 10
gives a value of
6
for
16
,
8
for
58
,
and
0
for
510
.)
The
number
variable is then divided by
10
. Because it is an integer, it is rounded down during the division, so
16
becomes
1
,
58
becomes
5
, and
510
becomes
51
.
The process is repeated until
number /= 10
is equal to
0
, at which point the
output
string is returned by
the closure, and is added to the output array by the
map
function.
The use of trailing closure syntax in the example above neatly encapsulates the closure’s functionality
immediately after the function that closure supports, without needing to wrap the entire closure within the
map
function’s outer parentheses.
​
Capturing Values
A closure can
capture
constants and variables from the surrounding context in which it is defined. The closure
can then refer to and modify the values of those constants and variables from within its body, even if the original
scope that defined the constants and variables no longer exists.
The simplest form of a closure in Swift is a nested function, written within the body of another function. A nested

function can capture any of its outer function’s arguments and can also capture any constants and variables
defined within the outer function.
Here’s an example of a function called
makeIncrementor
, which contains a nested function called
incrementor
. The nested
incrementor
function captures two values,
runningTotal
and
amount
,
from its surrounding context. After capturing these values,
incrementor
is returned by
makeIncrementor
as a closure that increments
runningTotal
by
amount
each time it is called.
1
func

makeIncrementor
(
forIncrement

amount
:
Int
) -> () ->
Int
{
2

var

runningTotal
=
0
3

func

incrementor
() ->
Int
{
4

runningTotal
+=
amount
5

return

runningTotal
6
}
7

return

incrementor
8
}
The return type of
makeIncrementor
is
() -> Int
. This means that it returns a
function
, rather than a
simple value. The function it returns has no parameters, and returns an
Int
value each time it is called. To
learn how functions can return other functions, see
Function Types as Return Types
.
The
makeIncrementor
function defines an integer variable called
runningTotal
, to store the current
running total of the incrementor that will be returned. This variable is initialized with a value of
0
.
The
makeIncrementor
function has a single
Int
parameter with an external name of
forIncrement
,
and a local name of
amount
. The argument value passed to this parameter specifies how much
runningTotal
should be incremented by each time the returned incrementor function is called.
makeIncrementor
defines a nested function called
incrementor
, which performs the actual
incrementing. This function simply adds
amount
to
runningTotal
, and returns the result.
When considered in isolation, the nested
incrementor
function might seem unusual:
1
func

incrementor
() ->
Int
{
2

runningTotal
+=
amount

3

return

runningTotal
4
}
The
incrementor
function doesn’t have any parameters, and yet it refers to
runningTotal
and
amount
from within its function body. It does this by capturing the
existing
values of
runningTotal
and
amount
from its surrounding function and using them within its own function body.
Because it does not modify
amount
,
incrementor
actually captures and stores a
copy
of the value stored in
amount
. This value is stored along with the new
incrementor
function.
However, because it modifies the
runningTotal
variable each time it is called,
incrementor
captures a
reference
to the current
runningTotal
variable, and not just a copy of its initial value. Capturing a reference
ensures sure that
runningTotal
does not disappear when the call to
makeIncrementor
ends, and
ensures that
runningTotal
will continue to be available the next time that the incrementor function is called.
NOTE
Swift determines what should be captured by reference and what should be copied by value. You
don’t need to annotate
amount
or
runningTotal
to say that they can be used within the nested
incrementor
function. Swift also handles all memory management involved in disposing of
runningTotal
when it is no longer needed by the incrementor function.
Here’s an example of
makeIncrementor
in action:
1
let

incrementByTen
=
makeIncrementor
(
forIncrement
:
10
)
This example sets a constant called
incrementByTen
to refer to an incrementor function that adds
10
to its
runningTotal
variable each time it is called. Calling the function multiple times shows this behavior in
action:
1
incrementByTen
()

2
// returns a value of 10
3
incrementByTen
()
4
// returns a value of 20
5
incrementByTen
()
6
// returns a value of 30
If you create another incrementor, it will have its own stored reference to a new, separate
runningTotal
variable. In the example below,
incrementBySeven
captures a reference to a new
runningTotal
variable, and this variable is unconnected to the one captured by
incrementByTen
:
1
let

incrementBySeven
=
makeIncrementor
(
forIncrement
:
7
)
2
incrementBySeven
()
3
// returns a value of 7
4
incrementByTen
()
5
// returns a value of 40
NOTE
If you assign a closure to a property of a class instance, and the closure captures that instance by
referring to the instance or its members, you will create a strong reference cycle between the
closure and the instance. Swift uses
capture lists
to break these strong reference cycles. For more
information, see
Strong Reference Cycles for Closures
.
​
Closures Are Reference Types
In the example above,
incrementBySeven
and
incrementByTen
are constants, but the closures these
constants refer to are still able to increment the
runningTotal
variables that they have captured. This is
because functions and closures are
reference types
.
Whenever you assign a function or a closure to a constant or a variable, you are actually setting that constant or
variable to be a
reference
to the function or closure. In the example above, it is the choice of closure that

incrementByTen

refers to
that is constant, and not the contents of the closure itself.
This also means that if you assign a closure to two different constants or variables, both of those constants or
variables will refer to the same closure:
1
let

alsoIncrementByTen
=
incrementByTen
2
alsoIncrementByTen
()
3
// returns a value of 50

​
​
Enumerations
An
enumeration
defines a common type for a group of related values and enables you to work with those values
in a type-safe way within your code.
If you are familiar with C, you will know that C enumerations assign related names to a set of integer values.
Enumerations in Swift are much more flexible, and do not have to provide a value for each member of the
enumeration. If a value (known as a “raw” value)
is
provided for each enumeration member, the value can be a
string, a character, or a value of any integer or floating-point type.
Alternatively, enumeration members can specify associated values of
any
type to be stored along with each
different member value, much as unions or variants do in other languages. You can define a common set of
related members as part of one enumeration, each of which has a different set of values of appropriate types
associated with it.
Enumerations in Swift are first-class types in their own right. They adopt many features traditionally supported
only by classes, such as computed properties to provide additional information about the enumeration’s current
value, and instance methods to provide functionality related to the values the enumeration represents.
Enumerations can also define initializers to provide an initial member value; can be extended to expand their
functionality beyond their original implementation; and can conform to protocols to provide standard
functionality.
For more on these capabilities, see
Properties
,
Methods
,
Initialization
,
Extensions
, and
Protocols
.
​
Enumeration Syntax
You introduce enumerations with the
enum
keyword and place their entire definition within a pair of braces:
1
enum

SomeEnumeration
{
2

// enumeration definition goes here
3
}

Here’s an example for the four main points of a compass:
1
enum

CompassPoint
{
2

case

North
3

case

South
4

case

East
5

case

West
6
}
The values defined in an enumeration (such as
North
,
South
,
East
, and
West
) are the
member values
(or
members
) of that enumeration. The
case
keyword indicates that a new line of member values is about to be
defined.
NOTE
Unlike C and Objective-C, Swift enumeration members are not assigned a default integer value
when they are created. In the
CompassPoints
example above,
North
,
South
,
East
and
West
do not implicitly equal
0
,
1
,
2
and
3
. Instead, the different enumeration members are fully-fledged
values in their own right, with an explicitly-defined type of
CompassPoint
.
Multiple member values can appear on a single line, separated by commas:
1
enum

Planet
{
2

case

Mercury
,
Venus
,
Earth
,
Mars
,
Jupiter
,
Saturn
,
Uranus
,
Neptune
3
}
Each enumeration definition defines a brand new type. Like other types in Swift, their names (such as
CompassPoint
and
Planet
) should start with a capital letter. Give enumeration types singular rather than
plural names, so that they read as self-evident:
1
var

directionToHead
=
CompassPoint
.
West

The type of
directionToHead
is inferred when it is initialized with one of the possible values of
CompassPoint
. Once
directionToHead
is declared as a
CompassPoint
, you can set it to a different
CompassPoint
value using a shorter dot syntax:
1
directionToHead
= .
East
The type of
directionToHead
is already known, and so you can drop the type when setting its value. This
makes for highly readable code when working with explicitly-typed enumeration values.
​
Matching Enumeration Values with a Switch Statement
You can match individual enumeration values with a
switch
statement:
1
directionToHead
= .
South
2
switch

directionToHead
{
3
case
.
North
:
4

println
(
"Lots of planets have a north"
)
5
case
.
South
:
6

println
(
"Watch out for penguins"
)
7
case
.
East
:
8

println
(
"Where the sun rises"
)
9
case
.
West
:
10

println
(
"Where the skies are blue"
)
11
}
12
// prints "Watch out for penguins"
You can read this code as:
“Consider the value of
directionToHead
. In the case where it equals
.North
, print
"Lots of
planets have a north"
. In the case where it equals
.South
, print
"Watch out for
penguins"
.”
…and so on.

As described in
Control Flow
, a
switch
statement must be exhaustive when considering an enumeration’s
members. If the
case
for
.West
is omitted, this code does not compile, because it does not consider the
complete list of
CompassPoint
members. Requiring exhaustiveness ensures that enumeration members
are not accidentally omitted.
When it is not appropriate to provide a
case
for every enumeration member, you can provide a
default
case to cover any members that are not addressed explicitly:
1
let

somePlanet
=
Planet
.
Earth
2
switch

somePlanet
{
3
case
.
Earth
:
4

println
(
"Mostly harmless"
)
5
default
:
6

println
(
"Not a safe place for humans"
)
7
}
8
// prints "Mostly harmless"
​
Associated Values
The examples in the previous section show how the members of an enumeration are a defined (and typed)
value in their own right. You can set a constant or variable to
Planet.Earth
, and check for this value later.
However, it is sometimes useful to be able to store
associated values
of other types alongside these member
values. This enables you to store additional custom information along with the member value, and permits this
information to vary each time you use that member in your code.
You can define Swift enumerations to store associated values of any given type, and the value types can be
different for each member of the enumeration if needed. Enumerations similar to these are known as
discriminated unions
,
tagged unions
, or
variants
in other programming languages.
For example, suppose an inventory tracking system needs to track products by two different types of barcode.
Some products are labeled with 1D barcodes in UPC-A format, which uses the numbers
0
to
9
. Each barcode
has a “number system” digit, followed by ten “identifier” digits. These are followed by a “check” digit to verify
that the code has been scanned correctly:

Other products are labeled with 2D barcodes in QR code format, which can use any ISO 8859-1 character and
can encode a string up to 2,953 characters long:
It would be convenient for an inventory tracking system to be able to store UPC-A barcodes as a tuple of three
integers, and QR code barcodes as a string of any length.
In Swift, an enumeration to define product barcodes of either type might look like this:
1
enum

Barcode
{
2

case

UPCA
(
Int
,
Int
,
Int
)
3

case

QRCode
(
String
)

4
}
This can be read as:
“Define an enumeration type called
Barcode
, which can take either a value of
UPCA
with an associated value
of type (
Int
,
Int
,
Int
), or a value of
QRCode
with an associated value of type
String
.”
This definition does not provide any actual
Int
or
String
values—it just defines the
type
of associated values
that
Barcode
constants and variables can store when they are equal to
Barcode.UPCA
or
Barcode.QRCode
.
New barcodes can then be created using either type:
1
var

productBarcode
=
Barcode
.
UPCA
(
8
,
85909_51226
,
3
)
This example creates a new variable called
productBarcode
and assigns it a value of
Barcode.UPCA
with an associated tuple value of
(8, 8590951226, 3)
. The provided “identifier” value has an underscore
within its integer literal—
85909_51226
—to make it easier to read as a barcode.
The same product can be assigned a different type of barcode:
1
productBarcode
= .
QRCode
(
"ABCDEFGHIJKLMNOP"
)
At this point, the original
Barcode.UPCA
and its integer values are replaced by the new
Barcode.QRCode
and its string value. Constants and variables of type
Barcode
can store either a
.UPCA
or a
.QRCode
(together with their associated values), but they can only store one of them at any given time.
The different barcode types can be checked using a switch statement, as before. This time, however, the
associated values can be extracted as part of the switch statement. You extract each associated value as a
constant (with the
let
prefix) or a variable (with the
var
prefix) for use within the
switch
case’s body:
1
switch

productBarcode
{
2
case
.
UPCA
(
let

numberSystem
,
let

identifier
,
let

check
):
3

println
(
"UPC-A with value of
\(
numberSystem
)
,
\(
identifier
)
,
\

(
check
)
."
)
4
case
.
QRCode
(
let

productCode
):
5

println
(
"QR code with value of
\(
productCode
)
."
)
6
}
7
// prints "QR code with value of ABCDEFGHIJKLMNOP."
If all of the associated values for a enumeration member are extracted as constants, or if all are extracted as
variables, you can place a single
var
or
let
annotation before the member name, for brevity:
1
switch

productBarcode
{
2
case

let
.
UPCA
(
numberSystem
,
identifier
,
check
):
3

println
(
"UPC-A with value of
\(
numberSystem
)
,
\(
identifier
)
,
\
(
check
)
."
)
4
case

let
.
QRCode
(
productCode
):
5

println
(
"QR code with value of
\(
productCode
)
."
)
6
}
7
// prints "QR code with value of ABCDEFGHIJKLMNOP."
​
Raw Values
The barcode example in
Associated Values
shows how members of an enumeration can declare that they store
associated values of different types. As an alternative to associated values, enumeration members can come
prepopulated with default values (called
raw values
), which are all of the same type.
Here’s an example that stores raw ASCII values alongside named enumeration members:
1
enum

ASCIIControlCharacter
:
Character
{
2

case

Tab
=
"\t"
3

case

LineFeed
=
"\n"
4

case

CarriageReturn
=
"\r"
5
}
Here, the raw values for an enumeration called
ASCIIControlCharacter
are defined to be of type

Character
, and are set to some of the more common ASCII control characters.
Character
values are
described in
Strings and Characters
.
Note that raw values are
not
the same as associated values. Raw values are set to prepopulated values when
you first define the enumeration in your code, like the three ASCII codes above. The raw value for a particular
enumeration member is always the same. Associated values are set when you create a new constant or
variable based on one of the enumeration’s members, and can be different each time you do so.
Raw values can be strings, characters, or any of the integer or floating-point number types. Each raw value
must be unique within its enumeration declaration. When integers are used for raw values, they auto-increment
if no value is specified for some of the enumeration members.
The enumeration below is a refinement of the earlier
Planet
enumeration, with raw integer values to
represent each planet’s order from the sun:
1
enum

Planet
:
Int
{
2

case

Mercury
=
1
,
Venus
,
Earth
,
Mars
,
Jupiter
,
Saturn
,
Uranus
,

Neptune
3
}
Auto-incrementation means that
Planet.Venus
has a raw value of
2
, and so on.
Access the raw value of an enumeration member with its
toRaw
method:
1
let

earthsOrder
=
Planet
.
Earth
.
toRaw
()
2
// earthsOrder is 3
Use an enumeration’s
fromRaw
method to try to find an enumeration member with a particular raw value.
This example identifies Uranus from its raw value of
7
:
1
let

possiblePlanet
=
Planet
.
fromRaw
(
7
)
2
// possiblePlanet is of type Planet? and equals Planet.Uranus
Not all possible
Int
values will find a matching planet, however. Because of this, the
fromRaw
method

returns an
optional
enumeration member. In the example above,
possiblePlanet
is of type
Planet?
, or
“optional
Planet
.”
If you try to find a Planet with a position of
9
, the optional
Planet
value returned by
fromRaw
will be
nil
:
1
let

positionToFind
=
9
2
if

let

somePlanet
=
Planet
.
fromRaw
(
positionToFind
) {
3

switch

somePlanet
{
4

case
.
Earth
:
5

println
(
"Mostly harmless"
)
6

default
:
7

println
(
"Not a safe place for humans"
)
8
}
9
}
else
{
10

println
(
"There isn't a planet at position
\
(
positionToFind
)
"
)
11
}
12
// prints "There isn't a planet at position 9"
This example uses optional binding to try to access a planet with a raw value of
9
. The statement
if let
somePlanet = Planet.fromRaw(9)
retrieves an optional
Planet
, and sets
somePlanet
to the
contents of that optional
Planet
if it can be retrieved. In this case, it is not possible to retrieve a planet with a
position of
9
, and so the
else
branch is executed instead.

​
​
Classes and Structures
Classes
and
structures
are general-purpose, flexible constructs that become the building blocks of your
program’s code. You define properties and methods to add functionality to your classes and structures by using
exactly the same syntax as for constants, variables, and functions.
Unlike other programming languages, Swift does not require you to create separate interface and
implementation files for custom classes and structures. In Swift, you define a class or a structure in a single
file, and the external interface to that class or structure is automatically made available for other code to use.
NOTE
An instance of a
class
is traditionally known as an
object
. However, Swift classes and structures are
much closer in functionality than in other languages, and much of this chapter describes functionality
that can apply to instances of
either
a class or a structure type. Because of this, the more general
term
instance
is used.
​
Comparing Classes and Structures
Classes and structures in Swift have many things in common. Both can:
Define properties to store values
Define methods to provide functionality
Define subscripts to provide access to their values using subscript syntax
Define initializers to set up their initial state
Be extended to expand their functionality beyond a default implementation
Conform to protocols to provide standard functionality of a certain kind

For more information, see
Properties
,
Methods
,
Subscripts
,
Initialization
,
Extensions
, and
Protocols
.
Classes have additional capabilities that structures do not:
For more information, see
Inheritance
,
Type Casting
,
Initialization
, and
Automatic Reference Counting
.
NOTE
Structures are always copied when they are passed around in your code, and do not use reference
counting.
​
Definition Syntax
Classes and structures have a similar definition syntax. You introduce classes with the
class
keyword and
structures with the
struct
keyword. Both place their entire definition within a pair of braces:
1
class

SomeClass
{
2

// class definition goes here
3
}
4
struct

SomeStructure
{
5

// structure definition goes here
6
}
Inheritance enables one class to inherit the characteristics of another.
Type casting enables you to check and interpret the type of a class instance at runtime.
Deinitializers enable an instance of a class to free up any resources it has assigned.
Reference counting allows more than one reference to a class instance.

NOTE
Whenever you define a new class or structure, you effectively define a brand new Swift type. Give
types
UpperCamelCase
names (such as
SomeClass
and
SomeStructure
here) to match
the capitalization of standard Swift types (such as
String
,
Int
, and
Bool
). Conversely, always
give properties and methods
lowerCamelCase
names (such as
frameRate
and
incrementCount
) to differentiate them from type names.
Here’s an example of a structure definition and a class definition:
1
struct

Resolution
{
2

var

width
=
0
3

var

height
=
0
4
}
5
class

VideoMode
{
6

var

resolution
=
Resolution
()
7

var

interlaced
=
false
8

var

frameRate
=
0.0
9

var

name
:
String
?
10
}
The example above defines a new structure called
Resolution
, to describe a pixel-based display resolution.
This structure has two stored properties called
width
and
height
. Stored properties are constants or
variables that are bundled up and stored as part of the class or structure. These two properties are inferred to be
of type
Int
by setting them to an initial integer value of
0
.
The example above also defines a new class called
VideoMode
, to describe a specific video mode for video
display. This class has four variable stored properties. The first,
resolution
, is initialized with a new
Resolution
structure instance, which infers a property type of
Resolution
. For the other three properties,
new
VideoMode
instances will be initialized with an
interlaced
setting of
false
(meaning “non-
interlaced video”), a playback frame rate of
0.0
, and an optional
String
value called
name
. The
name
property is automatically given a default value of
nil
, or “no
name
value”, because it is of an optional type.
​

Class and Structure Instances
The
Resolution
structure definition and the
VideoMode
class definition only describe what a
Resolution
or
VideoMode
will look like. They themselves do not describe a specific resolution or video
mode. To do that, you need to create an instance of the structure or class.
The syntax for creating instances is very similar for both structures and classes:
1
let

someResolution
=
Resolution
()
2
let

someVideoMode
=
VideoMode
()
Structures and classes both use initializer syntax for new instances. The simplest form of initializer syntax
uses the type name of the class or structure followed by empty parentheses, such as
Resolution()
or
VideoMode()
. This creates a new instance of the class or structure, with any properties initialized to their
default values. Class and structure initialization is described in more detail in
Initialization
.
​
Accessing Properties
You can access the properties of an instance using
dot syntax
. In dot syntax, you write the property name
immediately after the instance name, separated by a period (
.
), without any spaces:
1
println
(
"The width of someResolution is
\(
someResolution
.
width
)
"
)
2
// prints "The width of someResolution is 0"
In this example,
someResolution.width
refers to the
width
property of
someResolution
, and
returns its default initial value of
0
.
You can drill down into sub-properties, such as the
width
property in the
resolution
property of a
VideoMode
:
1
println
(
"The width of someVideoMode is
\
(
someVideoMode
.
resolution
.
width
)
"
)

2
// prints "The width of someVideoMode is 0"
You can also use dot syntax to assign a new value to a variable property:
1
someVideoMode
.
resolution
.
width
=
1280
2
println
(
"The width of someVideoMode is now
\
(
someVideoMode
.
resolution
.
width
)
"
)
3
// prints "The width of someVideoMode is now 1280"
NOTE
Unlike Objective-C, Swift enables you to set sub-properties of a structure property directly. In the
last example above, the
width
property of the
resolution
property of
someVideoMode
is set
directly, without your needing to set the entire
resolution
property to a new value.
​
Memberwise Initializers for Structure Types
All structures have an automatically-generated
memberwise initializer
, which you can use to initialize the
member properties of new structure instances. Initial values for the properties of the new instance can be
passed to the memberwise initializer by name:
1
let

vga
=
Resolution
(
width
:
640
,
height
:
480
)
Unlike structures, class instances do not receive a default memberwise initializer. Initializers are described in
more detail in
Initialization
.
​
Structures and Enumerations Are Value Types

A
value type
is a type that is
copied
when it is assigned to a variable or constant, or when it is passed to a
function.
You’ve actually been using value types extensively throughout the previous chapters. In fact, all of the basic
types in Swift—integers, floating-point numbers, Booleans, strings, arrays and dictionaries—are value types,
and are implemented as structures behind the scenes.
All structures and enumerations are value types in Swift. This means that any structure and enumeration
instances you create—and any value types they have as properties—are always copied when they are passed
around in your code.
Consider this example, which uses the
Resolution
structure from the previous example:
1
let

hd
=
Resolution
(
width
:
1920
,
height
:
1080
)
2
var

cinema
=
hd
This example declares a constant called
hd
and sets it to a
Resolution
instance initialized with the width
and height of full HD video (
1920
pixels wide by
1080
pixels high).
It then declares a variable called
cinema
and sets it to the current value of
hd
. Because
Resolution
is a
structure, a
copy
of the existing instance is made, and this new copy is assigned to
cinema
. Even though
hd
and
cinema
now have the same width and height, they are two completely different instances behind the
scenes.
Next, the
width
property of
cinema
is amended to be the width of the slightly-wider 2K standard used for
digital cinema projection (
2048
pixels wide and
1080
pixels high):
1
cinema
.
width
=
2048
Checking the
width
property of
cinema
shows that it has indeed changed to be
2048
:
1
println
(
"cinema is now
\(
cinema
.
width
)
pixels wide"
)
2
// prints "cinema is now 2048 pixels wide"

However, the
width
property of the original
hd
instance still has the old value of
1920
:
1
println
(
"hd is still
\(
hd
.
width
)
pixels wide"
)
2
// prints "hd is still 1920 pixels wide"
When
cinema
was given the current value of
hd
, the
values
stored in
hd
were copied into the new
cinema
instance. The end result is two completely separate instances, which just happened to contain the same
numeric values. Because they are separate instances, setting the width of
cinema
to
2048
doesn’t affect the
width stored in
hd
.
The same behavior applies to enumerations:
1
enum

CompassPoint
{
2

case

North
,
South
,
East
,
West
3
}
4
var

currentDirection
=
CompassPoint
.
West
5
let

rememberedDirection
=
currentDirection
6
currentDirection
= .
East
7
if

rememberedDirection
== .
West
{
8

println
(
"The remembered direction is still .West"
)
9
}
10
// prints "The remembered direction is still .West"
When
rememberedDirection
is assigned the value of
currentDirection
, it is actually set to a copy
of that value. Changing the value of
currentDirection
thereafter does not affect the copy of the original
value that was stored in
rememberedDirection
.
​
Classes Are Reference Types
Unlike value types,
reference types
are
not
copied when they are assigned to a variable or constant, or when
they are passed to a function. Rather than a copy, a reference to the same existing instance is used instead.
Here’s an example, using the
VideoMode
class defined above:

1
let

tenEighty
=
VideoMode
()
2
tenEighty
.
resolution
=
hd
3
tenEighty
.
interlaced
=
true
4
tenEighty
.
name
=
"1080i"
5
tenEighty
.
frameRate
=
25.0
This example declares a new constant called
tenEighty
and sets it to refer to a new instance of the
VideoMode
class. The video mode is assigned a copy of the HD resolution of
1920
by
1080
from before. It
is set to be interlaced, and is given a name of
"1080i"
. Finally, it is set to a frame rate of
25.0
frames per
second.
Next,
tenEighty
is assigned to a new constant, called
alsoTenEighty
, and the frame rate of
alsoTenEighty
is modified:
1
let

alsoTenEighty
=
tenEighty
2
alsoTenEighty
.
frameRate
=
30.0
Because classes are reference types,
tenEighty
and
alsoTenEighty
actually both refer to the
same
VideoMode
instance. Effectively, they are just two different names for the same single instance.
Checking the
frameRate
property of
tenEighty
shows that it correctly reports the new frame rate of
30.0
from the underlying
VideoMode
instance:
1
println
(
"The frameRate property of tenEighty is now
\
(
tenEighty
.
frameRate
)
"
)
2
// prints "The frameRate property of tenEighty is now 30.0"
Note that
tenEighty
and
alsoTenEighty
are declared as
constants
, rather than variables. However, you
can still change
tenEighty.frameRate
and
alsoTenEighty.frameRate
because the values of the
tenEighty
and
alsoTenEighty
constants themselves do not actually change.
tenEighty
and
alsoTenEighty
themselves do not “store” the
VideoMode
instance—instead, they both
refer
to a
VideoMode
instance behind the scenes. It is the
frameRate
property of the underlying
VideoMode
that is
changed, not the values of the constant references to that
VideoMode
.

​
Identity Operators
Because classes are reference types, it is possible for multiple constants and variables to refer to the same
single instance of a class behind the scenes. (The same is not true for structures and enumerations, because
they are value types and are always copied when they are assigned to a constant or variable, or passed to a
function.)
It can sometimes be useful to find out if two constants or variables refer to exactly the same instance of a class.
To enable this, Swift provides two identity operators:
Use these operators to check whether two constants or variables refer to the same single instance:
1
if

tenEighty
===
alsoTenEighty
{
2

println
(
"tenEighty and alsoTenEighty refer to the same Resolution

instance."
)
3
}
4
// prints "tenEighty and alsoTenEighty refer to the same Resolution

instance."
Note that “identical to” (represented by three equals signs, or
===
) does not mean the same thing as “equal to”
(represented by two equals signs, or
==
):
When you define your own custom classes and structures, it is your responsibility to decide what qualifies as
two instances being “equal”. The process of defining your own implementations of the “equal to” and “not equal
to” operators is described in
Equivalence Operators
.
Identical to (
===
)
Not identical to (
!==
)
“Identical to” means that two constants or variables of class type refer to exactly the same class
instance.
“Equal to” means that two instances are considered “equal” or “equivalent” in value, for some
appropriate meaning of “equal”, as defined by the type’s designer.

​
Pointers
If you have experience with C, C++, or Objective-C, you may know that these languages use
pointers
to refer to
addresses in memory. A Swift constant or variable that refers to an instance of some reference type is similar
to a pointer in C, but is not a direct pointer to an address in memory, and does not require you to write an
asterisk (
*
) to indicate that you are creating a reference. Instead, these references are defined like any other
constant or variable in Swift.
​
Choosing Between Classes and Structures
You can use both classes and structures to define custom data types to use as the building blocks of your
program’s code.
However, structure instances are always passed by
value
, and class instances are always passed by
reference
. This means that they are suited to different kinds of tasks. As you consider the data constructs and
functionality that you need for a project, decide whether each data construct should be defined as a class or as a
structure.
As a general guideline, consider creating a structure when one or more of these conditions apply:
Examples of good candidates for structures include:
The structure’s primary purpose is to encapsulate a few relatively simple data values.
It is reasonable to expect that the encapsulated values will be copied rather than referenced when
you assign or pass around an instance of that structure.
Any properties stored by the structure are themselves value types, which would also be
expected to be copied rather than referenced.
The structure does not need to inherit properties or behavior from another existing type.
The size of a geometric shape, perhaps encapsulating a
width
property and a
height
property, both of type
Double
.
A way to refer to ranges within a series, perhaps encapsulating a
start
property and a

In all other cases, define a class, and create instances of that class to be managed and passed by reference. In
practice, this means that most custom data constructs should be classes, not structures.
​
Assignment and Copy Behavior for Collection Types
Swift’s
Array
and
Dictionary
types are implemented as structures. However, arrays have slightly
different copying behavior from dictionaries and other structures when they are assigned to a constant or
variable, and when they are passed to a function or method.
The behavior described for
Array
and
Dictionary
below is different again from the behavior of
NSArray
and
NSDictionary
in Foundation, which are implemented as classes, not structures.
NSArray
and
NSDictionary
instances are always assigned and passed around as a reference to an existing instance,
rather than as a copy.
NOTE
The descriptions below refer to the “copying” of arrays, dictionaries, strings, and other values.
Where copying is mentioned, the behavior you see in your code will always be as if a copy took
place. However, Swift only performs an
actual
copy behind the scenes when it is absolutely
necessary to do so. Swift manages all value copying to ensure optimal performance, and you should
not avoid assignment to try to preempt this optimization.
​
Assignment and Copy Behavior for Dictionaries
Whenever you assign a
Dictionary
instance to a constant or variable, or pass a
Dictionary
instance as
an argument to a function or method call, the dictionary is
copied
at the point that the assignment or call takes
length
property, both of type
Int
.
A point in a 3D coordinate system, perhaps encapsulating
x
,
y
and
z
properties, each of type
Double
.

place. This process is described in
Structures and Enumerations Are Value Types
.
If the keys and/or values stored in the
Dictionary
instance are value types (structures or enumerations),
they too are copied when the assignment or call takes place. Conversely, if the keys and/or values are
reference types (classes or functions), the references are copied, but not the class instances or functions that
they refer to. This copy behavior for a dictionary’s keys and values is the same as the copy behavior for a
structure’s stored properties when the structure is copied.
The example below defines a dictionary called
ages
, which stores the names and ages of four people. The
ages
dictionary is then assigned to a new variable called
copiedAges
and is copied when this assignment
takes place. After the assignment,
ages
and
copiedAges
are two separate dictionaries.
1
var

ages
= [
"Peter"
:
23
,
"Wei"
:
35
,
"Anish"
:
65
,
"Katya"
:
19
]
2
var

copiedAges
=
ages
The keys for this dictionary are of type
String
, and the values are of type
Int
. Both types are value types in
Swift, and so the keys and values are also copied when the dictionary copy takes place.
You can prove that the
ages
dictionary has been copied by changing an age value in one of the dictionaries and
checking the corresponding value in the other. If you set the value for
"Peter"
in the
copiedAges
dictionary
to
24
, the
ages
dictionary still returns the old value of
23
from before the copy took place:
1
copiedAges
[
"Peter"
] =
24
2
println
(
ages
[
"Peter"
])
3
// prints "23"
​
Assignment and Copy Behavior for Arrays
The assignment and copy behavior for Swift’s
Array
type is more complex than for its
Dictionary
type.
Array
provides C-like performance when you work with an array’s contents and copies an array’s contents
only when copying is necessary.
If you assign an
Array
instance to a constant or variable, or pass an
Array
instance as an argument to a

function or method call, the contents of the array are
not
copied at the point that the assignment or call takes
place. Instead, both arrays share the same sequence of element values. When you modify an element value
through one array, the result is observable through the other.
For arrays, copying only takes place when you perform an action that has the potential to modify the
length
of
the array. This includes appending, inserting, or removing items, or using a ranged subscript to replace a range
of items in the array. If and when array copying does take place, the copy behavior for an array’s contents is the
same as for a dictionary’s keys and values, as described in
Assignment and Copy Behavior for Dictionaries
.
The example below assigns a new array of
Int
values to a variable called
a
. This array is also assigned to
two further variables called
b
and
c
:
1
var

a
= [
1
,
2
,
3
]
2
var

b
=
a
3
var

c
=
a
You can retrieve the first value in the array with subscript syntax on either
a
,
b
, or
c
:
1
println
(
a
[
0
])
2
// 1
3
println
(
b
[
0
])
4
// 1
5
println
(
c
[
0
])
6
// 1
If you set an item in the array to a new value with subscript syntax, all three of
a
,
b
, and
c
will return the new
value. Note that the array is not copied when you set a new value with subscript syntax, because setting a single
value with subscript syntax does not have the potential to change the array’s length:
1
a
[
0
] =
42
2
println
(
a
[
0
])
3
// 42
4
println
(
b
[
0
])
5
// 42
6
println
(
c
[
0
])

7
// 42
However, if you append a new item to
a
, you
do
modify the array’s length. This prompts Swift to create a new
copy of the array at the point that you append the new value. Henceforth,
a
is a separate, independent copy of the
array.
If you change a value in
a
after the copy is made,
a
will return a different value from
b
and
c
, which both still
reference the original array contents from before the copy took place:
1
a
.
append
(
4
)
2
a
[
0
] =
777
3
println
(
a
[
0
])
4
// 777
5
println
(
b
[
0
])
6
// 42
7
println
(
c
[
0
])
8
// 42
​
Ensuring That an Array Is Unique
It can be useful to ensure that you have a unique copy of an array before performing an action on that array’s
contents, or before passing that array to a function or method. You ensure the uniqueness of an array reference
by calling the
unshare
method on a variable of array type. (The
unshare
method cannot be called on a
constant array.)
If multiple variables currently refer to the same array, and you call the
unshare
method on one of those
variables, the array is copied, so that the variable has its own independent copy of the array. However, no
copying takes place if the variable is already the only reference to the array.
At the end of the previous example,
b
and
c
both reference the same array. Call the
unshare
method on
b
to
make it become a unique copy:
1
b
.
unshare
()

If you change the first value in
b
after calling the
unshare
method, all three arrays will now report a different
value:
1
b
[
0
] = –
105
2
println
(
a
[
0
])
3
// 777
4
println
(
b
[
0
])
5
// -105
6
println
(
c
[
0
])
7
// 42
​
Checking Whether Two Arrays Share the Same Elements
Check whether two arrays or subarrays share the same storage and elements by comparing them with the
identity operators (
===
and
!==
).
The example below uses the “identical to” operator (
===
) to check whether
b
and
c
still share the same array
elements:
1
if

b
===
c
{
2

println
(
"b and c still share the same array elements."
)
3
}
else
{
4

println
(
"b and c now refer to two independent sets of array

elements."
)
5
}
6
// prints "b and c now refer to two independent sets of array

elements."
Alternatively, use the identity operators to check whether two subarrays share the same elements. The
example below compares two identical subarrays from
b
and confirms that they refer to the same elements:
1
if

b
[
0
…
1
] ===
b
[
0
…
1
] {
2

println
(
"These two subarrays share the same elements."
)

3
}
else
{
4

println
(
"These two subarrays do not share the same elements."
)
5
}
6
// prints "These two subarrays share the same elements."
​
Forcing a Copy of an Array
Force an explicit copy of an array by calling the array’s
copy
method. This method performs a shallow copy of
the array and returns a new array containing the copied items.
The example below defines an array called
names
, which stores the names of seven people. A new variable
called
copiedNames
is set to the result of calling the
copy
method on the
names
array:
1
var

names
= [
"Mohsen"
,
"Hilary"
,
"Justyn"
,
"Amy"
,
"Rich"
,
"Graham"
,

"Vic"
]
2
var

copiedNames
=
names
.
copy
()
You can prove that the
names
array has been copied by changing an item in one of the arrays and checking the
corresponding item in the other. If you set the first item in the
copiedNames
array to
"Mo"
rather than
"Mohsen"
, the
names
array still returns the old value of
"Mohsen"
from before the copy took place:
1
copiedNames
[
0
] =
"Mo"
2
println
(
names
[
0
])
3
// prints "Mohsen"
NOTE
If you simply need to be sure that your reference to an array’s contents is the only reference in
existence, call the
unshare
method, not the
copy
method. The
unshare
method does not make
a copy of the array unless it is necessary to do so. The
copy
method always copies the array, even
if it is already unshared.

​
​
Properties
Properties
associate values with a particular class, structure, or enumeration. Stored properties store constant
and variable values as part of an instance, whereas computed properties calculate (rather than store) a value.
Computed properties are provided by classes, structures, and enumerations. Stored properties are provided
only by classes and structures.
Stored and computed properties are usually associated with instances of a particular type. However, properties
can also be associated with the type itself. Such properties are known as type properties.
In addition, you can define property observers to monitor changes in a property’s value, which you can respond
to with custom actions. Property observers can be added to stored properties you define yourself, and also to
properties that a subclass inherits from its superclass.
​
Stored Properties
In its simplest form, a stored property is a constant or variable that is stored as part of an instance of a
particular class or structure. Stored properties can be either
variable stored properties
(introduced by the
var
keyword) or
constant stored properties
(introduced by the
let
keyword).
You can provide a default value for a stored property as part of its definition, as described in
Default Property
Values
. You can also set and modify the initial value for a stored property during initialization. This is true even
for constant stored properties, as described in
Modifying Constant Properties During Initialization
.
The example below defines a structure called
FixedLengthRange
, which describes a range of integers
whose range length cannot be changed once it is created:
1
struct

FixedLengthRange
{
2

var

firstValue
:
Int
3

let

length
:
Int
4
}
5
var

rangeOfThreeItems
=
FixedLengthRange
(
firstValue
:
0
,
length
:
3
)

6
// the range represents integer values 0, 1, and 2
7
rangeOfThreeItems
.
firstValue
=
6
8
// the range now represents integer values 6, 7, and 8
Instances of
FixedLengthRange
have a variable stored property called
firstValue
and a constant
stored property called
length
. In the example above,
length
is initialized when the new range is created
and cannot be changed thereafter, because it is a constant property.
​
Stored Properties of Constant Structure Instances
If you create an instance of a structure and assign that instance to a constant, you cannot modify the instance’s
properties, even if they were declared as variable properties:
1
let

rangeOfFourItems
=
FixedLengthRange
(
firstValue
:
0
,
length
:
4
)
2
// this range represents integer values 0, 1, 2, and 3
3
rangeOfFourItems
.
firstValue
=
6
4
// this will report an error, even thought firstValue is a variable

property
Because
rangeOfFourItems
is declared as a constant (with the
let
keyword), it is not possible to change
its
firstValue
property, even though
firstValue
is a variable property.
This behavior is due to structures being
value types
. When an instance of a value type is marked as a constant,
so are all of its properties.
The same is not true for classes, which are
reference types
. If you assign an instance of a reference type to a
constant, you can still change that instance’s variable properties.
​
Lazy Stored Properties
A
lazy stored property
is a property whose initial value is not calculated until the first time it is used. You
indicate a lazy stored property by writing the
@lazy
attribute before its declaration.

NOTE
You must always declare a lazy property as a variable (with the
var
keyword), because its initial
value may not be retrieved until after instance initialization completes. Constant properties must
always have a value
before
initialization completes, and therefore cannot be declared as lazy.
Lazy properties are useful when the initial value for a property is dependent on outside factors whose values are
not known until after an instance’s initialization is complete. Lazy properties are also useful when the initial
value for a property requires complex or computationally expensive setup that should not be performed unless
or until it is needed.
The example below uses a lazy stored property to avoid unnecessary initialization of a complex class. This
example defines two classes called
DataImporter
and
DataManager
, neither of which is shown in full:
1
class

DataImporter
{
2

/*
3
DataImporter is a class to import data from an external file.
4
The class is assumed to take a non-trivial amount of time to

initialize.
5
*/
6

var

fileName
=
"data.txt"
7

// the DataImporter class would provide data importing

functionality here
8
}
9

10
class

DataManager
{
11

@lazy

var

importer
=
DataImporter
()
12

var

data
=
String
[]()
13

// the DataManager class would provide data management

functionality here
14
}
15

16
let

manager
=
DataManager
()
17
manager
.
data
+=
"Some data"

18
manager
.
data
+=
"Some more data"
19
// the DataImporter instance for the importer property has not

yet been created
The
DataManager
class has a stored property called
data
, which is initialized with a new, empty array of
String
values. Although the rest of its functionality is not shown, the purpose of this
DataManager
class is
to manage and provide access to this array of
String
data.
Part of the functionality of the
DataManager
class is the ability to import data from a file. This functionality is
provided by the
DataImporter
class, which is assumed to take a non-trivial amount of time to initialize. This
might be because a
DataImporter
instance needs to open a file and read its contents into memory when the
DataImporter
instance is initialized.
It is possible for a
DataManager
instance to manage its data without ever importing data from a file, so there
is no need to create a new
DataImporter
instance when the
DataManager
itself is created. Instead, it
makes more sense to create the
DataImporter
instance if and when it is first used.
Because it is marked with the
@lazy
attribute, the
DataImporter
instance for the
importer
property is
only created when the
importer
property is first accessed, such as when its
fileName
property is queried:
1
println
(
manager
.
importer
.
fileName
)
2
// the DataImporter instance for the importer property has now been

created
3
// prints "data.txt"
​
Stored Properties and Instance Variables
If you have experience with Objective-C, you may know that it provides
two
ways to store values and
references as part of a class instance. In addition to properties, you can use instance variables as a backing
store for the values stored in a property.
Swift unifies these concepts into a single property declaration. A Swift property does not have a corresponding
instance variable, and the backing store for a property is not accessed directly. This approach avoids confusion

about how the value is accessed in different contexts and simplifies the property’s declaration into a single,
definitive statement. All information about the property—including its name, type, and memory management
characteristics—is defined in a single location as part of the type’s definition.
​
Computed Properties
In addition to stored properties, classes, structures, and enumerations can define
computed properties
, which
do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other
properties and values indirectly.
1
struct

Point
{
2

var

x
=
0.0
,
y
=
0.0
3
}
4
struct

Size
{
5

var

width
=
0.0
,
height
=
0.0
6
}
7
struct

Rect
{
8

var

origin
=
Point
()
9

var

size
=
Size
()
10

var

center
:
Point
{
11

get
{
12

let

centerX
=
origin
.
x
+ (
size
.
width
/
2
)
13

let

centerY
=
origin
.
y
+ (
size
.
height
/
2
)
14

return

Point
(
x
:
centerX
,
y
:
centerY
)
15
}
16

set
(
newCenter
) {
17

origin
.
x
=
newCenter
.
x
– (
size
.
width
/
2
)
18

origin
.
y
=
newCenter
.
y
– (
size
.
height
/
2
)
19
}
20
}
21
}
22
var

square
=
Rect
(
origin
:
Point
(
x
:
0.0
,
y
:
0.0
),
23

size
:
Size
(
width
:
10.0
,
height
:
10.0
))
24
let

initialSquareCenter
=
square
.
center
25
square
.
center
=
Point
(
x
:
15.0
,
y
:
15.0
)

26
println
(
"square.origin is now at (
\(
square
.
origin
.
x
)
,
\
(
square
.
origin
.
y
)
)"
)
27
// prints "square.origin is now at (10.0, 10.0)"
This example defines three structures for working with geometric shapes:
The
Rect
structure also provides a computed property called
center
. The current center position of a
Rect
can always be determined from its
origin
and
size
, and so you don’t need to store the center point as an
explicit
Point
value. Instead,
Rect
defines a custom getter and setter for a computed variable called
center
, to enable you to work with the rectangle’s
center
as if it were a real stored property.
The preceding example creates a new
Rect
variable called
square
. The
square
variable is initialized with
an origin point of
(0, 0)
, and a width and height of
10
. This square is represented by the blue square in the
diagram below.
The
square
variable’s
center
property is then accessed through dot syntax (
square.center
), which
causes the getter for
center
to be called, to retrieve the current property value. Rather than returning an
existing value, the getter actually calculates and returns a new
Point
to represent the center of the square. As
can be seen above, the getter correctly returns a center point of
(5, 5)
.
The
center
property is then set to a new value of
(15, 15)
, which moves the square up and to the right, to
the new position shown by the orange square in the diagram below. Setting the
center
property calls the
setter for
center
, which modifies the
x
and
y
values of the stored
origin
property, and moves the square to
its new position.
Point
encapsulates an
(x, y)
coordinate.
Size
encapsulates a
width
and a
height
.
Rect
defines a rectangle by an origin point and a size.

​
Shorthand Setter Declaration
If a computed property’s setter does not define a name for the new value to be set, a default name of
newValue
is used. Here’s an alternative version of the
Rect
structure, which takes advantage of this shorthand notation:

1
struct

AlternativeRect
{
2

var

origin
=
Point
()
3

var

size
=
Size
()
4

var

center
:
Point
{
5

get
{
6

let

centerX
=
origin
.
x
+ (
size
.
width
/
2
)
7

let

centerY
=
origin
.
y
+ (
size
.
height
/
2
)
8

return

Point
(
x
:
centerX
,
y
:
centerY
)
9
}
10

set
{
11

origin
.
x
=
newValue
.
x
– (
size
.
width
/
2
)
12

origin
.
y
=
newValue
.
y
– (
size
.
height
/
2
)
13
}
14
}
15
}
​
Read-Only Computed Properties
A computed property with a getter but no setter is known as a
read-only computed property
. A read-only
computed property always returns a value, and can be accessed through dot syntax, but cannot be set to a
different value.
NOTE
You must declare computed properties—including read-only computed properties—as variable
properties with the
var
keyword, because their value is not fixed. The
let
keyword is only used for
constant properties, to indicate that their values cannot be changed once they are set as part of
instance initialization.
You can simplify the declaration of a read-only computed property by removing the
get
keyword and its
braces:

1
struct

Cuboid
{
2

var

width
=
0.0
,
height
=
0.0
,
depth
=
0.0
3

var

volume
:
Double
{
4

return

width
*
height
*
depth
5
}
6
}
7
let

fourByFiveByTwo
=
Cuboid
(
width
:
4.0
,
height
:
5.0
,
depth
:
2.0
)
8
println
(
"the volume of fourByFiveByTwo is
\(
fourByFiveByTwo
.
volume
)
"
)
9
// prints "the volume of fourByFiveByTwo is 40.0"
This example defines a new structure called
Cuboid
, which represents a 3D rectangular box with
width
,
height
, and
depth
properties. This structure also has a read-only computed property called
volume
, which
calculates and returns the current volume of the cuboid. It doesn’t make sense for
volume
to be settable,
because it would be ambiguous as to which values of
width
,
height
, and
depth
should be used for a
particular
volume
value. Nonetheless, it is useful for a
Cuboid
to provide a read-only computed property to
enable external users to discover its current calculated volume.
​
Property Observers
Property observers
observe and respond to changes in a property’s value. Property observers are called every
time a property’s value is set, even if the new value is the same as the property’s current value.
You can add property observers to any stored properties you define, apart from lazy stored properties. You can
also add property observers to any inherited property (whether stored or computed) by overriding the property
within a subclass. Property overriding is described in
Overriding
.
NOTE
You don’t need to define property observers for non-overridden computed properties, because you
can observe and respond to changes to their value from directly within the computed property’s
setter.

You have the option to define either or both of these observers on a property:
If you implement a
willSet
observer, it is passed the new property value as a constant parameter. You can
specify a name for this parameter as part of your
willSet
implementation. If you choose not to write the
parameter name and parentheses within your implementation, the parameter will still be made available with a
default parameter name of
newValue
.
Similarly, if you implement a
didSet
observer, it will be passed a constant parameter containing the old
property value. You can name the parameter if you wish, or use the default parameter name of
oldValue
.
NOTE
willSet
and
didSet
observers are not called when a property is first initialized. They are only
called when the property’s value is set outside of an initialization context.
Here’s an example of
willSet
and
didSet
in action. The example below defines a new class called
StepCounter
, which tracks the total number of steps that a person takes while walking. This class might be
used with input data from a pedometer or other step counter to keep track of a person’s exercise during their
daily routine.
1
class

StepCounter
{
2

var

totalSteps
:
Int
=
0
{
3

willSet
(
newTotalSteps
) {
4

println
(
"About to set totalSteps to
\(
newTotalSteps
)
"
)
5
}
6

didSet
{
7

if

totalSteps
>
oldValue
{
8

println
(
"Added
\(
totalSteps
–
oldValue
)
steps"
)
willSet
is called just before the value is stored.
didSet
is called immediately after the new value is stored.

9
}
10
}
11
}
12
}
13
let

stepCounter
=
StepCounter
()
14
stepCounter
.
totalSteps
=
200
15
// About to set totalSteps to 200
16
// Added 200 steps
17
stepCounter
.
totalSteps
=
360
18
// About to set totalSteps to 360
19
// Added 160 steps
20
stepCounter
.
totalSteps
=
896
21
// About to set totalSteps to 896
22
// Added 536 steps
The
StepCounter
class declares a
totalSteps
property of type
Int
. This is a stored property with
willSet
and
didSet
observers.
The
willSet
and
didSet
observers for
totalSteps
are called whenever the property is assigned a new
value. This is true even if the new value is the same as the current value.
This example’s
willSet
observer uses a custom parameter name of
newTotalSteps
for the upcoming
new value. In this example, it simply prints out the value that is about to be set.
The
didSet
observer is called after the value of
totalSteps
is updated. It compares the new value of
totalSteps
against the old value. If the total number of steps has increased, a message is printed to indicate
how many new steps have been taken. The
didSet
observer does not provide a custom parameter name for
the old value, and the default name of
oldValue
is used instead.
NOTE
If you assign a value to a property within its own
didSet
observer, the new value that you assign
will replace the one that was just set.

​
Global and Local Variables
The capabilities described above for computing and observing properties are also available to
global variables
and
local variables
. Global variables are variables that are defined outside of any function, method, closure, or
type context. Local variables are variables that are defined within a function, method, or closure context.
The global and local variables you have encountered in previous chapters have all been
stored variables
. Stored
variables, like stored properties, provide storage for a value of a certain type and allow that value to be set and
retrieved.
However, you can also define
computed variables
and define observers for stored variables, in either a global
or local scope. Computed variables calculate rather than store a value, and are written in the same way as
computed properties.
NOTE
Global constants and variables are always computed lazily, in a similar manner to
Lazy Stored
Properties
. Unlike lazy stored properties, global constants and variables do not need to be marked
with the
@lazy
attribute.
Local constants and variables are never computed lazily.
​
Type Properties
Instance properties are properties that belong to an instance of a particular type. Every time you create a new
instance of that type, it has its own set of property values, separate from any other instance.
You can also define properties that belong to the type itself, not to any one instance of that type. There will only
ever be one copy of these properties, no matter how many instances of that type you create. These kinds of
properties are called
type properties
.

Type properties are useful for defining values that are universal to
all
instances of a particular type, such as a
constant property that all instances can use (like a static constant in C), or a variable property that stores a
value that is global to all instances of that type (like a static variable in C).
For value types (that is, structures and enumerations), you can define stored and computed type properties. For
classes, you can define computed type properties only.
Stored type properties for value types can be variables or constants. Computed type properties are always
declared as variable properties, in the same way as computed instance properties.
NOTE
Unlike stored instance properties, you must always give stored type properties a default value. This
is because the type itself does not have an initializer that can assign a value to a stored type property
at initialization time.
​
Type Property Syntax
In C and Objective-C, you define static constants and variables associated with a type as
global
static variables.
In Swift, however, type properties are written as part of the type’s definition, within the type’s outer curly braces,
and each type property is explicitly scoped to the type it supports.
You define type properties for value types with the
static
keyword, and type properties for class types with
the
class
keyword. The example below shows the syntax for stored and computed type properties:
1
struct

SomeStructure
{
2

static

var

storedTypeProperty
=
"Some value."
3

static

var

computedTypeProperty
:
Int
{
4

// return an Int value here
5
}
6
}

7
enum

SomeEnumeration
{
8

static

var

storedTypeProperty
=
"Some value."
9

static

var

computedTypeProperty
:
Int
{
10

// return an Int value here
11
}
12
}
13
class

SomeClass
{
14

class

var

computedTypeProperty
:
Int
{
15

// return an Int value here
16
}
17
}
NOTE
The computed type property examples above are for read-only computed type properties, but you
can also define read-write computed type properties with the same syntax as for computed instance
properties.
​
Querying and Setting Type Properties
Type properties are queried and set with dot syntax, just like instance properties. However, type properties are
queried and set on the
type
, not on an instance of that type. For example:
1
println
(
SomeClass
.
computedTypeProperty
)
2
// prints "42"
3

4
println
(
SomeStructure
.
storedTypeProperty
)
5
// prints "Some value."
6
SomeStructure
.
storedTypeProperty
=
"Another value."
7
println
(
SomeStructure
.
storedTypeProperty
)
8
// prints "Another value."

The examples that follow use two stored type properties as part of a structure that models an audio level meter
for a number of audio channels. Each channel has an integer audio level between
0
and
10
inclusive.
The figure below illustrates how two of these audio channels can be combined to model a stereo audio level
meter. When a channel’s audio level is
0
, none of the lights for that channel are lit. When the audio level is
10
,
all of the lights for that channel are lit. In this figure, the left channel has a current level of
9
, and the right channel
has a current level of
7
:

The audio channels described above are represented by instances of the
AudioChannel
structure:
1
struct

AudioChannel
{
2

static

let

thresholdLevel
=
10
3

static

var

maxInputLevelForAllChannels
=
0
4

var

currentLevel
:
Int
=
0
{
5

didSet
{
6

if

currentLevel
>
AudioChannel
.
thresholdLevel
{
7

// cap the new audio level to the threshold level
8

currentLevel
=
AudioChannel
.
thresholdLevel
9
}
10

if

currentLevel
>

AudioChannel
.
maxInputLevelForAllChannels
{
11

// store this as the new overall maximum input

level
12

AudioChannel
.
maxInputLevelForAllChannels
=

currentLevel
13
}
14
}
15
}
16
}
The
AudioChannel
structure defines two stored type properties to support its functionality. The first,
thresholdLevel
, defines the maximum threshold value an audio level can take. This is a constant value of
10
for all
AudioChannel
instances. If an audio signal comes in with a higher value than
10
, it will be capped
to this threshold value (as described below).
The second type property is a variable stored property called
maxInputLevelForAllChannels
. This
keeps track of the maximum input value that has been received by
any

AudioChannel
instance. It starts with
an initial value of
0
.
The
AudioChannel
structure also defines a stored instance property called
currentLevel
, which
represents the channel’s current audio level on a scale of
0
to
10
.
The
currentLevel
property has a
didSet
property observer to check the value of
currentLevel

whenever it is set. This observer performs two checks:
NOTE
In the first of these two checks, the
didSet
observer sets
currentLevel
to a different value.
This does not, however, cause the observer to be called again.
You can use the
AudioChannel
structure to create two new audio channels called
leftChannel
and
rightChannel
, to represent the audio levels of a stereo sound system:
1
var

leftChannel
=
AudioChannel
()
2
var

rightChannel
=
AudioChannel
()
If you set the
currentLevel
of the
left
channel to
7
, you can see that the
maxInputLevelForAllChannels
type property is updated to equal
7
:
1
leftChannel
.
currentLevel
=
7
2
println
(
leftChannel
.
currentLevel
)
3
// prints "7"
4
println
(
AudioChannel
.
maxInputLevelForAllChannels
)
5
// prints "7"
If you try to set the
currentLevel
of the
right
channel to
11
, you can see that the right channel’s
currentLevel
property is capped to the maximum value of
10
, and the
If the new value of
currentLevel
is greater than the allowed
thresholdLevel
, the
property observer caps
currentLevel
to
thresholdLevel
.
If the new value of
currentLevel
(after any capping) is higher than any value previously
received by
any

AudioChannel
instance, the property observer stores the new
currentLevel
value in the
maxInputLevelForAllChannels
static property.

maxInputLevelForAllChannels
type property is updated to equal
10
:
1
rightChannel
.
currentLevel
=
11
2
println
(
rightChannel
.
currentLevel
)
3
// prints "10"
4
println
(
AudioChannel
.
maxInputLevelForAllChannels
)
5
// prints "10"

​
​
Methods
Methods
are functions that are associated with a particular type. Classes, structures, and enumerations can all
define instance methods, which encapsulate specific tasks and functionality for working with an instance of a
given type. Classes, structures, and enumerations can also define type methods, which are associated with the
type itself. Type methods are similar to class methods in Objective-C.
The fact that structures and enumerations can define methods in Swift is a major difference from C and
Objective-C. In Objective-C, classes are the only types that can define methods. In Swift, you can choose
whether to define a class, structure, or enumeration, and still have the flexibility to define methods on the type
you create.
​
Instance Methods
Instance methods
are functions that belong to instances of a particular class, structure, or enumeration. They
support the functionality of those instances, either by providing ways to access and modify instance properties,
or by providing functionality related to the instance’s purpose. Instance methods have exactly the same syntax
as functions, as described in
Functions
.
You write an instance method within the opening and closing braces of the type it belongs to. An instance
method has implicit access to all other instance methods and properties of that type. An instance method can be
called only on a specific instance of the type it belongs to. It cannot be called in isolation without an existing
instance.
Here’s an example that defines a simple
Counter
class, which can be used to count the number of times an
action occurs:
1
class

Counter
{
2

var

count
=
0
3

func

increment
() {
4

count
++
5
}
6

func

incrementBy
(
amount
:
Int
) {

7

count
+=
amount
8
}
9

func

reset
() {
10

count
=
0
11
}
12
}
The
Counter
class defines three instance methods:
The
Counter
class also declares a variable property,
count
, to keep track of the current counter value.
You call instance methods with the same dot syntax as properties:
1
let

counter
=
Counter
()
2
// the initial counter value is 0
3
counter
.
increment
()
4
// the counter's value is now 1
5
counter
.
incrementBy
(
5
)
6
// the counter's value is now 6
7
counter
.
reset
()
8
// the counter's value is now 0
​
Local and External Parameter Names for Methods
Function parameters can have both a local name (for use within the function’s body) and an external name (for
use when calling the function), as described in
External Parameter Names
. The same is true for method
parameters, because methods are just functions that are associated with a type. However, the default behavior
of local names and external names is different for functions and methods.
increment
increments the counter by
1
.
incrementBy(amount: Int)
increments the counter by an specified integer amount.
reset
resets the counter to zero.

Methods in Swift are very similar to their counterparts in Objective-C. As in Objective-C, the name of a method
in Swift typically refers to the method’s first parameter using a preposition such as
with
,
for
, or
by
, as seen
in the
incrementBy
method from the preceding
Counter
class example. The use of a preposition enables
the method to be read as a sentence when it is called. Swift makes this established method naming convention
easy to write by using a different default approach for method parameters than it uses for function parameters.
Specifically, Swift gives the
first
parameter name in a method a local parameter name by default, and gives the
second and subsequent parameter names both local
and
external parameter names by default. This convention
matches the typical naming and calling convention you will be familiar with from writing Objective-C methods,
and makes for expressive method calls without the need to qualify your parameter names.
Consider this alternative version of the
Counter
class, which defines a more complex form of the
incrementBy
method:
1
class

Counter
{
2

var

count
:
Int
=
0
3

func

incrementBy
(
amount
:
Int
,
numberOfTimes
:
Int
) {
4

count
+=
amount
*
numberOfTimes
5
}
6
}
This
incrementBy
method has two parameters—
amount
and
numberOfTimes
. By default, Swift treats
amount
as a local name only, but treats
numberOfTimes
as both a local
and
an external name. You call the
method as follows:
1
let

counter
=
Counter
()
2
counter
.
incrementBy
(
5
,
numberOfTimes
:
3
)
3
// counter value is now 15
You don’t need to define an external parameter name for the first argument value, because its purpose is clear
from the function name
incrementBy
. The second argument, however, is qualified by an external parameter
name to make its purpose clear when the method is called.
This default behavior effectively treats the method as if you had written a hash symbol (
#
) before the
numberOfTimes
parameter:

1
func

incrementBy
(
amount
:
Int
, #
numberOfTimes
:
Int
) {
2

count
+=
amount
*
numberOfTimes
3
}
The default behavior described above mean that method definitions in Swift are written with the same
grammatical style as Objective-C, and are called in a natural, expressive way.
​
Modifying External Parameter Name Behavior for Methods
Sometimes it’s useful to provide an external parameter name for a method’s first parameter, even though this is
not the default behavior. You can either add an explicit external name yourself, or you can prefix the first
parameter’s name with a hash symbol to use the local name as an external name too.
Conversely, if you do not want to provide an external name for the second or subsequent parameter of a
method, override the default behavior by using an underscore character (
_
) as an explicit external parameter
name for that parameter.
​
The self Property
Every instance of a type has an implicit property called
self
, which is exactly equivalent to the instance itself.
You use this implicit
self
property to refer to the current instance within its own instance methods.
The
increment
method in the example above could have been written like this:
1
func

increment
() {
2

self
.
count
++
3
}
In practice, you don’t need to write
self
in your code very often. If you don’t explicitly write
self
, Swift
assumes that you are referring to a property or method of the current instance whenever you use a known
property or method name within a method. This assumption is demonstrated by the use of
count
(rather than
self.count
) inside the three instance methods for
Counter
.

The main exception to this rule occurs when a parameter name for an instance method has the same name as
a property of that instance. In this situation, the parameter name takes precedence, and it becomes necessary to
refer to the property in a more qualified way. You use the implicit
self
property to distinguish between the
parameter name and the property name.
Here,
self
disambiguates between a method parameter called
x
and an instance property that is also called
x
:
1
struct

Point
{
2

var

x
=
0.0
,
y
=
0.0
3

func

isToTheRightOfX
(
x
:
Double
) ->
Bool
{
4

return

self
.
x
>
x
5
}
6
}
7
let

somePoint
=
Point
(
x
:
4.0
,
y
:
5.0
)
8
if

somePoint
.
isToTheRightOfX
(
1.0
) {
9

println
(
"This point is to the right of the line where x == 1.0"
)
10
}
11
// prints "This point is to the right of the line where x ==

1.0"
Without the
self
prefix, Swift would assume that both uses of
x
referred to the method parameter called
x
.
​
Modifying Value Types from Within Instance Methods
Structures and enumerations are
value types
. By default, the properties of a value type cannot be modified from
within its instance methods.
However, if you need to modify the properties of your structure or enumeration within a particular method, you
can opt in to
mutating
behavior for that method. The method can then mutate (that is, change) its properties from
within the method, and any changes that it makes are written back to the original structure when the method
ends. The method can also assign a completely new instance to its implicit
self
property, and this new
instance will replace the existing one when the method ends.

You can opt in to this behavior by placing the
mutating
keyword before the
func
keyword for that method:
1
struct

Point
{
2

var

x
=
0.0
,
y
=
0.0
3

mutating

func

moveByX
(
deltaX
:
Double
,
y

deltaY
:
Double
) {
4

x
+=
deltaX
5

y
+=
deltaY
6
}
7
}
8
var

somePoint
=
Point
(
x
:
1.0
,
y
:
1.0
)
9
somePoint
.
moveByX
(
2.0
,
y
:
3.0
)
10
println
(
"The point is now at (
\(
somePoint
.
x
)
,
\(
somePoint
.
y
)
)"
)
11
// prints "The point is now at (3.0, 4.0)"
The
Point
structure above defines a mutating
moveByX
method, which moves a
Point
instance by a
certain amount. Instead of returning a new point, this method actually modifies the point on which it is called.
The
mutating
keyword is added to its definition to enable it to modify its properties.
Note that you cannot call a mutating method on a constant of structure type, because its properties cannot be
changed, even if they are variable properties, as described in
Stored Properties of Constant Structure Instances
:
1
let

fixedPoint
=
Point
(
x
:
3.0
,
y
:
3.0
)
2
fixedPoint
.
moveByX
(
2.0
,
y
:
3.0
)
3
// this will report an error
​
Assigning to self Within a Mutating Method
Mutating methods can assign an entirely new instance to the implicit
self
property. The
Point
example
shown above could have been written in the following way instead:
1
struct

Point
{
2

var

x
=
0.0
,
y
=
0.0
3

mutating

func

moveByX
(
deltaX
:
Double
,
y

deltaY
:
Double
) {

4

self
=
Point
(
x
:
x
+
deltaX
,
y
:
y
+
deltaY
)
5
}
6
}
This version of the mutating
moveByX
method creates a brand new structure whose
x
and
y
values are set to
the target location. The end result of calling this alternative version of the method will be exactly the same as for
calling the earlier version.
Mutating methods for enumerations can set the implicit
self
parameter to be a different member from the
same enumeration:
1
enum

TriStateSwitch
{
2

case

Off
,
Low
,
High
3

mutating

func

next
() {
4

switch

self
{
5

case

Off
:
6

self
=
Low
7

case

Low
:
8

self
=
High
9

case

High
:
10

self
=
Off
11
}
12
}
13
}
14
var

ovenLight
=
TriStateSwitch
.
Low
15
ovenLight
.
next
()
16
// ovenLight is now equal to .High
17
ovenLight
.
next
()
18
// ovenLight is now equal to .Off
This example defines an enumeration for a three-state switch. The switch cycles between three different power
states (
Off
,
Low
and
High
) every time its
next
method is called.
​

Type Methods
Instance methods, as described above, are methods that are called on an instance of a particular type. You can
also define methods that are called on the type itself. These kinds of methods are called
type methods
. You
indicate type methods for classes by writing the keyword
class
before the method’s
func
keyword, and type
methods for structures and enumerations by writing the keyword
static
before the method’s
func
keyword.
NOTE
In Objective-C, you can define type-level methods only for Objective-C classes. In Swift, you can
define type-level methods for all classes, structures, and enumerations. Each type method is
explicitly scoped to the type it supports.
Type methods are called with dot syntax, like instance methods. However, you call type methods on the type,
not on an instance of that type. Here’s how you call a type method on a class called
SomeClass
:
1
class

SomeClass
{
2

class

func

someTypeMethod
() {
3

// type method implementation goes here
4
}
5
}
6
SomeClass
.
someTypeMethod
()
Within the body of a type method, the implicit
self
property refers to the type itself, rather than an instance of
that type. For structures and enumerations, this means that you can use
self
to disambiguate between static
properties and static method parameters, just as you do for instance properties and instance method
parameters.
More generally, any unqualified method and property names that you use within the body of a type method will
refer to other type-level methods and properties. A type method can call another type method with the other
method’s name, without needing to prefix it with the type name. Similarly, type methods on structures and
enumerations can access static properties by using the static property’s name without a type name prefix.

The example below defines a structure called
LevelTracker
, which tracks a player’s progress through the
different levels or stages of a game. It is a single-player game, but can store information for multiple players on
a single device.
All of the game’s levels (apart from level one) are locked when the game is first played. Every time a player
finishes a level, that level is unlocked for all players on the device. The
LevelTracker
structure uses static
properties and methods to keep track of which levels of the game have been unlocked. It also tracks the current
level for an individual player.
1
struct

LevelTracker
{
2

static

var

highestUnlockedLevel
=
1
3

static

func

unlockLevel
(
level
:
Int
) {
4

if

level
>
highestUnlockedLevel
{
highestUnlockedLevel
=
level
}
5
}
6

static

func

levelIsUnlocked
(
level
:
Int
) ->
Bool
{
7

return

level
<=
highestUnlockedLevel
8
}
9

var

currentLevel
=
1
10

mutating

func

advanceToLevel
(
level
:
Int
) ->
Bool
{
11

if

LevelTracker
.
levelIsUnlocked
(
level
) {
12

currentLevel
=
level
13

return

true
14
}
else
{
15

return

false
16
}
17
}
18
}
The
LevelTracker
structure keeps track of the highest level that any player has unlocked. This value is
stored in a static property called
highestUnlockedLevel
.
LevelTracker
also defines two type functions to work with the
highestUnlockedLevel
property. The
first is a type function called
unlockLevel
, which updates the value of
highestUnlockedLevel
whenever a new level is unlocked. The second is a convenience type function called
levelIsUnlocked
,
which returns
true
if a particular level number is already unlocked. (Note that these type methods can access

the
highestUnlockedLevel
static property without your needing to write it as
LevelTracker.highestUnlockedLevel
.)
In addition to its static property and type methods,
LevelTracker
tracks an individual player’s progress
through the game. It uses an instance property called
currentLevel
to track the level that a player is
currently playing.
To help manage the
currentLevel
property,
LevelTracker
defines an instance method called
advanceToLevel
. Before updating
currentLevel
, this method checks whether the requested new level
is already unlocked. The
advanceToLevel
method returns a Boolean value to indicate whether or not it was
actually able to set
currentLevel
.
The
LevelTracker
structure is used with the
Player
class, shown below, to track and update the progress
of an individual player:
1
class

Player
{
2

var

tracker
=
LevelTracker
()
3

let

playerName
:
String
4

func

completedLevel
(
level
:
Int
) {
5

LevelTracker
.
unlockLevel
(
level
+
1
)
6

tracker
.
advanceToLevel
(
level
+
1
)
7
}
8

init
(
name
:
String
) {
9

playerName
=
name
10
}
11
}
The
Player
class creates a new instance of
LevelTracker
to track that player’s progress. It also provides
a method called
completedLevel
, which is called whenever a player completes a particular level. This
method unlocks the next level for all players and updates the player’s progress to move them to the next level.
(The Boolean return value of
advanceToLevel
is ignored, because the level is known to have been unlocked
by the call to
LevelTracker.unlockLevel
on the previous line.)
You can create a instance of the
Player
class for a new player, and see what happens when the player
completes level one:

1
var

player
=
Player
(
name
:
"Argyrios"
)
2
player
.
completedLevel
(
1
)
3
println
(
"highest unlocked level is now
\
(
LevelTracker
.
highestUnlockedLevel
)
"
)
4
// prints "highest unlocked level is now 2"
If you create a second player, whom you try to move to a level that is not yet unlocked by any player in the
game, the attempt to set the player’s current level fails:
1
player
=
Player
(
name
:
"Beto"
)
2
if

player
.
tracker
.
advanceToLevel
(
6
) {
3

println
(
"player is now on level 6"
)
4
}
else
{
5

println
(
"level 6 has not yet been unlocked"
)
6
}
7
// prints "level 6 has not yet been unlocked"

​
​
Subscripts
Classes, structures, and enumerations can define
subscripts
, which are shortcuts for accessing the member
elements of a collection, list, or sequence. You use subscripts to set and retrieve values by index without
needing separate methods for setting and retrieval. For example, you access elements in an
Array
instance
as
someArray[index]
and elements in a
Dictionary
instance as
someDictionary[key]
.
You can define multiple subscripts for a single type, and the appropriate subscript overload to use is selected
based on the type of index value you pass to the subscript. Subscripts are not limited to a single dimension, and
you can define subscripts with multiple input parameters to suit your custom type’s needs.
​
Subscript Syntax
Subscripts enable you to query instances of a type by writing one or more values in square brackets after the
instance name. Their syntax is similar to both instance method syntax and computed property syntax. You write
subscript definitions with the
subscript
keyword, and specify one or more input parameters and a return
type, in the same way as instance methods. Unlike instance methods, subscripts can be read-write or read-
only. This behavior is communicated by a getter and setter in the same way as for computed properties:
1
subscript
(
index
:
Int
) ->
Int
{
2

get
{
3

// return an appropriate subscript value here
4
}
5

set
(
newValue
) {
6

// perform a suitable setting action here
7
}
8
}
The type of
newValue
is the same as the return value of the subscript. As with computed properties, you can
choose not to specify the setter’s
(newValue)
parameter. A default parameter called
newValue
is provided
to your setter if you do not provide one yourself.

As with read-only computed properties, you can drop the
get
keyword for read-only subscripts:
1
subscript
(
index
:
Int
) ->
Int
{
2

// return an appropriate subscript value here
3
}
Here’s an example of a read-only subscript implementation, which defines a
TimesTable
structure to
represent an
n
-times-table of integers:
1
struct

TimesTable
{
2

let

multiplier
:
Int
3

subscript
(
index
:
Int
) ->
Int
{
4

return

multiplier
*
index
5
}
6
}
7
let

threeTimesTable
=
TimesTable
(
multiplier
:
3
)
8
println
(
"six times three is
\(
threeTimesTable
[
6
])
"
)
9
// prints "six times three is 18"
In this example, a new instance of
TimesTable
is created to represent the three-times-table. This is indicated
by passing a value of
3
to the structure’s
initializer
as the value to use for the instance’s
multiplier
parameter.
You can query the
threeTimesTable
instance by calling its subscript, as shown in the call to
threeTimesTable[6]
. This requests the sixth entry in the three-times-table, which returns a value of
18
,
or
3
times
6
.
NOTE
An
n
-times-table is based on a fixed mathematical rule. It is not appropriate to set
threeTimesTable[someIndex]
to a new value, and so the subscript for
TimesTable
is
defined as a read-only subscript.

​
Subscript Usage
The exact meaning of “subscript” depends on the context in which it is used. Subscripts are typically used as a
shortcut for accessing the member elements in a collection, list, or sequence. You are free to implement
subscripts in the most appropriate way for your particular class or structure’s functionality.
For example, Swift’s
Dictionary
type implements a subscript to set and retrieve the values stored in a
Dictionary
instance. You can set a value in a dictionary by providing a key of the dictionary’s key type
within subscript braces, and assigning a value of the dictionary’s value type to the subscript:
1
var

numberOfLegs
= [
"spider"
:
8
,
"ant"
:
6
,
"cat"
:
4
]
2
numberOfLegs
[
"bird"
] =
2
The example above defines a variable called
numberOfLegs
and initializes it with a dictionary literal
containing three key-value pairs. The type of the
numberOfLegs
dictionary is inferred to be
Dictionary<String, Int>
. After creating the dictionary, this example uses subscript assignment to add
a
String
key of
"bird"
and an
Int
value of
2
to the dictionary.
For more information about
Dictionary
subscripting, see
Accessing and Modifying a Dictionary
.
NOTE
Swift’s
Dictionary
type implements its key-value subscripting as a subscript that takes and
receives an
optional
type. For the
numberOfLegs
dictionary above, the key-value subscript takes
and returns a value of type
Int?
, or “optional int”. The
Dictionary
type uses an optional
subscript type to model the fact that not every key will have a value, and to give a way to delete a
value for a key by assigning a
nil
value for that key.
​
Subscript Options

Subscripts can take any number of input parameters, and these input parameters can be of any type. Subscripts
can also return any type. Subscripts can use variable parameters and variadic parameters, but cannot use in-
out parameters or provide default parameter values.
A class or structure can provide as many subscript implementations as it needs, and the appropriate subscript
to be used will be inferred based on the types of the value or values that are contained within the subscript
braces at the point that the subscript is used. This definition of multiple subscripts is known as
subscript
overloading
.
While it is most common for a subscript to take a single parameter, you can also define a subscript with
multiple parameters if it is appropriate for your type. The following example defines a
Matrix
structure, which
represents a two-dimensional matrix of
Double
values. The
Matrix
structure’s subscript takes two integer
parameters:
1
struct

Matrix
{
2

let

rows
:
Int
,
columns
:
Int
3

var

grid
:
Double
[]
4

init
(
rows
:
Int
,
columns
:
Int
) {
5

self
.
rows
=
rows
6

self
.
columns
=
columns
7

grid
=
Array
(
count
:
rows
*
columns
,
repeatedValue
:
0.0
)
8
}
9

func

indexIsValidForRow
(
row
:
Int
,
column
:
Int
) ->
Bool
{
10

return

row
>=
0
&&
row
<
rows
&&
column
>=
0
&&
column

<
columns
11
}
12

subscript
(
row
:
Int
,
column
:
Int
) ->
Double
{
13

get
{
14

assert
(
indexIsValidForRow
(
row
,
column
:
column
),

"Index out of range"
)
15

return

grid
[(
row
*
columns
) +
column
]
16
}
17

set
{
18

assert
(
indexIsValidForRow
(
row
,
column
:
column
),

"Index out of range"
)
19

grid
[(
row
*
columns
) +
column
] =
newValue

20
}
21
}
22
}
Matrix
provides an initializer that takes two parameters called
rows
and
columns
, and creates an array
that is large enough to store
rows * columns
values of type
Double
. Each position in the matrix is given
an initial value of
0.0
. To achieve this, the array’s size, and an initial cell value of
0.0
, are passed to an array
initializer that creates and initializes a new array of the correct size. This initializer is described in more detail
in
Creating and Initializing an Array
.
You can construct a new
Matrix
instance by passing an appropriate row and column count to its initializer:
1
var

matrix
=
Matrix
(
rows
:
2
,
columns
:
2
)
The preceding example creates a new
Matrix
instance with two rows and two columns. The
grid
array for
this
Matrix
instance is effectively a flattened version of the matrix, as read from top left to bottom right:
Values in the matrix can be set by passing row and column values into the subscript, separated by a comma:
1
matrix
[
0
,
1
] =
1.5

2
matrix
[
1
,
0
] =
3.2
These two statements call the subscript’s setter to set a value of
1.5
in the top right position of the matrix
(where
row
is
0
and
column
is
1
), and
3.2
in the bottom left position (where
row
is
1
and
column
is
0
):
The
Matrix
subscript’s getter and setter both contain an assertion to check that the subscript’s
row
and
column
values are valid. To assist with these assertions,
Matrix
includes a convenience method called
indexIsValid
, which checks whether the requested
row
or
column
is outside the bounds of the matrix:
1
func

indexIsValidForRow
(
row
:
Int
,
column
:
Int
) ->
Bool
{
2

return

row
>=
0
&&
row
<
rows
&&
column
>=
0
&&
column
<
columns
3
}
An assertion is triggered if you try to access a subscript that is outside of the matrix bounds:
1
let

someValue
=
matrix
[
2
,
2
]
2
// this triggers an assert, because [2, 2] is outside of the matrix

bounds

​
​
Inheritance
A class can
inherit
methods, properties, and other characteristics from another class. When one class inherits
from another, the inheriting class is known as a
subclass
, and the class it inherits from is known as its
superclass
. Inheritance is a fundamental behavior that differentiates classes from other types in Swift.
Classes in Swift can call and access methods, properties, and subscripts belonging to their superclass and can
provide their own overriding versions of those methods, properties, and subscripts to refine or modify their
behavior. Swift helps to ensure your overrides are correct by checking that the override definition has a
matching superclass definition.
Classes can also add property observers to inherited properties in order to be notified when the value of a
property changes. Property observers can be added to any property, regardless of whether it was originally
defined as a stored or computed property.
​
Defining a Base Class
Any class that does not inherit from another class is known as a
base class
.
NOTE
Swift classes do not inherit from a universal base class. Classes you define without specifying a
superclass automatically become base classes for you to build upon.
The example below defines a base class called
Vehicle
. This base class declares two properties
(
numberOfWheels
and
maxPassengers
) that are universal to all vehicles. These properties are used by a
method called
description
, which returns a
String
description of the vehicle’s characteristics:

1
class

Vehicle
{
2

var

numberOfWheels
:
Int
3

var

maxPassengers
:
Int
4

func

description
() ->
String
{
5

return

"
\(
numberOfWheels
)
wheels; up to
\(
maxPassengers
)

passengers"
6
}
7

init
() {
8

numberOfWheels
=
0
9

maxPassengers
=
1
10
}
11
}
The
Vehicle
class also defines an
initializer
to set up its properties. Initializers are described in detail in
Initialization
, but a brief introduction is required here in order to illustrate how inherited properties can be
modified by subclasses.
You use initializers to create a new instance of a type. Although initializers are not methods, they are written in a
very similar syntax to instance methods. An initializer prepares a new instance for use, and ensures that all
properties of the instance have valid initial values.
In its simplest form, an initializer is like an instance method with no parameters, written using the
init
keyword:
1
init
() {
2

// perform some initialization here
3
}
To create a new instance of
Vehicle
, call this initializer with
initializer syntax
, written as
TypeName
followed by empty parentheses:
1
let

someVehicle
=
Vehicle
()
The initializer for
Vehicle
sets some initial property values (
numberOfWheels = 0
and
maxPassengers = 1
) for an arbitrary vehicle.

The
Vehicle
class defines common characteristics for an arbitrary vehicle, but is not much use in itself. To
make it more useful, you need to refine it to describe more specific kinds of vehicle.
​
Subclassing
Subclassing
is the act of basing a new class on an existing class. The subclass inherits characteristics from the
existing class, which you can refine. You can also add new characteristics to the subclass.
To indicate that a class has a superclass, write the superclass name after the original class name, separated by
a colon:
1
class

SomeClass
:
SomeSuperclass
{
2

// class definition goes here
3
}
The next example defines a second, more specific vehicle called
Bicycle
. This new class is based on the
existing capabilities of
Vehicle
. You indicate this by placing the name of the class the subclass builds upon
(
Vehicle
) after its own name (
Bicycle
), separated by a colon.
This can be read as:
“Define a new class called
Bicycle
, which inherits the characteristics of
Vehicle
”:
1
class

Bicycle
:
Vehicle
{
2

init
() {
3

super
.
init
()
4

numberOfWheels
=
2
5
}
6
}
Bicycle
is a subclass of
Vehicle
, and
Vehicle
is the superclass of
Bicycle
. The new
Bicycle
class
automatically gains all characteristics of
Vehicle
, such as its
maxPassengers
and
numberOfWheels

properties. You can tailor those characteristics and add new ones to better match the requirements of the
Bicycle
class.
The
Bicycle
class also defines an initializer to set up its tailored characteristics. The initializer for
Bicycle
calls
super.init()
, the initializer for the
Bicycle
class’s superclass,
Vehicle
, and ensures that all of
the inherited properties are initialized by
Vehicle
before
Bicycle
tries to modify them.
NOTE
Unlike Objective-C, initializers are not inherited by default in Swift. For more information, see
Initializer Inheritance and Overriding
.
The default value of
maxPassengers
provided by
Vehicle
is already correct for a bicycle, and so it is not
changed within the initializer for
Bicycle
. The original value of
numberOfWheels
is not correct, however,
and is replaced with a new value of
2
.
As well as inheriting the properties of
Vehicle
,
Bicycle
also inherits its methods. If you create an instance
of
Bicycle
, you can call its inherited
description
method to see how its properties have been updated:
1
let

bicycle
=
Bicycle
()
2
println
(
"Bicycle:
\(
bicycle
.
description
())
"
)
3
// Bicycle: 2 wheels; up to 1 passengers
Subclasses can themselves be subclassed:
1
class

Tandem
:
Bicycle
{
2

init
() {
3

super
.
init
()
4

maxPassengers
=
2
5
}
6
}

This example creates a subclass of
Bicycle
for a two-seater bicycle known as a “tandem”.
Tandem
inherits
the two properties from
Bicycle
, which in turn inherits these properties from
Vehicle
.
Tandem
doesn’t
change the number of wheels—it’s still a bicycle, after all—but it does update
maxPassengers
to have the
correct value for a tandem.
NOTE
Subclasses are only allowed to modify
variable
properties of superclasses during initialization. You
can’t modify inherited constant properties of subclasses.
Creating an instance of
Tandem
and printing its description shows how its properties have been updated:
1
let

tandem
=
Tandem
()
2
println
(
"Tandem:
\(
tandem
.
description
())
"
)
3
// Tandem: 2 wheels; up to 2 passengers
Note that the
description
method is also inherited by
Tandem
. Instance methods of a class are inherited
by any and all subclasses of that class.
​
Overriding
A subclass can provide its own custom implementation of an instance method, class method, instance property,
or subscript that it would otherwise inherit from a superclass. This is known as
overriding
.
To override a characteristic that would otherwise be inherited, you prefix your overriding definition with the
override
keyword. Doing so clarifies that you intend to provide an override and have not provided a matching
definition by mistake. Overriding by accident can cause unexpected behavior, and any overrides without the
override
keyword are diagnosed as an error when your code is compiled.
The
override
keyword also prompts the Swift compiler to check that your overriding class’s superclass (or

one of its parents) has a declaration that matches the one you provided for the override. This check ensures that
your overriding definition is correct.
​
Accessing Superclass Methods, Properties, and Subscripts
When you provide a method, property, or subscript override for a subclass, it is sometimes useful to use the
existing superclass implementation as part of your override. For example, you can refine the behavior of that
existing implementation or store a modified value in an existing inherited variable.
Where this is appropriate, you access the superclass version of a method, property, or subscript by using the
super
prefix:
​
Overriding Methods
You can override an inherited instance or class method to provide a tailored or alternative implementation of the
method within your subclass.
The following example defines a new subclass of
Vehicle
called
Car
, which overrides the
description
method it inherits from
Vehicle
:
1
class

Car
:
Vehicle
{
2

var

speed
:
Double
=
0.0
3

init
() {
4

super
.
init
()
An overridden method named
someMethod
can call the superclass version of
someMethod
by calling
super.someMethod()
within the overriding method implementation.
An overridden property called
someProperty
can access the superclass version of
someProperty
as
super.someProperty
within the overriding getter or setter
implementation.
An overridden subscript for
someIndex
can access the superclass version of the same
subscript as
super[someIndex]
from within the overriding subscript implementation.

5

maxPassengers
=
5
6

numberOfWheels
=
4
7
}
8

override

func

description
() ->
String
{
9

return

super
.
description
() +
"; "
10
+
"traveling at
\(
speed
)
mph"
11
}
12
}
Car
declares a new stored
Double
property called
speed
. This property defaults to
0.0
, meaning “zero
miles per hour”.
Car
also has a custom initializer, which sets the maximum number of passengers to
5
, and
the default number of wheels to
4
.
Car
overrides its inherited
description
method by providing a method with the same declaration as the
description
method from
Vehicle
. The overriding method definition is prefixed with the
override
keyword.
Rather than providing a completely custom implementation of
description
, the overriding method actually
starts by calling
super.description
to retrieve the description provided by
Vehicle
. It then appends
some additional information about the car’s current speed.
If you create a new instance of
Car
, and print the output of its
description
method, you can see that the
description has indeed changed:
1
let

car
=
Car
()
2
println
(
"Car:
\(
car
.
description
())
"
)
3
// Car: 4 wheels; up to 5 passengers; traveling at 0.0 mph
​
Overriding Properties
You can override an inherited instance or class property to provide your own custom getter and setter for that
property, or to add property observers to enable the overriding property to observe when the underlying property
value changes.

​
Overriding Property Getters and Setters
You can provide a custom getter (and setter, if appropriate) to override
any
inherited property, regardless of
whether the inherited property is implemented as a stored or computed property at its source. The stored or
computed nature of an inherited property is not known by a subclass—it only knows that the inherited property
has a certain name and type. You must always state both the name and the type of the property you are
overriding, to enable the compiler to check that your override matches a superclass property with the same
name and type.
You can present an inherited read-only property as a read-write property by providing both a getter and a setter
in your subclass property override. You cannot, however, present an inherited read-write property as a read-
only property.
NOTE
If you provide a setter as part of a property override, you must also provide a getter for that override.
If you don’t want to modify the inherited property’s value within the overriding getter, you can simply
pass through the inherited value by returning
super.someProperty
from the getter, as in the
SpeedLimitedCar
example below.
The following example defines a new class called
SpeedLimitedCar
, which is a subclass of
Car
. The
SpeedLimitedCar
class represents a car that has been fitted with a speed-limiting device, which prevents
the car from traveling faster than 40mph. You implement this limitation by overriding the inherited
speed
property:
1
class

SpeedLimitedCar
:
Car
{
2

override

var

speed
:
Double
{
3

get
{
4

return

super
.
speed
5
}
6

set
{

7

super
.
speed
=
min
(
newValue
,
40.0
)
8
}
9
}
10
}
Whenever you set the
speed
property of a
SpeedLimitedCar
instance, the property’s setter
implementation checks the new value and limits it to 40mph. It does this by setting the underlying
speed
property of its superclass to be the smaller of
newValue
and
40.0
. The smaller of these two values is
determined by passing them to the
min
function, which is a global function provided by the Swift standard
library. The
min
function takes two or more values and returns the smallest one of those values.
If you try to set the
speed
property of a
SpeedLimitedCar
instance to more than 40mph, and then print the
output of its
description
method, you see that the speed has been limited:
1
let

limitedCar
=
SpeedLimitedCar
()
2
limitedCar
.
speed
=
60.0
3
println
(
"SpeedLimitedCar:
\(
limitedCar
.
description
())
"
)
4
// SpeedLimitedCar: 4 wheels; up to 5 passengers; traveling at 40.0

mph
​
Overriding Property Observers
You can use property overriding to add property observers to an inherited property. This enables you to be
notified when the value of the inherited property changes, regardless of how that property was originally
implemented. For more information on property observers, see
Property Observers
.
NOTE
You cannot add property observers to inherited constant stored properties or inherited read-only
computed properties. The value of these properties cannot be set, and so it is not appropriate to
provide a
willSet
or
didSet
implementation as part of an override.
Note also that you cannot provide both an overriding setter and an overriding property observer. If

you want to observe changes to a property’s value, and you are already providing a custom setter for
that property, you can simply observe any value changes from within the custom setter.
The following example defines a new class called
AutomaticCar
, which is a subclass of
Car
. The
AutomaticCar
class represents a car with an automatic gearbox, which automatically selects an
appropriate gear to use based on the current speed.
AutomaticCar
also provides a custom
description
method to print the current gear.
1
class

AutomaticCar
:
Car
{
2

var

gear
=
1
3

override

var

speed
:
Double
{
4

didSet
{
5

gear
=
Int
(
speed
/
10.0
) +
1
6
}
7
}
8

override

func

description
() ->
String
{
9

return

super
.
description
() +
" in gear
\(
gear
)
"
10
}
11
}
Whenever you set the
speed
property of an
AutomaticCar
instance, the property’s
didSet
observer
automatically sets the
gear
property to an appropriate choice of gear for the new speed. Specifically, the
property observer chooses a gear which is the new
speed
value divided by
10
, rounded down to the nearest
integer, plus
1
. A speed of
10.0
produces a gear of
1
, and a speed of
35.0
produces a gear of
4
:
1
let

automatic
=
AutomaticCar
()
2
automatic
.
speed
=
35.0
3
println
(
"AutomaticCar:
\(
automatic
.
description
())
"
)
4
// AutomaticCar: 4 wheels; up to 5 passengers; traveling at 35.0 mph

in gear 4
​

Preventing Overrides
You can prevent a method, property, or subscript from being overridden by marking it as
final
. Do this by
writing the
@final
attribute before its introducer keyword (such as
@final var
,
@final func
,
@final
class func
, and
@final subscript
).
Any attempts to override a final method, property, or subscript in a subclass are reported as a compile-time
error. Methods, properties or subscripts that you add to a class in an extension can also be marked as final
within the extension’s definition.
You can mark an entire class as final by writing the
@final
attribute before the
class
keyword in its class
definition (
@final class
). Any attempts to subclass a final class will be reported as a compile-time error.

​
​
Initialization
Initialization
is the process of preparing an instance of a class, structure, or enumeration for use. This process
involves setting an initial value for each stored property on that instance and performing any other setup or
initialization that is required before the new instance is ready to for use.
You implement this initialization process by defining
initializers
, which are like special methods that can be
called to create a new instance of a particular type. Unlike Objective-C initializers, Swift initializers do not
return a value. Their primary role is to ensure that new instances of a type are correctly initialized before they
are used for the first time.
Instances of class types can also implement a
deinitializer
, which performs any custom cleanup just before an
instance of that class is deallocated. For more information about deinitializers, see
Deinitialization
.
​
Setting Initial Values for Stored Properties
Classes and structures
must
set all of their stored properties to an appropriate initial value by the time an
instance of that class or structure is created. Stored properties cannot be left in an indeterminate state.
You can set an initial value for a stored property within an initializer, or by assigning a default property value as
part of the property’s definition. These actions are described in the following sections.
NOTE
When you assign a default value to a stored property, or set its initial value within an initializer, the
value of that property is set directly, without calling any property observers.
​

Initializers
Initializers
are called to create a new instance of a particular type. In its simplest form, an initializer is like an
instance method with no parameters, written using the
init
keyword.
The example below defines a new structure called
Fahrenheit
to store temperatures expressed in the
Fahrenheit scale. The
Fahrenheit
structure has one stored property,
temperature
, which is of type
Double
:
1
struct

Fahrenheit
{
2

var

temperature
:
Double
3

init
() {
4

temperature
=
32.0
5
}
6
}
7
var

f
=
Fahrenheit
()
8
println
(
"The default temperature is
\(
f
.
temperature
)
° Fahrenheit"
)
9
// prints "The default temperature is 32.0° Fahrenheit"
The structure defines a single initializer,
init
, with no parameters, which initializes the stored temperature
with a value of
32.0
(the freezing point of water when expressed in the Fahrenheit scale).
​
Default Property Values
You can set the initial value of a stored property from within an initializer, as shown above. Alternatively, specify
a
default property value
as part of the property’s declaration. You specify a default property value by assigning
an initial value to the property when it is defined.
NOTE
If a property always takes the same initial value, provide a default value rather than setting a value
within an initializer. The end result is the same, but the default value ties the property’s initialization
more closely to its declaration. It makes for shorter, clearer initializers and enables you to infer the

type of the property from its default value. The default value also makes it easier for you to take
advantage of default initializers and initializer inheritance, as described later in this chapter.
You can write the
Fahrenheit
structure from above in a simpler form by providing a default value for its
temperature
property at the point that the property is declared:
1
struct

Fahrenheit
{
2

var

temperature
=
32.0
3
}
​
Customizing Initialization
You can customize the initialization process with input parameters and optional property types, or by modifying
constant properties during initialization, as described in the following sections.
​
Initialization Parameters
You can provide
initialization parameters
as part of an initializer’s definition, to define the types and names of
values that customize the initialization process. Initialization parameters have the same capabilities and syntax
as function and method parameters.
The following example defines a structure called
Celsius
, which stores temperatures expressed in the
Celsius scale. The
Celsius
structure implements two custom initializers called
init(fromFahrenheit:)
and
init(fromKelvin:)
, which initialize a new instance of the structure
with a value from a different temperature scale:
1
struct

Celsius
{
2

var

temperatureInCelsius
:
Double
=
0.0
3

init
(
fromFahrenheit

fahrenheit
:
Double
) {
4

temperatureInCelsius
= (
fahrenheit
–
32.0
) /
1.8

5
}
6

init
(
fromKelvin

kelvin
:
Double
) {
7

temperatureInCelsius
=
kelvin
–
273.15
8
}
9
}
10
let

boilingPointOfWater
=
Celsius
(
fromFahrenheit
:
212.0
)
11
// boilingPointOfWater.temperatureInCelsius is 100.0
12
let

freezingPointOfWater
=
Celsius
(
fromKelvin
:
273.15
)
13
// freezingPointOfWater.temperatureInCelsius is 0.0
The first initializer has a single initialization parameter with an external name of
fromFahrenheit
and a
local name of
fahrenheit
. The second initializer has a single initialization parameter with an external name
of
fromKelvin
and a local name of
kelvin
. Both initializers convert their single argument into a value in the
Celsius scale and store this value in a property called
temperatureInCelsius
.
​
Local and External Parameter Names
As with function and method parameters, initialization parameters can have both a local name for use within the
initializer’s body and an external name for use when calling the initializer.
However, initializers do not have an identifying function name before their parentheses in the way that functions
and methods do. Therefore, the names and types of an initializer’s parameters play a particularly important role
in identifying which initializer should be called. Because of this, Swift provides an automatic external name for
every
parameter in an initializer if you don’t provide an external name yourself. This automatic external name is
the same as the local name, as if you had written a hash symbol before every initialization parameter.
NOTE
If you do not want to provide an external name for a parameter in an initializer, provide an
underscore (
_
) as an explicit external name for that parameter to override the default behavior
described above.

The following example defines a structure called
Color
, with three constant properties called
red
,
green
,
and
blue
. These properties store a value between
0.0
and
1.0
to indicate the amount of red, green, and blue
in the color.
Color
provides an initializer with three appropriately named parameters of type
Double
:
1
struct

Color
{
2

let

red
=
0.0
,
green
=
0.0
,
blue
=
0.0
3

init
(
red
:
Double
,
green
:
Double
,
blue
:
Double
) {
4

self
.
red
=
red
5

self
.
green
=
green
6

self
.
blue
=
blue
7
}
8
}
Whenever you create a new
Color
instance, you call its initializer using external names for each of the three
color components:
1
let

magenta
=
Color
(
red
:
1.0
,
green
:
0.0
,
blue
:
1.0
)
Note that it is not possible to call this initializer without using the external names. External names must always
be used in an intializer if they are defined, and omitting them is a compile-time error:
1
let

veryGreen
=
Color
(
0.0
,
1.0
,
0.0
)
2
// this reports a compile-time error – external names are required
​
Optional Property Types
If your custom type has a stored property that is logically allowed to have “no value”—perhaps because its
value cannot be set during initialization, or because it is allowed to have “no value” at some later point—declare
the property with an
optional
type. Properties of optional type are automatically initialized with a value of
nil
,
indicating that the property is deliberately intended to have “no value yet” during initialization.

The following example defines a class called
SurveyQuestion
, with an optional
String
property called
response
:
1
class

SurveyQuestion
{
2

var

text
:
String
3

var

response
:
String
?
4

init
(
text
:
String
) {
5

self
.
text
=
text
6
}
7

func

ask
() {
8

println
(
text
)
9
}
10
}
11
let

cheeseQuestion
=
SurveyQuestion
(
text
:
"Do you like

cheese?"
)
12
cheeseQuestion
.
ask
()
13
// prints "Do you like cheese?"
14
cheeseQuestion
.
response
=
"Yes, I do like cheese."
The response to a survey question cannot be known until it is asked, and so the
response
property is
declared with a type of
String?
, or “optional
String
”. It is automatically assigned a default value of
nil
,
meaning “no string yet”, when a new instance of
SurveyQuestion
is initialized.
​
Modifying Constant Properties During Initialization
You can modify the value of a constant property at any point during initialization, as long as it is set to a definite
value by the time initialization finishes.
NOTE
For class instances, a constant property can only be modified during initialization by the class that
introduces it. It cannot be modified by a subclass.

You can revise the
SurveyQuestion
example from above to use a constant property rather than a variable
property for the
text
property of the question, to indicate that the question does not change once an instance of
SurveyQuestion
is created. Even though the
text
property is now a constant, it can still be set within the
class’s initializer:
1
class

SurveyQuestion
{
2

let

text
:
String
3

var

response
:
String
?
4

init
(
text
:
String
) {
5

self
.
text
=
text
6
}
7

func

ask
() {
8

println
(
text
)
9
}
10
}
11
let

beetsQuestion
=
SurveyQuestion
(
text
:
"How about beets?"
)
12
beetsQuestion
.
ask
()
13
// prints "How about beets?"
14
beetsQuestion
.
response
=
"I also like beets. (But not with

cheese.)"
​
Default Initializers
Swift provides a
default initializer
for any structure or base class that provides default values for all of its
properties and does not provide at least one initializer itself. The default initializer simply creates a new
instance with all of its properties set to their default values.
This example defines a class called
ShoppingListItem
, which encapsulates the name, quantity, and
purchase state of an item in a shopping list:
1
class

ShoppingListItem
{

2

var

name
:
String
?
3

var

quantity
=
1
4

var

purchased
=
false
5
}
6
var

item
=
ShoppingListItem
()
Because all properties of the
ShoppingListItem
class have default values, and because it is a base class
with no superclass,
ShoppingListItem
automatically gains a default initializer implementation that creates
a new instance with all of its properties set to their default values. (The
name
property is an optional
String
property, and so it automatically receives a default value of
nil
, even though this value is not written in the
code.) The example above uses the default initializer for the
ShoppingListItem
class to create a new
instance of the class with initializer syntax, written as
ShoppingListItem()
, and assigns this new
instance to a variable called
item
.
​
Memberwise Initializers for Structure Types
In addition to the default initializers mentioned above, structure types automatically receive a
memberwise
initializer
if they provide default values for all of their stored properties and do not define any of their own custom
initializers.
The memberwise initializer is a shorthand way to initialize the member properties of new structure instances.
Initial values for the properties of the new instance can be passed to the memberwise initializer by name.
The example below defines a structure called
Size
with two properties called
width
and
height
. Both
properties are inferred to be of type
Double
by assigning a default value of
0.0
.
Because both stored properties have a default value, the
Size
structure automatically receives an
init(width:height:)
memberwise initializer, which you can use to initialize a new
Size
instance:
1
struct

Size
{
2

var

width
=
0.0
,
height
=
0.0
3
}
4
let

twoByTwo
=
Size
(
width
:
2.0
,
height
:
2.0
)

​
Initializer Delegation for Value Types
Initializers can call other initializers to perform part of an instance’s initialization. This process, known as
initializer delegation
, avoids duplicating code across multiple initializers.
The rules for how initializer delegation works, and for what forms of delegation are allowed, are different for
value types and class types. Value types (structures and enumerations) do not support inheritance, and so their
initializer delegation process is relatively simple, because they can only delegate to another initializer that they
provide themselves. Classes, however, can inherit from other classes, as described in
Inheritance
. This means
that classes have additional responsibilities for ensuring that all stored properties they inherit are assigned a
suitable value during initialization. These responsibilities are described in
Class Inheritance and Initialization
below.
For value types, you use
self.init
to refer to other initializers from the same value type when writing your
own custom initializers. You can only call
self.init
from within an initializer.
Note that if you define a custom initializer for a value type, you will no longer have access to the default
initializer (or the memberwise structure initializer, if it is a structure) for that type. This constraint prevents a
situation in which you provide a more complex initializer that performs additional essential setup is
circumvented by someone accidentally using one of the automatic initializers instead.
NOTE
If you want your custom value type to be initializable with the default initializer and memberwise
initializer, and also with your own custom initializers, write your custom initializers in an extension
rather than as part of the value type’s original implementation. For more information, see
Extensions
.
The following example defines a custom
Rect
structure to represent a geometric rectangle. The example
requires two supporting structures called
Size
and
Point
, both of which provide default values of
0.0
for all
of their properties:

1
struct

Size
{
2

var

width
=
0.0
,
height
=
0.0
3
}
4
struct

Point
{
5

var

x
=
0.0
,
y
=
0.0
6
}
You can initialize the
Rect
structure below in one of three ways—by using its default zero-initialized
origin
and
size
property values, by providing a specific origin point and size, or by providing a specific center point
and size. These initialization options are represented by three custom initializers that are part of the
Rect
structure’s definition:
1
struct

Rect
{
2

var

origin
=
Point
()
3

var

size
=
Size
()
4

init
() {}
5

init
(
origin
:
Point
,
size
:
Size
) {
6

self
.
origin
=
origin
7

self
.
size
=
size
8
}
9

init
(
center
:
Point
,
size
:
Size
) {
10

let

originX
=
center
.
x
– (
size
.
width
/
2
)
11

let

originY
=
center
.
y
– (
size
.
height
/
2
)
12

self
.
init
(
origin
:
Point
(
x
:
originX
,
y
:
originY
),
size
:

size
)
13
}
14
}
The first
Rect
initializer,
init()
, is functionally the same as the default initializer that the structure would
have received if it did not have its own custom initializers. This initializer has an empty body, represented by an
empty pair of curly braces
{}
, and does not perfom any initialization. Calling this initializer returns a
Rect
instance whose
origin
and
size
properties are both initialized with the default values of
Point(x: 0.0,
y: 0.0)
and
Size(width: 0.0, height: 0.0)
from their property definitions:
1
let

basicRect
=
Rect
()

2
// basicRect's origin is (0.0, 0.0) and its size is (0.0, 0.0)
The second
Rect
initializer,
init(origin:size:)
, is functionally the same as the memberwise initializer
that the structure would have received if it did not have its own custom initializers. This initializer simply
assigns the
origin
and
size
argument values to the appropriate stored properties:
1
let

originRect
=
Rect
(
origin
:
Point
(
x
:
2.0
,
y
:
2.0
),
2

size
:
Size
(
width
:
5.0
,
height
:
5.0
))
3
// originRect's origin is (2.0, 2.0) and its size is (5.0, 5.0)
The third
Rect
initializer,
init(center:size:)
, is slightly more complex. It starts by calculating an
appropriate origin point based on a
center
point and a
size
value. It then calls (or
delegates
) to the
init(origin:size:)
initializer, which stores the new origin and size values in the appropriate properties:
1
let

centerRect
=
Rect
(
center
:
Point
(
x
:
4.0
,
y
:
4.0
),
2

size
:
Size
(
width
:
3.0
,
height
:
3.0
))
3
// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)
The
init(center:size:)
initializer could have assigned the new values of
origin
and
size
to the
appropriate properties itself. However, it is more convenient (and clearer in intent) for the
init(center:size:)
initializer to take advantage of an existing initializer that already provides exactly
that functionality.
NOTE
For an alternative way to write this example without defining the
init()
and
init(origin:size:)
initializers yourself, see
Extensions
.
​
Class Inheritance and Initialization

All of a class’s stored properties—including any properties the class inherits from its superclass—
must
be
assigned an initial value during initialization.
Swift defines two kinds of initializers for class types to help ensure all stored properties receive an initial value.
These are known as designated initializers and convenience initializers.
​
Designated Initializers and Convenience Initializers
Designated initializers
are the primary initializers for a class. A designated initializer fully initializes all
properties introduced by that class and calls an appropriate superclass initializer to continue the initialization
process up the superclass chain.
Classes tend to have very few designated initializers, and it is quite common for a class to have only one.
Designated initializers are “funnel” points through which initialization takes place, and through which the
initialization process continues up the superclass chain.
Every class must have at least one designated initializer. In some cases, this requirement is satisfied by
inheriting one or more designated initializers from a superclass, as described in
Automatic Initializer
Inheritance
below.
Convenience initializers
are secondary, supporting initializers for a class. You can define a convenience
initializer to call a designated initializer from the same class as the convenience initializer with some of the
designated initializer’s parameters set to default values. You can also define a convenience initializer to create
an instance of that class for a specific use case or input value type.
You do not have to provide convenience initializers if your class does not require them. Create convenience
initializers whenever a shortcut to a common initialization pattern will save time or make initialization of the
class clearer in intent.
​
Initializer Chaining
To simplify the relationships between designated and convenience initializers, Swift applies the following three
rules for delegation calls between initializers:

Designated initializers must call a designated initializer from their immediate superclass.
Convenience initializers must call another initializer available in the
same
class.
Convenience initializers must ultimately end up calling a designated initializer.
A simple way to remember this is:
These rules are illustrated in the figure below:
Here, the superclass has a single designated initializer and two convenience initializers. One convenience
initializer calls another convenience initializer, which in turn calls the single designated initializer. This satisfies
Rule 1
Rule 2
Rule 3
Designated initializers must always delegate
up
.
Convenience initializers must always delegate
across
.

rules 2 and 3 from above. The superclass does not itself have a further superclass, and so rule 1 does not apply.
The subclass in this figure has two designated initializers and one convenience initializer. The convenience
initializer must call one of the two designated initializers, because it can only call another initializer from the
same class. This satisfies rules 2 and 3 from above. Both designated initializers must call the single designated
initializer from the superclass, to satisfy rule 1 from above.
NOTE
These rules don’t affect how users of your classes
create
instances of each class. Any initializer in
the diagram above can be used to create a fully-initialized instance of the class they belong to. The
rules only affect how you write the class’s implementation.
The figure below shows a more complex class hierarchy for four classes. It illustrates how the designated
initializers in this hierarchy act as “funnel” points for class initialization, simplifying the interrelationships
among classes in the chain:

​
Two-Phase Initialization

Class initialization in Swift is a two-phase process. In the first phase, each stored property is assigned an initial
value by the class that introduced it. Once the initial state for every stored property has been determined, the
second phase begins, and each class is given the opportunity to customize its stored properties further before
the new instance is considered ready for use.
The use of a two-phase initialization process makes initialization safe, while still giving complete flexibility to
each class in a class hierarchy. Two-phase initialization prevents property values from being accessed before
they are initialized, and prevents property values from being set to a different value by another initializer
unexpectedly.
NOTE
Swift’s two-phase initialization process is similar to initialization in Objective-C. The main
difference is that during phase 1, Objective-C assigns zero or null values (such as
0
or
nil
) to
every property. Swift’s initialization flow is more flexible in that it lets you set custom initial values,
and can cope with types for which
0
or
nil
is not a valid default value.
Swift’s compiler performs four helpful safety-checks to make sure that two-phase initialization is completed
without error:
A designated initializer must ensure that all of the properties introduced by its class are initialized
before it delegates up to a superclass initializer.
As mentioned above, the memory for an object is only considered fully initialized once the initial state of all of its
stored properties is known. In order for this rule to be satisfied, a designated initializer must make sure that all
its own properties are initialized before it hands off up the chain.
A designated initializer must delegate up to a superclass initializer before assigning a value to an
inherited property. If it doesn’t, the new value the designated initializer assigns will be overwritten
by the superclass as part of its own initialization.
Safety check 1
Safety check 2
Safety check 3

A convenience initializer must delegate to another initializer before assigning a value to
any
property (including properties defined by the same class). If it doesn’t, the new value the
convenience initializer assigns will be overwritten by its own class’s designated initializer.
An initializer cannot call any instance methods, read the values of any instance properties, or refer
to
self
as a value until after the first phase of initialization is complete.
The class instance is not fully valid until the first phase ends. Properties can only be accessed, and methods
can only be called, once the class instance is known to be valid at the end of the first phase.
Here’s how two-phase initialization plays out, based on the four safety checks above:
Phase 1
Phase 2
Here’s how phase 1 looks for an initialization call for a hypothetical subclass and superclass:
Safety check 4
A designated or convenience initializer is called on a class.
Memory for a new instance of that class is allocated. The memory is not yet initialized.
A designated initializer for that class confirms that all stored properties introduced by that class
have a value. The memory for these stored properties is now initialized.
The designated initializer hands off to a superclass initializer to perform the same task for its
own stored properties.
This continues up the class inheritance chain until the top of the chain is reached.
Once the top of the chain is reached, and the final class in the chain has ensured that all of its
stored properties have a value, the instance’s memory is considered to be fully initialized, and
phase 1 is complete.
Working back down from the top of the chain, each designated initializer in the chain has the
option to customize the instance further. Initializers are now able to access
self
and can
modify its properties, call its instance methods, and so on.
Finally, any convenience initializers in the chain have the option to customize the instance and to
work with
self
.

In this example, initialization begins with a call to a convenience initializer on the subclass. This convenience
initializer cannot yet modify any properties. It delegates across to a designated initializer from the same class.
The designated initializer makes sure that all of the subclass’s properties have a value, as per safety check 1. It
then calls a designated initializer on its superclass to continue the initialization up the chain.
The superclass’s designated initializer makes sure that all of the superclass properties have a value. There are
no further superclasses to initialize, and so no further delegation is needed.
As soon as all properties of the superclass have an initial value, its memory is considered fully initialized, and
Phase 1 is complete.
Here’s how phase 2 looks for the same initialization call:

The superclass’s designated initializer now has an opportunity to customize the instance further (although it
does not have to).
Once the superclass’s designated initializer is finished, the subclass’s designated initializer can perform
additional customization (although again, it does not have to).
Finally, once the subclass’s designated initializer is finished, the convenience initializer that was originally
called can perform additional customization.
​
Initializer Inheritance and Overriding
Unlike subclasses in Objective-C, Swift subclasses do not not inherit their superclass initializers by default.
Swift’s approach prevents a situation in which a simple initializer from a superclass is automatically inherited
by a more specialized subclass and is used to create a new instance of the subclass that is not fully or correctly
initialized.
If you want your custom subclass to present one or more of the same initializers as its superclass—perhaps to
perform some customization during initialization—you can provide an overriding implementation of the same

initializer within your custom subclass.
If the initializer you are overriding is a
designated
initializer, you can override its implementation in your
subclass and call the superclass version of the initializer from within your overriding version.
If the initializer you are overriding is a
convenience
initializer, your override must call another designated
initializer from its own subclass, as per the rules described above in
Initializer Chaining
.
NOTE
Unlike methods, properties, and subscripts, you do not need to write the
override
keyword when
overriding an initializer.
​
Automatic Initializer Inheritance
As mentioned above, subclasses do not not inherit their superclass initializers by default. However, superclass
initializers
are
automatically inherited if certain conditions are met. In practice, this means that you do not need
to write initializer overrides in many common scenarios, and can inherit your superclass initializers with
minimal effort whenever it is safe to do so.
Assuming that you provide default values for any new properties you introduce in a subclass, the following two
rules apply:
If your subclass doesn’t define any designated initializers, it automatically inherits all of its
superclass designated initializers.
If your subclass provides an implementation of
all
of its superclass designated initializers—either
by inheriting them as per rule 1, or by providing a custom implementation as part of its definition—
then it automatically inherits all of the superclass convenience initializers.
Rule 1
Rule 2

These rules apply even if your subclass adds further convenience initializers.
NOTE
A subclass can implement a superclass designated initializer as a subclass convenience initializer
as part of satisfying rule 2.
​
Syntax for Designated and Convenience Initializers
Designated initializers for classes are written in the same way as simple initializers for value types:
init
(
parameters
) {

statements
}
Convenience initializers are written in the same style, but with the
convenience
keyword placed before the
init
keyword, separated by a space:
convenience

init
(
parameters
) {

statements
}
​
Designated and Convenience Initializers in Action
The following example shows designated initializers, convenience initializers, and automatic initializer

inheritance in action. This example defines a hierarchy of three classes called
Food
,
RecipeIngredient
,
and
ShoppingListItem
, and demonstrates how their initializers interact.
The base class in the hierarchy is called
Food
, which is a simple class to encapsulate the name of a foodstuff.
The
Food
class introduces a single
String
property called
name
and provides two initializers for creating
Food
instances:
1
class

Food
{
2

var

name
:
String
3

init
(
name
:
String
) {
4

self
.
name
=
name
5
}
6

convenience

init
() {
7

self
.
init
(
name
:
"[Unnamed]"
)
8
}
9
}
The figure below shows the initializer chain for the
Food
class:
Classes do not have a default memberwise initializer, and so the
Food
class provides a designated initializer
that takes a single argument called
name
. This initializer can be used to create a new
Food
instance with a
specific name:

1
let

namedMeat
=
Food
(
name
:
"Bacon"
)
2
// namedMeat's name is "Bacon"
The
init(name: String)
initializer from the
Food
class is provided as a
designated
initializer, because
it ensures that all stored properties of a new
Food
instance are fully initialized. The
Food
class does not have
a superclass, and so the
init(name: String)
initializer does not need to call
super.init()
to
complete its initialization.
The
Food
class also provides a
convenience
initializer,
init()
, with no arguments. The
init()
initializer
provides a default placeholder name for a new food by delegating across to the
Food
class’s
init(name:
String)
with a
name
value of
[Unnamed]
:
1
let

mysteryMeat
=
Food
()
2
// mysteryMeat's name is "[Unnamed]"
The second class in the hierarchy is a subclass of
Food
called
RecipeIngredient
. The
RecipeIngredient
class models an ingredient in a cooking recipe. It introduces an
Int
property called
quantity
(in addition to the
name
property it inherits from
Food
) and defines two initializers for creating
RecipeIngredient
instances:
1
class

RecipeIngredient
:
Food
{
2

var

quantity
:
Int
3

init
(
name
:
String
,
quantity
:
Int
) {
4

self
.
quantity
=
quantity
5

super
.
init
(
name
:
name
)
6
}
7

convenience

init
(
name
:
String
) {
8

self
.
init
(
name
:
name
,
quantity
:
1
)
9
}
10
}
The figure below shows the initializer chain for the
RecipeIngredient
class:

The
RecipeIngredient
class has a single designated initializer,
init(name: String, quantity:
Int)
, which can be used to populate all of the properties of a new
RecipeIngredient
instance. This
initializer starts by assigning the passed
quantity
argument to the
quantity
property, which is the only
new property introduced by
RecipeIngredient
. After doing so, the initializer delegates up to the
init(name: String)
initializer of the
Food
class. This process satisfies safety check 1 from
Two-Phase
Initialization
above.
RecipeIngredient
also defines a convenience initializer,
init(name: String)
, which is used to
create a
RecipeIngredient
instance by name alone. This convenience initializer assumes a quantity of
1
for any
RecipeIngredient
instance that is created without an explicit quantity. The definition of this
convenience initializer makes
RecipeIngredient
instances quicker and more convenient to create, and
avoids code duplication when creating several single-quantity
RecipeIngredient
instances. This
convenience initializer simply delegates across to the class’s designated initializer.

Note that the
init(name: String)
convenience initializer provided by
RecipeIngredient
takes the
same parameters as the
init(name: String)

designated
initializer from
Food
. Even though
RecipeIngredient
provides this initializer as a convenience initializer,
RecipeIngredient
has
nonetheless provided an implementation of all of its superclass’s designated initializers. Therefore,
RecipeIngredient
automatically inherits all of its superclass’s convenience initializers too.
In this example, the superclass for
RecipeIngredient
is
Food
, which has a single convenience initializer
called
init()
. This initializer is therefore inherited by
RecipeIngredient
. The inherited version of
init()
functions in exactly the same way as the
Food
version, except that it delegates to the
RecipeIngredient
version of
init(name: String)
rather than the
Food
version.
All three of these initializers can be used to create new
RecipeIngredient
instances:
1
let

oneMysteryItem
=
RecipeIngredient
()
2
let

oneBacon
=
RecipeIngredient
(
name
:
"Bacon"
)
3
let

sixEggs
=
RecipeIngredient
(
name
:
"Eggs"
,
quantity
:
6
)
The third and final class in the hierarchy is a subclass of
RecipeIngredient
called
ShoppingListItem
. The
ShoppingListItem
class models a recipe ingredient as it appears in a
shopping list.
Every item in the shopping list starts out as “unpurchased”. To represent this fact,
ShoppingListItem
introduces a Boolean property called
purchased
, with a default value of
false
.
ShoppingListItem
also
adds a computed
description
property, which provides a textual description of a
ShoppingListItem
instance:
1
class

ShoppingListItem
:
RecipeIngredient
{
2

var

purchased
=
false
3

var

description
:
String
{
4

var

output
=
"
\(
quantity
)
x
\(
name
.
lowercaseString
)
"
5

output
+=
purchased
?
" ✔"
:
" ✘"
6

return

output
7
}
8
}

NOTE
ShoppingListItem
does not define an initializer to provide an initial value for
purchased
,
because items in a shopping list (as modeled here) always start out unpurchased.
Because it provides a default value for all of the properties it introduces and does not define any initializers itself,
ShoppingListItem
automatically inherits
all
of the designated and convenience initializers from its
superclass.
The figure below shows the overall initializer chain for all three classes:

You can use all three of the inherited initializers to create a new
ShoppingListItem
instance:
1
var

breakfastList
= [

2

ShoppingListItem
(),
3

ShoppingListItem
(
name
:
"Bacon"
),
4

ShoppingListItem
(
name
:
"Eggs"
,
quantity
:
6
),
5
]
6
breakfastList
[
0
].
name
=
"Orange juice"
7
breakfastList
[
0
].
purchased
=
true
8
for

item

in

breakfastList
{
9

println
(
item
.
description
)
10
}
11
// 1 x orange juice ✔
12
// 1 x bacon ✘
13
// 6 x eggs ✘
Here, a new array called
breakfastList
is created from an array literal containing three new
ShoppingListItem
instances. The type of the array is inferred to be
ShoppingListItem[]
. After the
array is created, the name of the
ShoppingListItem
at the start of the array is changed from
"
[Unnamed]"
to
"Orange juice"
and it is marked as having been purchased. Printing the description of
each item in the array shows that their default states have been set as expected.
​
Setting a Default Property Value with a Closure or Function
If a stored property’s default value requires some customization or setup, you can use a closure or global
function to provide a customized default value for that property. Whenever a new instance of the type that the
property belongs to is initialized, the closure or function is called, and its return value is assigned as the
property’s default value.
These kinds of closures or functions typically create a temporary value of the same type as the property, tailor
that value to represent the desired initial state, and then return that temporary value to be used as the property’s
default value.
Here’s a skeleton outline of how a closure can be used to provide a default property value:
1
class

SomeClass
{
2

let

someProperty
:
SomeType
= {

3

// create a default value for someProperty inside this closure
4

// someValue must be of the same type as SomeType
5

return

someValue
6
}()
7
}
Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute
the closure immediately. If you omit these parentheses, you are trying to assign the closure itself to the
property, and not the return value of the closure.
NOTE
If you use a closure to initialize a property, remember that the rest of the instance has not yet been
initialized at the point that the closure is executed. This means that you cannot access any other
property values from within your closure, even if those properties have default values. You also
cannot use the implicit
self
property, or call any of the instance’s methods.
The example below defines a structure called
Checkerboard
, which models a board for the game of
Checkers
(also known as
Draughts
):

The game of
Checkers
is played on a ten-by-ten board, with alternating black and white squares. To represent
this game board, the
Checkerboard
structure has a single property called
boardColors
, which is an
array of 100
Bool
values. A value of
true
in the array represents a black square and a value of
false
represents a white square. The first item in the array represents the top left square on the board and the last
item in the array represents the bottom right square on the board.
The
boardColors
array is initialized with a closure to set up its color values:
1
struct

Checkerboard
{
2

let

boardColors
:
Bool
[] = {
3

var

temporaryBoard
=
Bool
[]()
4

var

isBlack
=
false
5

for

i

in

1
…
10
{
6

for

j

in

1
…
10
{

7

temporaryBoard
.
append
(
isBlack
)
8

isBlack
= !
isBlack
9
}
10

isBlack
= !
isBlack
11
}
12

return

temporaryBoard
13
}()
14

func

squareIsBlackAtRow
(
row
:
Int
,
column
:
Int
) ->
Bool
{
15

return

boardColors
[(
row
*
10
) +
column
]
16
}
17
}
Whenever a new
Checkerboard
instance is created, the closure is executed, and the default value of
boardColors
is calculated and returned. The closure in the example above calculates and sets the
appropriate color for each square on the board in a temporary array called
temporaryBoard
, and returns
this temporary array as the closure’s return value once its setup is complete. The returned array value is stored
in
boardColors
and can be queried with the
squareIsBlackAtRow
utility function:
1
let

board
=
Checkerboard
()
2
println
(
board
.
squareIsBlackAtRow
(
0
,
column
:
1
))
3
// prints "true"
4
println
(
board
.
squareIsBlackAtRow
(
9
,
column
:
9
))
5
// prints "false"

​
​
Deinitialization
A
deinitializer
is called immediately before a class instance is deallocated. You write deinitializers with the
deinit
keyword, similar to how intializers are written with the
init
keyword. Deinitializers are only
available on class types.
​
How Deinitialization Works
Swift automatically deallocates your instances when they are no longer needed, to free up resources. Swift
handles the memory management of instances through
automatic reference counting
(
ARC
), as described in
Automatic Reference Counting
. Typically you don’t need to perform manual clean-up when your instances are
deallocated. However, when you are working with your own resources, you might need to perform some
additional clean-up yourself. For example, if you create a custom class to open a file and write some data to it,
you might need to close the file before the class instance is deallocated.
Class definitions can have at most one deinitializer per class. The deinitializer does not take any parameters
and is written without parentheses:
1
deinit
{
2

// perform the deinitialization
3
}
Deinitializers are called automatically, just before instance deallocation takes place. You are not allowed to call
a deinitializer yourself. Superclass deinitializers are inherited by their subclasses, and the superclass
deinitializer is called automatically at the end of a subclass deinitializer implementation. Superclass
deinitializers are always called, even if a subclass does not provide its own deinitializer.
Because an instance is not deallocated until after its deinitializer is called, a deinitializer can access all
properties of the instance it is called on and can modify its behavior based on those properties (such as looking
up the name of a file that needs to be closed).
​

Deinitializers in Action
Here’s an example of a deinitializer in action. This example defines two new types,
Bank
and
Player
, for a
simple game. The
Bank
structure manages a made-up currency, which can never have more than 10,000
coins in circulation. There can only ever be one
Bank
in the game, and so the
Bank
is implemented as a
structure with static properties and methods to store and manage its current state:
1
struct

Bank
{
2

static

var

coinsInBank
=
10_000
3

static

func

vendCoins
(
var

numberOfCoinsToVend
:
Int
) ->
Int
{
4

numberOfCoinsToVend
=
min
(
numberOfCoinsToVend
,
coinsInBank
)
5

coinsInBank
-=
numberOfCoinsToVend
6

return

numberOfCoinsToVend
7
}
8

static

func

receiveCoins
(
coins
:
Int
) {
9

coinsInBank
+=
coins
10
}
11
}
Bank
keeps track of the current number of coins it holds with its
coinsInBank
property. It also offers two
methods—
vendCoins
and
receiveCoins
—to handle the distribution and collection of coins.
vendCoins
checks that there are enough coins in the bank before distributing them. If there are not enough
coins,
Bank
returns a smaller number than the number that was requested (and returns zero if no coins are left
in the bank).
vendCoins
declares
numberOfCoinsToVend
as a variable parameter, so that the number
can be modified within the method’s body without the need to declare a new variable. It returns an integer value
to indicate the actual number of coins that were provided.
The
receiveCoins
method simply adds the received number of coins back into the bank’s coin store.
The
Player
class describes a player in the game. Each player has a certain number of coins stored in their
purse at any time. This is represented by the player’s
coinsInPurse
property:
1
class

Player
{
2

var

coinsInPurse
:
Int

3

init
(
coins
:
Int
) {
4

coinsInPurse
=
Bank
.
vendCoins
(
coins
)
5
}
6

func

winCoins
(
coins
:
Int
) {
7

coinsInPurse
+=
Bank
.
vendCoins
(
coins
)
8
}
9

deinit
{
10

Bank
.
receiveCoins
(
coinsInPurse
)
11
}
12
}
Each
Player
instance is initialized with a starting allowance of a specified number of coins from the bank
during initialization, although a
Player
instance may receive fewer than that number if not enough coins are
available.
The
Player
class defines a
winCoins
method, which retrieves a certain number of coins from the bank and
adds them to the player’s purse. The
Player
class also implements a deinitializer, which is called just before
a
Player
instance is deallocated. Here, the deinitializer simply returns all of the player’s coins to the bank:
1
var

playerOne
:
Player
? =
Player
(
coins
:
100
)
2
println
(
"A new player has joined the game with
\
(
playerOne
!.
coinsInPurse
)
coins"
)
3
// prints "A new player has joined the game with 100 coins"
4
println
(
"There are now
\(
Bank
.
coinsInBank
)
coins left in the bank"
)
5
// prints "There are now 9900 coins left in the bank"
A new
Player
instance is created, with a request for 100 coins if they are available. This
Player
instance is
stored in an optional
Player
variable called
playerOne
. An optional variable is used here, because players
can leave the game at any point. The optional lets you track whether there is currently a player in the game.
Because
playerOne
is an optional, it is qualified with an exclamation mark (
!
) when its
coinsInPurse
property is accessed to print its default number of coins, and whenever its
winCoins
method is called:
1
playerOne
!.
winCoins
(
2_000
)
2
println
(
"PlayerOne won 2000 coins & now has
\(
playerOne
!.
coinsInPurse
)

coins"
)
3
// prints "PlayerOne won 2000 coins & now has 2100 coins"
4
println
(
"The bank now only has
\(
Bank
.
coinsInBank
)
coins left"
)
5
// prints "The bank now only has 7900 coins left"
Here, the player has won 2,000 coins. The player’s purse now contains 2,100 coins, and the bank has only 7,900
coins left.
1
playerOne
=
nil
2
println
(
"PlayerOne has left the game"
)
3
// prints "PlayerOne has left the game"
4
println
(
"The bank now has
\(
Bank
.
coinsInBank
)
coins"
)
5
// prints "The bank now has 10000 coins"
The player has now left the game. This is indicated by setting the optional
playerOne
variable to
nil
,
meaning “no
Player
instance.” At the point that this happens, the
playerOne
variable’s reference to the
Player
instance is broken. No other properties or variables are still referring to the
Player
instance, and so
it is deallocated in order to free up its memory. Just before this happens, its deinitializer is called automatically,
and its coins are returned to the bank.

​
​
Automatic Reference Counting
Swift uses
Automatic Reference Counting
(ARC) to track and manage your app’s memory usage. In most
cases, this means that memory management “just works” in Swift, and you do not need to think about memory
management yourself. ARC automatically frees up the memory used by class instances when those instances
are no longer needed.
However, in a few cases ARC requires more information about the relationships between parts of your code in
order to manage memory for you. This chapter describes those situations and shows how you enable ARC to
manage all of your app’s memory.
NOTE
Reference counting only applies to instances of classes. Structures and enumerations are value
types, not reference types, and are not stored and passed by reference.
​
How ARC Works
Every time you create a new instance of a class, ARC allocates a chunk of memory to store information about
that instance. This memory holds information about the type of the instance, together with the values of any
stored properties associated with that instance.
Additionally, when an instance is no longer needed, ARC frees up the memory used by that instance so that the
memory can be used for other purposes instead. This ensures that class instances do not take up space in
memory when they are no longer needed.
However, if ARC were to deallocate an instance that was still in use, it would no longer be possible to access
that instance’s properties, or call that instance’s methods. Indeed, if you tried to access the instance, your app
would most likely crash.

To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties,
constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as
long as at least one active reference to that instance still exists.
To make this possible, whenever you assign a class instance to a property, constant, or variable, that property,
constant, or variable makes a
strong reference
to the instance. The reference is called a “strong“ reference
because it keeps a firm hold on that instance, and does not allow it to be deallocated for as long as that strong
reference remains.
​
ARC in Action
Here’s an example of how Automatic Reference Counting works. This example starts with a simple class
called
Person
, which defines a stored constant property called
name
:
1
class

Person
{
2

let

name
:
String
3

init
(
name
:
String
) {
4

self
.
name
=
name
5

println
(
"
\(
name
)
is being initialized"
)
6
}
7

deinit
{
8

println
(
"
\(
name
)
is being deinitialized"
)
9
}
10
}
The
Person
class has an initializer that sets the instance’s
name
property and prints a message to indicate
that initialization is underway. The
Person
class also has a deinitializer that prints a message when an
instance of the class is deallocated.
The next code snippet defines three variables of type
Person?
, which are used to set up multiple references to
a new
Person
instance in subsequent code snippets. Because these variables are of an optional type
(
Person?
, not
Person
), they are automatically initialized with a value of
nil
, and do not currently reference
a
Person
instance.

1
var

reference1
:
Person
?
2
var

reference2
:
Person
?
3
var

reference3
:
Person
?
You can now create a new
Person
instance and assign it to one of these three variables:
1
reference1
=
Person
(
name
:
"John Appleseed"
)
2
// prints "John Appleseed is being initialized"
Note that the message
"John Appleseed is being initialized"
is printed at the point that you
call the
Person
class’s initializer. This confirms that initialization has taken place.
Because the new
Person
instance has been assigned to the
reference1
variable, there is now a strong
reference from
reference1
to the new
Person
instance. Because there is at least one strong reference,
ARC makes sure that this
Person
is kept in memory and is not deallocated.
If you assign the same
Person
instance to two more variables, two more strong references to that instance
are established:
1
reference2
=
reference1
2
reference3
=
reference1
There are now
three
strong references to this single
Person
instance.
If you break two of these strong references (including the original reference) by assigning
nil
to two of the
variables, a single strong reference remains, and the
Person
instance is not deallocated:
1
reference1
=
nil
2
reference2
=
nil
ARC does not deallocate the
Person
instance until the third and final strong reference is broken, at which point
it is clear that you are no longer using the
Person
instance:

1
reference3
=
nil
2
// prints "John Appleseed is being deinitialized"
​
Strong Reference Cycles Between Class Instances
In the examples above, ARC is able to track the number of references to the new
Person
instance you create
and to deallocate that
Person
instance when it is no longer needed.
However, it is possible to write code in which an instance of a class
never
gets to a point where it has zero
strong references. This can happen if two class instances hold a strong reference to each other, such that each
instance keeps the other alive. This is known as a
strong reference cycle
.
You resolve strong reference cycles by defining some of the relationships between classes as weak or
unowned references instead of as strong references. This process is described in
Resolving Strong Reference
Cycles Between Class Instances
. However, before you learn how to resolve a strong reference cycle, it is
useful to understand how such a cycle is caused.
Here’s an example of how a strong reference cycle can be created by accident. This example defines two
classes called
Person
and
Apartment
, which model a block of apartments and its residents:
1
class

Person
{
2

let

name
:
String
3

init
(
name
:
String
) {
self
.
name
=
name
}
4

var

apartment
:
Apartment
?
5

deinit
{
println
(
"
\(
name
)
is being deinitialized"
) }
6
}
7

8
class

Apartment
{
9

let

number
:
Int
10

init
(
number
:
Int
) {
self
.
number
=
number
}
11

var

tenant
:
Person
?
12

deinit
{
println
(
"Apartment #
\(
number
)
is being

deinitialized"
) }
13
}

Every
Person
instance has a
name
property of type
String
and an optional
apartment
property that is
initially
nil
. The
apartment
property is optional, because a person may not always have an apartment.
Similarly, every
Apartment
instance has a
number
property of type
Int
and has an optional
tenant
property that is initially
nil
. The tenant property is optional because an apartment may not always have a
tenant.
Both of these classes also define a deinitializer, which prints the fact that an instance of that class is being
deinitialized. This enables you to see whether instances of
Person
and
Apartment
are being deallocated as
expected.
This next code snippet defines two variables of optional type called
john
and
number73
, which will be set to
a specific
Apartment
and
Person
instance below. Both of these variables have an initial value of
nil
, by
virtue of being optional:
1
var

john
:
Person
?
2
var

number73
:
Apartment
?
You can now create a specific
Person
instance and
Apartment
instance and assign these new instances to
the
john
and
number73
variables:
1
john
=
Person
(
name
:
"John Appleseed"
)
2
number73
=
Apartment
(
number
:
73
)
Here’s how the strong references look after creating and assigning these two instances. The
john
variable
now has a strong reference to the new
Person
instance, and the
number73
variable has a strong reference to
the new
Apartment
instance:

You can now link the two instances together so that the person has an apartment, and the apartment has a
tenant. Note that an exclamation mark (
!
) is used to unwrap and access the instances stored inside the
john
and
number73
optional variables, so that the properties of those instances can be set:
1
john
!.
apartment
=
number73
2
number73
!.
tenant
=
john
Here’s how the strong references look after you link the two instances together:

Unfortunately, linking these two instances creates a strong reference cycle between them. The
Person
instance now has a strong reference to the
Apartment
instance, and the
Apartment
instance has a strong
reference to the
Person
instance. Therefore, when you break the strong references held by the
john
and
number73
variables, the reference counts do not drop to zero, and the instances are not deallocated by ARC:
1
john
=
nil
2
number73
=
nil
Note that neither deinitializer was called when you set these two variables to
nil
. The strong reference cycle
prevents the
Person
and
Apartment
instances from ever being deallocated, causing a memory leak in your
app.
Here’s how the strong references look after you set the
john
and
number73
variables to
nil
:

The strong references between the
Person
instance and the
Apartment
instance remain and cannot be
broken.
​
Resolving Strong Reference Cycles Between Class Instances
Swift provides two ways to resolve strong reference cycles when you work with properties of class type: weak
references and unowned references.
Weak and unowned references enable one instance in a reference cycle to refer to the other instance
without
keeping a strong hold on it. The instances can then refer to each other without creating a strong reference cycle.
Use a weak reference whenever it is valid for that reference to become
nil
at some point during its lifetime.
Conversely, use an unowned reference when you know that the reference will never be
nil
once it has been
set during initialization.
​
Weak References

A
weak reference
is a reference that does not keep a strong hold on the instance it refers to, and so does not
stop ARC from disposing of the referenced instance. This behavior prevents the reference from becoming part
of a strong reference cycle. You indicate a weak reference by placing the
weak
keyword before a property or
variable declaration.
Use a weak reference to avoid reference cycles whenever it is possible for that reference to have “no value” at
some point in its life. If the reference will
always
have a value, use an unowned reference instead, as described
in
Unowned References
. In the
Apartment
example above, it is appropriate for an apartment to be able to
have “no tenant” at some point in its lifetime, and so a weak reference is an appropriate way to break the
reference cycle in this case.
NOTE
Weak references must be declared as variables, to indicate that their value can change at runtime. A
weak reference cannot be declared as a constant.
Because weak references are allowed to have “no value”, you must declare every weak reference as having an
optional type. Optional types are the preferred way to represent the possibility for “no value” in Swift.
Because a weak reference does not keep a strong hold on the instance it refers to, it is possible for that instance
to be deallocated while the weak reference is still referring to it. Therefore, ARC automatically sets a weak
reference to
nil
when the instance that it refers to is deallocated. You can check for the existence of a value in
the weak reference, just like any other optional value, and you will never end up with a reference to an invalid
instance that no longer exists.
The example below is identical to the
Person
and
Apartment
example from above, with one important
difference. This time around, the
Apartment
type’s
tenant
property is declared as a weak reference:
1
class

Person
{
2

let

name
:
String
3

init
(
name
:
String
) {
self
.
name
=
name
}
4

var

apartment
:
Apartment
?
5

deinit
{
println
(
"
\(
name
)
is being deinitialized"
) }

6
}
7

8
class

Apartment
{
9

let

number
:
Int
10

init
(
number
:
Int
) {
self
.
number
=
number
}
11

weak

var

tenant
:
Person
?
12

deinit
{
println
(
"Apartment #
\(
number
)
is being

deinitialized"
) }
13
}
The strong references from the two variables (
john
and
number73
) and the links between the two instances
are created as before:
1
var

john
:
Person
?
2
var

number73
:
Apartment
?
3

4
john
=
Person
(
name
:
"John Appleseed"
)
5
number73
=
Apartment
(
number
:
73
)
6

7
john
!.
apartment
=
number73
8
number73
!.
tenant
=
john
Here’s how the references look now that you’ve linked the two instances together:

The
Person
instance still has a strong reference to the
Apartment
instance, but the
Apartment
instance
now has a
weak
reference to the
Person
instance. This means that when you break the strong reference held
by the
john
variables, there are no more strong references to the
Person
instance:

Because there are no more strong references to the
Person
instance, it is deallocated:
1
john
=
nil
2
// prints "John Appleseed is being deinitialized"
The only remaining strong reference to the
Apartment
instance is from the
number73
variable. If you break
that
strong reference, there are no more strong references to the
Apartment
instance:
Because there are no more strong references to the
Apartment
instance, it too is deallocated:
1
number73
=
nil
2
// prints "Apartment #73 is being deinitialized"
The final two code snippets above show that the deinitializers for the
Person
instance and
Apartment
instance print their “deinitialized” messages after the
john
and
number73
variables are set to
nil
. This
proves that the reference cycle has been broken.
​

Unowned References
Like weak references, an
unowned reference
does not keep a strong hold on the instance it refers to. Unlike a
weak reference, however, an unowned reference is assumed to
always
have a value. Because of this, an
unowned reference is always defined as a non-optional type. You indicate an unowned reference by placing the
unowned
keyword before a property or variable declaration.
Because an unowned reference is non-optional, you don’t need to unwrap the unowned reference each time it is
used. An unowned reference can always be accessed directly. However, ARC cannot set the reference to
nil
when the instance it refers to is deallocated, because variables of a non-optional type cannot be set to
nil
.
NOTE
If you try to access an unowned reference after the instance that it references is deallocated, you will
trigger a runtime error. Use unowned references only when you are sure that the reference will
always
refer to an instance.
Note also that Swift guarantees your app will crash if you try to access an unowned reference after
the instance it references is deallocated. You will never encounter unexpected behavior in this
situation. Your app will always crash reliably, although you should, of course, prevent it from doing
so.
The following example defines two classes,
Customer
and
CreditCard
, which model a bank customer and
a possible credit card for that customer. These two classes each store an instance of the other class as a
property. This relationship has the potential to create a strong reference cycle.
The relationship between
Customer
and
CreditCard
is slightly different from the relationship between
Apartment
and
Person
seen in the weak reference example above. In this data model, a customer may or
may not have a credit card, but a credit card will
always
be associated with a customer. To represent this, the
Customer
class has an optional
card
property, but the
CreditCard
class has a non-optional
customer
property.
Furthermore, a new
CreditCard
instance can
only
be created by passing a
number
value and a
customer
instance to a custom
CreditCard
initializer. This ensures that a
CreditCard
instance always
has a
customer
instance associated with it when the
CreditCard
instance is created.

Because a credit card will always have a customer, you define its
customer
property as an unowned
reference, to avoid a strong reference cycle:
1
class

Customer
{
2

let

name
:
String
3

var

card
:
CreditCard
?
4

init
(
name
:
String
) {
5

self
.
name
=
name
6
}
7

deinit
{
println
(
"
\(
name
)
is being deinitialized"
) }
8
}
9

10
class

CreditCard
{
11

let

number
:
Int
12

unowned

let

customer
:
Customer
13

init
(
number
:
Int
,
customer
:
Customer
) {
14

self
.
number
=
number
15

self
.
customer
=
customer
16
}
17

deinit
{
println
(
"Card #
\(
number
)
is being deinitialized"
)

}
18
}
This next code snippet defines an optional
Customer
variable called
john
, which will be used to store a
reference to a specific customer. This variable has an initial value of nil, by virtue of being optional:
1
var

john
:
Customer
?
You can now create a
Customer
instance, and use it to initialize and assign a new
CreditCard
instance as
that customer’s
card
property:
1
john
=
Customer
(
name
:
"John Appleseed"
)
2
john
!.
card
=
CreditCard
(
number
:
1234_5678_9012_3456
,
customer
:
john
!)

Here’s how the references look, now that you’ve linked the two instances:
The
Customer
instance now has a strong reference to the
CreditCard
instance, and the
CreditCard
instance has an unowned reference to the
Customer
instance.
Because of the unowned
customer
reference, when you break the strong reference held by the
john
variable, there are no more strong references to the
Customer
instance:

Because there are no more strong references to the
Customer
instance, it is deallocated. After this happens,
there are no more strong references to the
CreditCard
instance, and it too is deallocated:
1
john
=
nil
2
// prints "John Appleseed is being deinitialized"
3
// prints "Card #1234567890123456 is being deinitialized"
The final code snippet above shows that the deinitializers for the
Customer
instance and
CreditCard
instance both print their “deinitialized” messages after the
john
variable is set to
nil
.
​
Unowned References and Implicitly Unwrapped Optional
Properties
The examples for weak and unowned references above cover two of the more common scenarios in which it is
necessary to break a strong reference cycle.
The
Person
and
Apartment
example shows a situation where two properties, both of which are allowed to
be
nil
, have the potential to cause a strong reference cycle. This scenario is best resolved with a weak

reference.
The
Customer
and
CreditCard
example shows a situation where one property that is allowed to be
nil
and another property that cannot be
nil
have the potential to cause a strong reference cycle. This scenario is
best resolved with an unowned reference.
However, there is a third scenario, in which
both
properties should always have a value, and neither property
should ever be
nil
once initialization is complete. In this scenario, it is useful to combine an unowned property
on one class with an implicitly unwrapped optional property on the other class.
This enables both properties to be accessed directly (without optional unwrapping) once initialization is
complete, while still avoiding a reference cycle. This section shows you how to set up such a relationship.
The example below defines two classes,
Country
and
City
, each of which stores an instance of the other
class as a property. In this data model, every country must always have a capital city, and every city must
always belong to a country. To represent this, the
Country
class has a
capitalCity
property, and the
City
class has a
country
property:
1
class

Country
{
2

let

name
:
String
3

let

capitalCity
:
City
!
4

init
(
name
:
String
,
capitalName
:
String
) {
5

self
.
name
=
name
6

self
.
capitalCity
=
City
(
name
:
capitalName
,
country
:
self
)
7
}
8
}
9

10
class

City
{
11

let

name
:
String
12

unowned

let

country
:
Country
13

init
(
name
:
String
,
country
:
Country
) {
14

self
.
name
=
name
15

self
.
country
=
country
16
}
17
}

To set up the interdependency between the two classes, the initializer for
City
takes a
Country
instance, and
stores this instance in its
country
property.
The initializer for
City
is called from within the initializer for
Country
. However, the initializer for
Country
cannot pass
self
to the
City
initializer until a new
Country
instance is fully initialized, as described in
Two-Phase Initialization
.
To cope with this requirement, you declare the
capitalCity
property of
Country
as an implicitly
unwrapped optional property, indicated by the exclamation mark at the end of its type annotation (
City!
). This
means that the
capitalCity
property has a default value of
nil
, like any other optional, but can be
accessed without the need to unwrap its value as described in
Implicitly Unwrapped Optionals
.
Because
capitalCity
has a default
nil
value, a new
Country
instance is considered fully initialized as
soon as the
Country
instance sets its
name
property within its initializer. This means that the
Country
initializer can start to reference and pass around the implicit
self
property as soon as the
name
property is
set. The
Country
initializer can therefore pass
self
as one of the parameters for the
City
initializer when
the
Country
initializer is setting its own
capitalCity
property.
All of this means that you can create the
Country
and
City
instances in a single statement, without creating
a strong reference cycle, and the
capitalCity
property can be accessed directly, without needing to use an
exclamation mark to unwrap its optional value:
1
var

country
=
Country
(
name
:
"Canada"
,
capitalName
:
"Ottawa"
)
2
println
(
"
\(
country
.
name
)
's capital city is called
\
(
country
.
capitalCity
.
name
)
"
)
3
// prints "Canada's capital city is called Ottawa"
In the example above, the use of an implicitly unwrapped optional means that all of the two-phase class
initializer requirements are satisfied. The
capitalCity
property can be used and accessed like a non-
optional value once initialization is complete, while still avoiding a strong reference cycle.
​
Strong Reference Cycles for Closures
You saw above how a strong reference cycle can be created when two class instance properties hold a strong

reference to each other. You also saw how to use weak and unowned references to break these strong
reference cycles.
A strong reference cycle can also occur if you assign a closure to a property of a class instance, and the body of
that closure captures the instance. This capture might occur because the closure’s body accesses a property of
the instance, such as
self.someProperty
, or because the closure calls a method on the instance, such as
self.someMethod()
. In either case, these accesses cause the closure to “capture”
self
, creating a
strong reference cycle.
This strong reference cycle occurs because closures, like classes, are
reference types
. When you assign a
closure to a property, you are assigning a
reference
to that closure. In essence, it’s the same problem as above
—two strong references are keeping each other alive. However, rather than two class instances, this time it’s a
class instance and a closure that are keeping each other alive.
Swift provides an elegant solution to this problem, known as a
closure capture list
. However, before you learn
how to break a strong reference cycle with a closure capture list, it is useful to understand how such a cycle can
be caused.
The example below shows how you can create a strong reference cycle when using a closure that references
self
. This example defines a class called
HTMLElement
, which provides a simple model for an individual
element within an HTML document:
1
class

HTMLElement
{
2

3

let

name
:
String
4

let

text
:
String
?
5

6

@lazy

var

asHTML
: () ->
String
= {
7

if

let

text
=
self
.
text
{
8

return

"<
\(
self
.
name
)
>
\(
text
)
</
\(
self
.
name
)
>"
9
}
else
{
10

return

"<
\(
self
.
name
)
/>"
11
}
12
}
13

14

init
(
name
:
String
,
text
:
String
? =
nil
) {
15

self
.
name
=
name

16

self
.
text
=
text
17
}
18

19

deinit
{
20

println
(
"
\(
name
)
is being deinitialized"
)
21
}
22

23
}
The
HTMLElement
class defines a
name
property, which indicates the name of the element, such as
"p"
for
a paragraph element, or
"br"
for a line break element.
HTMLElement
also defines an optional
text
property, which you can set to a string that represents the text to be rendered within that HTML element.
In addition to these two simple properties, the
HTMLElement
class defines a lazy property called
asHTML
.
This property references a closure that combines
name
and
text
into an HTML string fragment. The
asHTML
property is of type
() -> String
, or “a function that takes no parameters, and returns a
String
value”.
By default, the
asHTML
property is assigned a closure that returns a string representation of an HTML tag. This
tag contains the optional
text
value if it exists, or no text content if
text
does not exist. For a paragraph
element, the closure would return
"<p>some text</p>"
or
"<p />"
, depending on whether the
text
property equals
"some text"
or
nil
.
The
asHTML
property is named and used somewhat like an instance method. However, because
asHTML
is a
closure property rather than an instance method, you can replace the default value of the
asHTML
property with
a custom closure, if you want to change the HTML rendering for a particular HTML element.
NOTE
The
asHTML
property is declared as a lazy property, because it is only needed if and when the
element actually needs to be rendered as a string value for some HTML output target. The fact that
asHTML
is a lazy property means that you can refer to
self
within the default closure, because the
lazy property will not be accessed until after initialization has been completed and
self
is known to
exist.

The
HTMLElement
class provides a single initializer, which takes a
name
argument and (if desired) a
text
argument to initialize a new element. The class also defines a deinitializer, which prints a message to show
when an
HTMLElement
instance is deallocated.
Here’s how you use the
HTMLElement
class to create and print a new instance:
1
var

paragraph
:
HTMLElement
? =
HTMLElement
(
name
:
"p"
,
text
:
"hello,

world"
)
2
println
(
paragraph
!.
asHTML
())
3
// prints "<p>hello, world</p>"
NOTE
The
paragraph
variable above is defined as an
optional

HTMLElement
, so that it can be set to
nil
below to demonstrate the presence of a strong reference cycle.
Unfortunately, the
HTMLElement
class, as written above, creates a strong reference cycle between an
HTMLElement
instance and the closure used for its default
asHTML
value. Here’s how the cycle looks:

The instance’s
asHTML
property holds a strong reference to its closure. However, because the closure refers
to
self
within its body (as a way to reference
self.name
and
self.text
), the closure
captures
self,
which means that it holds a strong reference back to the
HTMLElement
instance. A strong reference cycle is
created between the two. (For more information about capturing values in a closure, see
Capturing Values
.)
NOTE
Even though the closure refers to
self
multiple times, it only captures one strong reference to the
HTMLElement
instance.
If you set the
paragraph
variable to
nil
and break its strong reference to the
HTMLElement
instance,
neither the
HTMLElement
instance nor its closure are deallocated, because of the strong reference cycle:
1
paragraph
=
nil

Note that the message in the
HTMLElement
deinitializer is not printed, which shows that the
HTMLElement
instance is not deallocated.
​
Resolving Strong Reference Cycles for Closures
You resolve a strong reference cycle between a closure and a class instance by defining a
capture list
as part of
the closure’s definition. A capture list defines the rules to use when capturing one or more reference types
within the closure’s body. As with strong reference cycles between two class instances, you declare each
captured reference to be a weak or unowned reference rather than a strong reference. The appropriate choice of
weak or unowned depends on the relationships between the different parts of your code.
NOTE
Swift requires you to write
self.someProperty
or
self.someMethod
(rather than just
someProperty
or
someMethod
) whenever you refer to a member of
self
within a closure.
This helps you remember that it’s possible to capture
self
by accident.
​
Defining a Capture List
Each item in a capture list is a pairing of the
weak
or
unowned
keyword with a reference to a class instance
(such as
self
or
someInstance
). These pairings are written within a pair of square braces, separated by
commas.
Place the capture list before a closure’s parameter list and return type if they are provided:
1
@lazy

var

someClosure
: (
Int
,
String
) ->
String
= {
2
[
unowned

self
] (
index
:
Int
,
stringToProcess
:
String
) ->
String

in
3

// closure body goes here
4
}

If a closure does not specify a parameter list or return type because they can be inferred from context, place the
capture list at the very start of the closure, followed by the
in
keyword:
1
@lazy

var

someClosure
: () ->
String
= {
2
[
unowned

self
]
in
3

// closure body goes here
4
}
​
Weak and Unowned References
Define a capture in a closure as an unowned reference when the closure and the instance it captures will
always refer to each other, and will always be deallocated at the same time.
Conversely, define a capture as a weak reference when the captured reference may become
nil
at some point
in the future. Weak references are always of an optional type, and automatically become
nil
when the instance
they reference is deallocated. This enables you to check for their existence within the closure’s body.
NOTE
If the captured reference will never become
nil
, it should always be captured as an unowned
reference, rather than a weak reference.
An unowned reference is the appropriate capture method to use to resolve the strong reference cycle in the
HTMLElement
example from earlier. Here’s how you write the
HTMLElement
class to avoid the cycle:
1
class

HTMLElement
{
2

3

let

name
:
String
4

let

text
:
String
?
5

6

@lazy

var

asHTML
: () ->
String
= {
7
[
unowned

self
]
in
8

if

let

text
=
self
.
text
{
9

return

"<
\(
self
.
name
)
>
\(
text
)
</
\(
self
.
name
)
>"
10
}
else
{
11

return

"<
\(
self
.
name
)
/>"
12
}
13
}
14

15

init
(
name
:
String
,
text
:
String
? =
nil
) {
16

self
.
name
=
name
17

self
.
text
=
text
18
}
19

20

deinit
{
21

println
(
"
\(
name
)
is being deinitialized"
)
22
}
23

24
}
This implementation of
HTMLElement
is identical to the previous implementation, apart from the addition of a
capture list within the
asHTML
closure. In this case, the capture list is
[unowned self]
, which means
“capture self as an unowned reference rather than a strong reference”.
You can create and print an
HTMLElement
instance as before:
1
var

paragraph
:
HTMLElement
? =
HTMLElement
(
name
:
"p"
,
text
:
"hello,

world"
)
2
println
(
paragraph
!.
asHTML
())
3
// prints "<p>hello, world</p>"
Here’s how the references look with the capture list in place:

This time, the capture of
self
by the closure is an unowned reference, and does not keep a strong hold on the
HTMLElement
instance it has captured. If you set the strong reference from the
paragraph
variable to
nil
,
the
HTMLElement
instance is deallocated, as can be seen from the printing of its deinitializer message in the
example below:
1
paragraph
=
nil
2
// prints "p is being deinitialized"

​
​
Optional Chaining
Optional chaining
is a process for querying and calling properties, methods, and subscripts on an optional that
might currently be
nil
. If the optional contains a value, the property, method, or subscript call succeeds; if the
optional is
nil
, the property, method, or subscript call returns
nil
. Multiple queries can be chained together,
and the entire chain fails gracefully if any link in the chain is
nil
.
NOTE
Optional chaining in Swift is similar to messaging
nil
in Objective-C, but in a way that works for
any type, and that can be checked for success or failure.
​
Optional Chaining as an Alternative to Forced Unwrapping
You specify optional chaining by placing a question mark (
?
) after the optional value on which you wish to call a
property, method or subscript if the optional is non-
nil
. This is very similar to placing an exclamation mark
(
!
) after an optional value to force the unwrapping of its value. The main difference is that optional chaining fails
gracefully when the optional is
nil
, whereas forced unwrapping triggers a runtime error when the optional is
nil
.
To reflect the fact that optional chaining can be called on a
nil
value, the result of an optional chaining call is
always an optional value, even if the property, method, or subscript you are querying returns a non-optional
value. You can use this optional return value to check whether the optional chaining call was successful (the
returned optional contains a value), or did not succeed due to a
nil
value in the chain (the returned optional
value is
nil
).
Specifically, the result of an optional chaining call is of the same type as the expected return value, but wrapped
in an optional. A property that normally returns an
Int
will return an
Int?
when accessed through optional

chaining.
The next several code snippets demonstrate how optional chaining differs from forced unwrapping and enables
you to check for success.
First, two classes called
Person
and
Residence
are defined:
1
class

Person
{
2

var

residence
:
Residence
?
3
}
4

5
class

Residence
{
6

var

numberOfRooms
=
1
7
}
Residence
instances have a single
Int
property called
numberOfRooms
, with a default value of
1
.
Person
instances have an optional
residence
property of type
Residence?
.
If you create a new
Person
instance, its
residence
property is default initialized to
nil
, by virtue of being
optional. In the code below,
john
has a
residence
property value of
nil
:
1
let

john
=
Person
()
If you try to access the
numberOfRooms
property of this person’s
residence
, by placing an exclamation
mark after
residence
to force the unwrapping of its value, you trigger a runtime error, because there is no
residence
value to unwrap:
1
let

roomCount
=
john
.
residence
!.
numberOfRooms
2
// this triggers a runtime error
The code above succeeds when
john.residence
has a non-
nil
value and will set
roomCount
to an
Int
value containing the appropriate number of rooms. However, this code always triggers a runtime error when
residence
is
nil
, as illustrated above.

Optional chaining provides an alternative way to access the value of
numberOfRooms
. To use optional
chaining, use a question mark in place of the exclamation mark:
1
if

let

roomCount
=
john
.
residence
?.
numberOfRooms
{
2

println
(
"John's residence has
\(
roomCount
)
room(s)."
)
3
}
else
{
4

println
(
"Unable to retrieve the number of rooms."
)
5
}
6
// prints "Unable to retrieve the number of rooms."
This tells Swift to “chain” on the optional
residence
property and to retrieve the value of
numberOfRooms
if
residence
exists.
Because the attempt to access
numberOfRooms
has the potential to fail, the optional chaining attempt returns
a value of type
Int?
, or “optional
Int
”. When
residence
is
nil
, as in the example above, this optional
Int
will also be
nil
, to reflect the fact that it was not possible to access
numberOfRooms
.
Note that this is true even though
numberOfRooms
is a non-optional
Int
. The fact that it is queried through an
optional chain means that the call to
numberOfRooms
will always return an
Int?
instead of an
Int
.
You can assign a
Residence
instance to
john.residence
, so that it no longer has a
nil
value:
1
john
.
residence
=
Residence
()
john.residence
now contains an actual
Residence
instance, rather than
nil
. If you try to access
numberOfRooms
with the same optional chaining as before, it will now return an
Int?
that contains the
default
numberOfRooms
value of
1
:
1
if

let

roomCount
=
john
.
residence
?.
numberOfRooms
{
2

println
(
"John's residence has
\(
roomCount
)
room(s)."
)
3
}
else
{
4

println
(
"Unable to retrieve the number of rooms."
)
5
}
6
// prints "John's residence has 1 room(s)."

​
Defining Model Classes for Optional Chaining
You can use optional chaining with calls to properties, methods, and subscripts that are more than one level
deep. This enables you to drill down into subproperties within complex models of interrelated types, and to
check whether it is possible to access properties, methods, and subscripts on those subproperties.
The code snippets below define four model classes for use in several subsequent examples, including
examples of multilevel optional chaining. These classes expand upon the
Person
and
Residence
model
from above by adding a
Room
and
Address
class, with associated properties, methods, and subscripts.
The
Person
class is defined in the same way as before:
1
class

Person
{
2

var

residence
:
Residence
?
3
}
The
Residence
class is more complex than before. This time, the
Residence
class defines a variable
property called
rooms
, which is initialized with an empty array of type
Room[]
:
1
class

Residence
{
2

var

rooms
=
Room
[]()
3

var

numberOfRooms
:
Int
{
4

return

rooms
.
count
5
}
6

subscript
(
i
:
Int
) ->
Room
{
7

return

rooms
[
i
]
8
}
9

func

printNumberOfRooms
() {
10

println
(
"The number of rooms is
\(
numberOfRooms
)
"
)
11
}
12

var

address
:
Address
?
13
}

Because this version of
Residence
stores an array of
Room
instances, its
numberOfRooms
property is
implemented as a computed property, not a stored property. The computed
numberOfRooms
property simply
returns the value of the
count
property from the
rooms
array.
As a shortcut to accessing its
rooms
array, this version of
Residence
provides a read-only subscript, which
starts by asserting that the index passed to the subscript is valid. If the index is valid, the subscript returns the
room at the requested index in the
rooms
array.
This version of
Residence
also provides a method called
printNumberOfRooms
, which simply prints the
number of rooms in the residence.
Finally,
Residence
defines an optional property called
address
, with a type of
Address?
. The
Address
class type for this property is defined below.
The
Room
class used for the
rooms
array is a simple class with one property called
name
, and an initializer to
set that property to a suitable room name:
1
class

Room
{
2

let

name
:
String
3

init
(
name
:
String
) {
self
.
name
=
name
}
4
}
The final class in this model is called
Address
. This class has three optional properties of type
String?
.
The first two properties,
buildingName
and
buildingNumber
, are alternative ways to identify a
particular building as part of an address. The third property,
street
, is used to name the street for that
address:
1
class

Address
{
2

var

buildingName
:
String
?
3

var

buildingNumber
:
String
?
4

var

street
:
String
?
5

func

buildingIdentifier
() ->
String
? {
6

if

buildingName
{
7

return

buildingName
8
}
else

if

buildingNumber
{

9

return

buildingNumber
10
}
else
{
11

return

nil
12
}
13
}
14
}
The
Address
class also provides a method called
buildingIdentifier
, which has a return type of
String?
. This method checks the
buildingName
and
buildingNumber
properties and returns
buildingName
if it has a value, or
buildingNumber
if it has a value, or
nil
if neither property has a
value.
​
Calling Properties Through Optional Chaining
As demonstrated in
Optional Chaining as an Alternative to Forced Unwrapping
, you can use optional chaining to
access a property on an optional value, and to check if that property access is successful. You cannot, however,
set a property’s value through optional chaining.
Use the classes defined above to create a new
Person
instance, and try to access its
numberOfRooms
property as before:
1
let

john
=
Person
()
2
if

let

roomCount
=
john
.
residence
?.
numberOfRooms
{
3

println
(
"John's residence has
\(
roomCount
)
room(s)."
)
4
}
else
{
5

println
(
"Unable to retrieve the number of rooms."
)
6
}
7
// prints "Unable to retrieve the number of rooms."
Because
john.residence
is
nil
, this optional chaining call fails in the same way as before, without error.
​
Calling Methods Through Optional Chaining

You can use optional chaining to call a method on an optional value, and to check whether that method call is
successful. You can do this even if that method does not define a return value.
The
printNumberOfRooms
method on the
Residence
class prints the current value of
numberOfRooms
. Here’s how the method looks:
1
func

printNumberOfRooms
() {
2

println
(
"The number of rooms is
\(
numberOfRooms
)
"
)
3
}
This method does not specify a return type. However, functions and methods with no return type have an
implicit return type of
Void
, as described in
Functions Without Return Values
.
If you call this method on an optional value with optional chaining, the method’s return type will be
Void?
, not
Void
, because return values are always of an optional type when called through optional chaining. This enables
you to use an
if
statement to check whether it was possible to call the
printNumberOfRooms
method,
even though the method does not itself define a return value. The implicit return value from the
printNumberOfRooms
will be equal to
Void
if the method was called succesfully through optional
chaining, or
nil
if was not:
1
if

john
.
residence
?.
printNumberOfRooms
() {
2

println
(
"It was possible to print the number of rooms."
)
3
}
else
{
4

println
(
"It was not possible to print the number of rooms."
)
5
}
6
// prints "It was not possible to print the number of rooms."
​
Calling Subscripts Through Optional Chaining
You can use optional chaining to try to retrieve a value from a subscript on an optional value, and to check
whether that subscript call is successful. You cannot, however, set a subscript through optional chaining.

NOTE
When you access a subscript on an optional value through optional chaining, you place the question
mark
before
the subscript’s braces, not after. The optional chaining question mark always follows
immediately after the part of the expression that is optional.
The example below tries to retrieve the name of the first room in the
rooms
array of the
john.residence
property using the subscript defined on the
Residence
class. Because
john.residence
is currently
nil
,
the subscript call fails:
1
if

let

firstRoomName
=
john
.
residence
?[
0
].
name
{
2

println
(
"The first room name is
\(
firstRoomName
)
."
)
3
}
else
{
4

println
(
"Unable to retrieve the first room name."
)
5
}
6
// prints "Unable to retrieve the first room name."
The optional chaining question mark in this subscript call is placed immediately after
john.residence
,
before the subscript brackets, because
john.residence
is the optional value on which optional chaining is
being attempted.
If you create and assign an actual
Residence
instance to
john.residence
, with one or more
Room
instances in its
rooms
array, you can use the
Residence
subscript to access the actual items in the
rooms
array through optional chaining:
1
let

johnsHouse
=
Residence
()
2
johnsHouse
.
rooms
+=
Room
(
name
:
"Living Room"
)
3
johnsHouse
.
rooms
+=
Room
(
name
:
"Kitchen"
)
4
john
.
residence
=
johnsHouse
5

6
if

let

firstRoomName
=
john
.
residence
?[
0
].
name
{
7

println
(
"The first room name is
\(
firstRoomName
)
."
)
8
}
else
{

9

println
(
"Unable to retrieve the first room name."
)
10
}
11
// prints "The first room name is Living Room."
​
Linking Multiple Levels of Chaining
You can link together multiple levels of optional chaining to drill down to properties, methods, and subscripts
deeper within a model. However, multiple levels of optional chaining do not add more levels of optionality to the
returned value.
To put it another way:
Therefore:
The example below tries to access the
street
property of the
address
property of the
residence
property of
john
. There are
two
levels of optional chaining in use here, to chain through the
residence
and
address
properties, both of which are of optional type:
1
if

let

johnsStreet
=
john
.
residence
?.
address
?.
street
{
2

println
(
"John's street name is
\(
johnsStreet
)
."
)
3
}
else
{
4

println
(
"Unable to retrieve the address."
)
5
}
If the type you are trying to retrieve is not optional, it will become optional because of the optional
chaining.
If the type you are trying to retrieve is
already
optional, it will not become
more
optional because
of the chaining.
If you try to retrieve an
Int
value through optional chaining, an
Int?
is always returned, no
matter how many levels of chaining are used.
Similarly, if you try to retrieve an
Int?
value through optional chaining, an
Int?
is always
returned, no matter how many levels of chaining are used.

6
// prints "Unable to retrieve the address."
The value of
john.residence
currently contains a valid
Residence
instance. However, the value of
john.residence.address
is currently
nil
. Because of this, the call to
john.residence?.address?.street
fails.
Note that in the example above, you are trying to retrieve the value of the
street
property. The type of this
property is
String?
. The return value of
john.residence?.address?.street
is therefore also
String?
, even though two levels of optional chaining are applied in addition to the underlying optional type of
the property.
If you set an actual
Address
instance as the value for
john.street.address
, and set an an actual value
for the address’s
street
property, you can access the value of property through the multi-level optional
chaining:
1
let

johnsAddress
=
Address
()
2
johnsAddress
.
buildingName
=
"The Larches"
3
johnsAddress
.
street
=
"Laurel Street"
4
john
.
residence
!.
address
=
johnsAddress
5

6
if

let

johnsStreet
=
john
.
residence
?.
address
?.
street
{
7

println
(
"John's street name is
\(
johnsStreet
)
."
)
8
}
else
{
9

println
(
"Unable to retrieve the address."
)
10
}
11
// prints "John's street name is Laurel Street."
Note the use of an exclamation mark during the assignment of an address instance to
john.residence.address
. The
john.residence
property has an optional type, and so you need to
unwrap its actual value with an exclamation mark before accessing the residence’s
address
property.
​
Chaining on Methods With Optional Return Values
The previous example shows how to retrieve the value of a property of optional type through optional chaining.

You can also use optional chaining to call a method that returns a value of optional type, and to chain on that
method’s return value if needed.
The example below calls the
Address
class’s
buildingIdentifier
method through optional chaining.
This method returns a value of type
String?
. As described above, the ultimate return type of this method call
after optional chaining is also
String?
:
1
if

let

buildingIdentifier
=

john
.
residence
?.
address
?.
buildingIdentifier
() {
2

println
(
"John's building identifier is
\(
buildingIdentifier
)
."
)
3
}
4
// prints "John's building identifier is The Larches."
If you want to perform further optional chaining on this method’s return value, place the optional chaining
question mark
after
the method’s parentheses:
1
if

let

upper
=

john
.
residence
?.
address
?.
buildingIdentifier
()?.
uppercaseString
{
2

println
(
"John's uppercase building identifier is
\(
upper
)
."
)
3
}
4
// prints "John's uppercase building identifier is THE LARCHES."
NOTE
In the example above, you place the optional chaining question mark
after
the parentheses, because
the optional value you are chaining on is the
buildingIdentifier
method’s return value, and
not the
buildingIdentifier
method itself.

​
​
Type Casting
Type casting
is a way to check the type of an instance, and/or to treat that instance as if it is a different
superclass or subclass from somewhere else in its own class hierarchy.
Type casting in Swift is implemented with the
is
and
as
operators. These two operators provide a simple and
expressive way to check the type of a value or cast a value to a different type.
You can also use type casting to check whether a type conforms to a protocol, as described in
Checking for
Protocol Conformance
.
​
Defining a Class Hierarchy for Type Casting
You can use type casting with a hierarchy of classes and subclasses to check the type of a particular class
instance and to cast that instance to another class within the same hierarchy. The three code snippets below
define a hierarchy of classes and an array containing instances of those classes, for use in an example of type
casting.
The first snippet defines a new base class called
MediaItem
. This class provides basic functionality for any
kind of item that appears in a digital media library. Specifically, it declares a
name
property of type
String
,
and an
init name
initializer. (It is assumed that all media items, including all movies and songs, will have a
name.)
1
class

MediaItem
{
2

var

name
:
String
3

init
(
name
:
String
) {
4

self
.
name
=
name
5
}
6
}
The next snippet defines two subclasses of
MediaItem
. The first subclass,
Movie
, encapsulates additional
information about a movie or film. It adds a
director
property on top of the base
MediaItem
class, with a

corresponding initializer. The second subclass,
Song
, adds an
artist
property and initializer on top of the
base class:
1
class

Movie
:
MediaItem
{
2

var

director
:
String
3

init
(
name
:
String
,
director
:
String
) {
4

self
.
director
=
director
5

super
.
init
(
name
:
name
)
6
}
7
}
8

9
class

Song
:
MediaItem
{
10

var

artist
:
String
11

init
(
name
:
String
,
artist
:
String
) {
12

self
.
artist
=
artist
13

super
.
init
(
name
:
name
)
14
}
15
}
The final snippet creates a constant array called
library
, which contains two
Movie
instances and three
Song
instances. The type of the
library
array is inferred by initializing it with the contents of an array literal.
Swift’s type checker is able to deduce that
Movie
and
Song
have a common superclass of
MediaItem
, and
so it infers a type of
MediaItem[]
for the
library
array:
1
let

library
= [
2

Movie
(
name
:
"Casablanca"
,
director
:
"Michael Curtiz"
),
3

Song
(
name
:
"Blue Suede Shoes"
,
artist
:
"Elvis Presley"
),
4

Movie
(
name
:
"Citizen Kane"
,
director
:
"Orson Welles"
),
5

Song
(
name
:
"The One And Only"
,
artist
:
"Chesney Hawkes"
),
6

Song
(
name
:
"Never Gonna Give You Up"
,
artist
:
"Rick Astley"
)
7
]
8
// the type of "library" is inferred to be MediaItem[]
The items stored in
library
are still
Movie
and
Song
instances behind the scenes. However, if you iterate
over the contents of this array, the items you receive back are typed as
MediaItem
, and not as
Movie
or

Song
. In order to work with them as their native type, you need to
check
their type, or
downcast
them to a
different type, as described below.
​
Checking Type
Use the
type check operator
(
is
) to check whether an instance is of a certain subclass type. The type check
operator returns
true
if the instance is of that subclass type and
false
if it is not.
The example below defines two variables,
movieCount
and
songCount
, which count the number of
Movie
and
Song
instances in the
library
array:
1
var

movieCount
=
0
2
var

songCount
=
0
3

4
for

item

in

library
{
5

if

item

is

Movie
{
6
++
movieCount
7
}
else

if

item

is

Song
{
8
++
songCount
9
}
10
}
11

12
println
(
"Media library contains
\(
movieCount
)
movies and
\
(
songCount
)
songs"
)
13
// prints "Media library contains 2 movies and 3 songs"
This example iterates through all items in the
library
array. On each pass, the
for
–
in
loop sets the
item
constant to the next
MediaItem
in the array.
item is Movie
returns
true
if the current
MediaItem
is a
Movie
instance and
false
if it is not.
Similarly,
item is Song
checks whether the item is a
Song
instance. At the end of the
for
–
in
loop, the
values of
movieCount
and
songCount
contain a count of how many
MediaItem
instances were found of
each type.

​
Downcasting
A constant or variable of a certain class type may actually refer to an instance of a subclass behind the scenes.
Where you believe this is the case, you can try to
downcast
to the subclass type with the
type cast operator
(
as
).
Because downcasting can fail, the type cast operator comes in two different forms. The optional form,
as?
,
returns an optional value of the type you are trying to downcast to. The forced form,
as
, attempts the downcast
and force-unwraps the result as a single compound action.
Use the optional form of the type cast operator (
as?
) when you are not sure if the downcast will succeed. This
form of the operator will always return an optional value, and the value will be
nil
if the downcast was not
possible. This enables you to check for a successful downcast.
Use the forced form of the type cast operator (
as
) only when you are sure that the downcast will always
succeed. This form of the operator will trigger a runtime error if you try to downcast to an incorrect class type.
The example below iterates over each
MediaItem
in
library
, and prints an appropriate description for
each item. To do this, it needs to access each item as a true
Movie
or
Song
, and not just as a
MediaItem
.
This is necessary in order for it to be able to access the
director
or
artist
property of a
Movie
or
Song
for use in the description.
In this example, each item in the array might be a
Movie
, or it might be a
Song
. You don’t know in advance
which actual class to use for each item, and so it is appropriate to use the optional form of the type cast operator
(
as?
) to check the downcast each time through the loop:
1
for

item

in

library
{
2

if

let

movie
=
item

as
?
Movie
{
3

println
(
"Movie: '
\(
movie
.
name
)
', dir.
\(
movie
.
director
)
"
)
4
}
else

if

let

song
=
item

as
?
Song
{
5

println
(
"Song: '
\(
song
.
name
)
', by
\(
song
.
artist
)
"
)
6
}
7
}
8

9
// Movie: 'Casablanca', dir. Michael Curtiz
10
// Song: 'Blue Suede Shoes', by Elvis Presley
11
// Movie: 'Citizen Kane', dir. Orson Welles
12
// Song: 'The One And Only', by Chesney Hawkes
13
// Song: 'Never Gonna Give You Up', by Rick Astley
The example starts by trying to downcast the current
item
as a
Movie
. Because
item
is a
MediaItem
instance, it’s possible that it
might
be a
Movie
; equally, it’s also possible that it might a
Song
, or even just a
base
MediaItem
. Because of this uncertainty, the
as?
form of the type cast operator returns an
optional
value
when attempting to downcast to a subclass type. The result of
item as Movie
is of type
Movie?
, or
“optional
Movie
”.
Downcasting to
Movie
fails when applied to the two
Song
instances in the library array. To cope with this, the
example above uses optional binding to check whether the optional
Movie
actually contains a value (that is, to
find out whether the downcast succeeded.) This optional binding is written “
if let movie = item as?
Movie
”, which can be read as:
“Try to access
item
as a
Movie
. If this is successful, set a new temporary constant called
movie
to the
value stored in the returned optional
Movie
.”
If the downcasting succeeds, the properties of
movie
are then used to print a description for that
Movie
instance, including the name of its
director
. A similar principle is used to check for
Song
instances, and to
print an appropriate description (including
artist
name) whenever a
Song
is found in the library.
NOTE
Casting does not actually modify the instance or change its values. The underlying instance remains
the same; it is simply treated and accessed as an instance of the type to which it has been cast.
​
Type Casting for Any and AnyObject

Swift provides two special type aliases for working with non-specific types:
NOTE
Use
Any
and
AnyObject
only when you explicitly need the behavior and capabilities they provide.
It is always better to be specific about the types you expect to work with in your code.
​
AnyObject
When working with Cocoa APIs, it is common to receive an array with a type of
AnyObject[]
, or “an array
of values of any object type”. This is because Objective-C does not have explicitly typed arrays. However, you
can often be confident about the type of objects contained in such an array just from the information you know
about the API that provided the array.
In these situations, you can use the forced version of the type cast operator (
as
) to downcast each item in the
array to a more specific class type than
AnyObject
, without the need for optional unwrapping.
The example below defines an array of type
AnyObject[]
and populates this array with three instances of
the
Movie
class:
1
let

someObjects
:
AnyObject
[] = [
2

Movie
(
name
:
"2001: A Space Odyssey"
,
director
:
"Stanley Kubrick"
),
3

Movie
(
name
:
"Moon"
,
director
:
"Duncan Jones"
),
4

Movie
(
name
:
"Alien"
,
director
:
"Ridley Scott"
)
5
]
AnyObject
can represent an instance of any class type.
Any
can represent an instance of any type at all, apart from function types.

Because this array is known to contain only
Movie
instances, you can downcast and unwrap directly to a non-
optional
Movie
with the forced version of the type cast operator (
as
):
1
for

object

in

someObjects
{
2

let

movie
=
object

as

Movie
3

println
(
"Movie: '
\(
movie
.
name
)
', dir.
\(
movie
.
director
)
"
)
4
}
5
// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
6
// Movie: 'Moon', dir. Duncan Jones
7
// Movie: 'Alien', dir. Ridley Scott
For an even shorter form of this loop, downcast the
someObjects
array to a type of
Movie[]
instead of
downcasting each item:
1
for

movie

in

someObjects

as

Movie
[] {
2

println
(
"Movie: '
\(
movie
.
name
)
', dir.
\(
movie
.
director
)
"
)
3
}
4
// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
5
// Movie: 'Moon', dir. Duncan Jones
6
// Movie: 'Alien', dir. Ridley Scott
​
Any
Here’s an example of using
Any
to work with a mix of different types, including non-class types. The example
creates an array called
things
, which can store values of type
Any
:
1
var

things
=
Any
[]()
2

3
things
.
append
(
0
)
4
things
.
append
(
0.0
)
5
things
.
append
(
42
)
6
things
.
append
(
3.14159
)
7
things
.
append
(
"hello"
)

8
things
.
append
((
3.0
,
5.0
))
9
things
.
append
(
Movie
(
name
:
"Ghostbusters"
,
director
:
"Ivan Reitman"
))
The
things
array contains two
Int
values, two
Double
values, a
String
value, a tuple of type
(Double, Double)
, and the movie “Ghostbusters”, directed by Ivan Reitman.
You can use the
is
and
as
operators in a
switch
statement’s cases to discover the specific type of a constant
or variable that is known only to be of type
Any
or
AnyObject
. The example below iterates over the items in
the
things
array and queries the type of each item with a
switch
statement. Several of the
switch
statement’s cases bind their matched value to a constant of the specified type to enable its value to be printed:
1
for

thing

in

things
{
2

switch

thing
{
3

case

0

as

Int
:
4

println
(
"zero as an Int"
)
5

case

0

as

Double
:
6

println
(
"zero as a Double"
)
7

case

let

someInt

as

Int
:
8

println
(
"an integer value of
\(
someInt
)
"
)
9

case

let

someDouble

as

Double

where

someDouble
>
0
:
10

println
(
"a positive double value of
\(
someDouble
)
"
)
11

case

is

Double
:
12

println
(
"some other double value that I don't want to

print"
)
13

case

let

someString

as

String
:
14

println
(
"a string value of \"
\(
someString
)
\""
)
15

case

let
(
x
,
y
)
as
(
Double
,
Double
):
16

println
(
"an (x, y) point at
\(
x
)
,
\(
y
)
"
)
17

case

let

movie

as

Movie
:
18

println
(
"a movie called '
\(
movie
.
name
)
', dir.
\
(
movie
.
director
)
"
)
19

default
:
20

println
(
"something else"
)
21
}
22
}

23

24
// zero as an Int
25
// zero as a Double
26
// an integer value of 42
27
// a positive double value of 3.14159
28
// a string value of "hello"
29
// an (x, y) point at 3.0, 5.0
30
// a movie called 'Ghostbusters', dir. Ivan Reitman
NOTE
The cases of a
switch
statement use the forced version of the type cast operator (
as
, not
as?
) to
check and cast to a specific type. This check is always safe within the context of a
switch
case
statement.

​
​
Nested Types
Enumerations are often created to support a specific class or structure’s functionality. Similarly, it can be
convenient to define utility classes and structures purely for use within the context of a more complex type. To
accomplish this, Swift enables you to define
nested types
, whereby you nest supporting enumerations, classes,
and structures within the definition of the type they support.
To nest a type within another type, write its definition within the outer braces of the type it supports. Types can
be nested to as many levels as are required.
​
Nested Types in Action
The example below defines a structure called
BlackjackCard
, which models a playing card as used in the
game of Blackjack. The
BlackJack
structure contains two nested enumeration types called
Suit
and
Rank
.
In Blackjack, the Ace cards have a value of either one or eleven. This feature is represented by a structure
called
Values
, which is nested within the
Rank
enumeration:
1
struct

BlackjackCard
{
2

3

// nested Suit enumeration
4

enum

Suit
:
Character
{
5

case

Spades
=
"♠"
,
Hearts
=
"♡"
,
Diamonds
=
"♢"
,
Clubs
=
"♣"
6
}
7

8

// nested Rank enumeration
9

enum

Rank
:
Int
{
10

case

Two
=
2
,
Three
,
Four
,
Five
,
Six
,
Seven
,
Eight
,

Nine
,
Ten
11

case

Jack
,
Queen
,
King
,
Ace
12

struct

Values
{
13

let

first
:
Int
,
second
:
Int
?

14
}
15

var

values
:
Values
{
16

switch

self
{
17

case
.
Ace
:
18

return

Values
(
first
:
1
,
second
:
11
)
19

case
.
Jack
, .
Queen
, .
King
:
20

return

Values
(
first
:
10
,
second
:
nil
)
21

default
:
22

return

Values
(
first
:
self
.
toRaw
(),
second
:
nil
)
23
}
24
}
25
}
26

27

// BlackjackCard properties and methods
28

let

rank
:
Rank
,
suit
:
Suit
29

var

description
:
String
{
30

var

output
=
"suit is
\(
suit
.
toRaw
())
,"
31

output
+=
" value is
\(
rank
.
values
.
first
)
"
32

if

let

second
=
rank
.
values
.
second
{
33

output
+=
" or
\(
second
)
"
34
}
35

return

output
36
}
37
}
The
Suit
enumeration describes the four common playing card suits, together with a raw
Character
value
to represent their symbol.
The
Rank
enumeration describes the thirteen possible playing card ranks, together with a raw
Int
value to
represent their face value. (This raw
Int
value is not used for the Jack, Queen, King, and Ace cards.)
As mentioned above, the
Rank
enumeration defines a further nested structure of its own, called
Values
. This
structure encapsulates the fact that most cards have one value, but the Ace card has two values. The
Values
structure defines two properties to represent this:
first
, of type
Int

Rank
also defines a computed property,
values
, which returns an instance of the
Values
structure. This
computed property considers the rank of the card and initializes a new
Values
instance with appropriate
values based on its rank. It uses special values for
Jack
,
Queen
,
King
, and
Ace
. For the numeric cards, it
uses the rank’s raw
Int
value.
The
BlackjackCard
structure itself has two properties—
rank
and
suit
. It also defines a computed
property called
description
, which uses the values stored in
rank
and
suit
to build a description of the
name and value of the card. The
description
property uses optional binding to check whether there is a
second value to display, and if so, inserts additional description detail for that second value.
Because
BlackjackCard
is a structure with no custom initializers, it has an implicit memberwise initializer,
as described in
Memberwise Initializers for Structure Types
. You can use this initializer to initialize a new
constant called
theAceOfSpades
:
1
let

theAceOfSpades
=
BlackjackCard
(
rank
: .
Ace
,
suit
: .
Spades
)
2
println
(
"theAceOfSpades:
\(
theAceOfSpades
.
description
)
"
)
3
// prints "theAceOfSpades: suit is ♠, value is 1 or 11"
Even though
Rank
and
Suit
are nested within
BlackjackCard
, their type can be inferred from context, and
so the initialization of this instance is able to refer to the enumeration members by their member names (
.Ace
and
.Spades
) alone. In the example above, the
description
property correctly reports that the Ace of
Spades has a value of
1
or
11
.
​
Referring to Nested Types
To use a nested type outside of its definition context, prefix its name with the name of the type it is nested within:
1
let

heartsSymbol
=
BlackjackCard
.
Suit
.
Hearts
.
toRaw
()
2
// heartsSymbol is "♡"
second
, of type
Int?
, or “optional
Int
”

For the example above, this enables the names of
Suit
,
Rank
, and
Values
to be kept deliberately short,
because their names are naturally qualified by the context in which they are defined.

​
​
Extensions
Extensions
add new functionality to an existing class, structure, or enumeration type. This includes the ability to
extend types for which you do not have access to the original source code (known as
retroactive modeling
).
Extensions are similar to categories in Objective-C. (Unlike Objective-C categories, Swift extensions do not
have names.)
Extensions in Swift can:
NOTE
If you define an extension to add new functionality to an existing type, the new functionality will be
available on all existing instances of that type, even if they were created before the extension was
defined.
​
Extension Syntax
Declare extensions with the
extension
keyword:
Add computed properties and computed static properties
Define instance methods and type methods
Provide new initializers
Define subscripts
Define and use new nested types
Make an existing type conform to a protocol

1
extension

SomeType
{
2

// new functionality to add to SomeType goes here
3
}
An extension can extend an existing type to make it adopt one or more protocols. Where this is the case, the
protocol names are written in exactly the same way as for a class or structure:
1
extension

SomeType
:
SomeProtocol
,
AnotherProtocol
{
2

// implementation of protocol requirements goes here
3
}
Adding protocol conformance in this way is described in
Adding Protocol Conformance with an Extension
.
​
Computed Properties
Extensions can add computed instance properties and computed type properties to existing types. This example
adds five computed instance properties to Swift’s built-in
Double
type, to provide basic support for working
with distance units:
1
extension

Double
{
2

var

km
:
Double
{
return

self
*
1_000.0
}
3

var

m
:
Double
{
return

self
}
4

var

cm
:
Double
{
return

self
/
100.0
}
5

var

mm
:
Double
{
return

self
/
1_000.0
}
6

var

ft
:
Double
{
return

self
/
3.28084
}
7
}
8
let

oneInch
=
25.4
.
mm
9
println
(
"One inch is
\(
oneInch
)
meters"
)
10
// prints "One inch is 0.0254 meters"
11
let

threeFeet
=
3
.
ft
12
println
(
"Three feet is
\(
threeFeet
)
meters"
)
13
// prints "Three feet is 0.914399970739201 meters"

These computed properties express that a
Double
value should be considered as a certain unit of length.
Although they are implemented as computed properties, the names of these properties can be appended to a
floating-point literal value with dot syntax, as a way to use that literal value to perform distance conversions.
In this example, a
Double
value of
1.0
is considered to represent “one meter”. This is why the
m
computed
property returns
self
—the expression
1.m
is considered to calculate a
Double
value of
1.0
.
Other units require some conversion to be expressed as a value measured in meters. One kilometer is the
same as 1,000 meters, so the
km
computed property multiplies the value by
1_000.00
to convert into a
number expressed in meters. Similarly, there are 3.28024 feet in a meter, and so the
ft
computed property
divides the underlying
Double
value by
3.28024
, to convert it from feet to meters.
These properties are read-only computed properties, and so they are expressed without the
get
keyword, for
brevity. Their return value is of type
Double
, and can be used within mathematical calculations wherever a
Double
is accepted:
1
let

aMarathon
=
42
.
km
+
195
.
m
2
println
(
"A marathon is
\(
aMarathon
)
meters long"
)
3
// prints "A marathon is 42195.0 meters long"
NOTE
Extensions can add new computed properties, but they cannot add stored properties, or add property
observers to existing properties.
​
Initializers
Extensions can add new initializers to existing types. This enables you to extend other types to accept your own
custom types as initializer parameters, or to provide additional initialization options that were not included as
part of the type’s original implementation.

Extensions can add new convenience initializers to a class, but they cannot add new designated initializers or
deinitializers to a class. Designated initializers and deinitializers must always be provided by the original class
implementation.
NOTE
If you use an extension to add an initializer to a value type that provides default values for all of its
stored properties and does not define any custom initializers, you can call the default initializer and
memberwise initializer for that value type from within your extension’s initializer.
This would not be the case if you had written the initializer as part of the value type’s original
implementation, as described in
Initializer Delegation for Value Types
.
The example below defines a custom
Rect
structure to represent a geometric rectangle. The example also
defines two supporting structures called
Size
and
Point
, both of which provide default values of
0.0
for all of
their properties:
1
struct

Size
{
2

var

width
=
0.0
,
height
=
0.0
3
}
4
struct

Point
{
5

var

x
=
0.0
,
y
=
0.0
6
}
7
struct

Rect
{
8

var

origin
=
Point
()
9

var

size
=
Size
()
10
}
Because the
Rect
structure provides default values for all of its properties, it receives a default initializer and a
memberwise initializer automatically, as described in
Default Initializers
. These initializers can be used to
create new
Rect
instances:
1
let

defaultRect
=
Rect
()

2
let

memberwiseRect
=
Rect
(
origin
:
Point
(
x
:
2.0
,
y
:
2.0
),
3

size
:
Size
(
width
:
5.0
,
height
:
5.0
))
You can extend the
Rect
structure to provide an additional initializer that takes a specific center point and size:
1
extension

Rect
{
2

init
(
center
:
Point
,
size
:
Size
) {
3

let

originX
=
center
.
x
– (
size
.
width
/
2
)
4

let

originY
=
center
.
y
– (
size
.
height
/
2
)
5

self
.
init
(
origin
:
Point
(
x
:
originX
,
y
:
originY
),
size
:
size
)
6
}
7
}
This new initializer starts by calculating an appropriate origin point based on the provided
center
point and
size
value. The initializer then calls the structure’s automatic memberwise initializer
init(origin:size:)
, which stores the new origin and size values in the appropriate properties:
1
let

centerRect
=
Rect
(
center
:
Point
(
x
:
4.0
,
y
:
4.0
),
2

size
:
Size
(
width
:
3.0
,
height
:
3.0
))
3
// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)
NOTE
If you provide a new initializer with an extension, you are still responsible for making sure that each
instance is fully initialized once the initializer completes.
​
Methods
Extensions can add new instance methods and type methods to existing types. The following example adds a

new instance method called
repetitions
to the
Int
type:
1
extension

Int
{
2

func

repetitions
(
task
: () -> ()) {
3

for

i

in

0
..
self
{
4

task
()
5
}
6
}
7
}
The
repetitions
method takes a single argument of type
() -> ()
, which indicates a function that has no
parameters and does not return a value.
After defining this extension, you can call the
repetitions
method on any integer number to perform a task
that many number of times:
1
3
.
repetitions
({
2

println
(
"Hello!"
)
3
})
4
// Hello!
5
// Hello!
6
// Hello!
Use trailing closure syntax to make the call more succinct:
1
3
.
repetitions
{
2

println
(
"Goodbye!"
)
3
}
4
// Goodbye!
5
// Goodbye!
6
// Goodbye!
​

Mutating Instance Methods
Instance methods added with an extension can also modify (or
mutate
) the instance itself. Structure and
enumeration methods that modify
self
or its properties must mark the instance method as
mutating
, just
like mutating methods from an original implementation.
The example below adds a new mutating method called
square
to Swift’s
Int
type, which squares the
original value:
1
extension

Int
{
2

mutating

func

square
() {
3

self
=
self
*
self
4
}
5
}
6
var

someInt
=
3
7
someInt
.
square
()
8
// someInt is now 9
​
Subscripts
Extensions can add new subscripts to an existing type. This example adds an integer subscript to Swift’s built-
in
Int
type. This subscript
[n]
returns the decimal digit
n
places in from the right of the number:
…and so on:
1
extension

Int
{
2

subscript
(
digitIndex
:
Int
) ->
Int
{
3

var

decimalBase
=
1
4

for

_

in

1
…
digitIndex
{
5

decimalBase
*=
10
123456789[0]
returns
9
123456789[1]
returns
8

6
}
7

return
(
self
/
decimalBase
) %
10
8
}
9
}
10
746381295
[
0
]
11
// returns 5
12
746381295
[
1
]
13
// returns 9
14
746381295
[
2
]
15
// returns 2
16
746381295
[
8
]
17
// returns 7
If the
Int
value does not have enough digits for the requested index, the subscript implementation returns
0
, as
if the number had been padded with zeroes to the left:
1
746381295
[
9
]
2
// returns 0, as if you had requested:
3
0746381295
[
9
]
​
Nested Types
Extensions can add new nested types to existing classes, structures and enumerations:
1
extension

Character
{
2

enum

Kind
{
3

case

Vowel
,
Consonant
,
Other
4
}
5

var

kind
:
Kind
{
6

switch

String
(
self
).
lowercaseString
{
7

case

"a"
,
"e"
,
"i"
,
"o"
,
"u"
:
8

return
.
Vowel
9

case

"b"
,
"c"
,
"d"
,
"f"
,
"g"
,
"h"
,
"j"
,
"k"
,
"l"
,
"m"
,

10

"n"
,
"p"
,
"q"
,
"r"
,
"s"
,
"t"
,
"v"
,
"w"
,
"x"
,
"y"
,
"z"
:
11

return
.
Consonant
12

default
:
13

return
.
Other
14
}
15
}
16
}
This example adds a new nested enumeration to
Character
. This enumeration, called
Kind
, expresses the
kind of letter that a particular character represents. Specifically, it expresses whether the character is a vowel
or a consonant in a standard Latin script (without taking into account accents or regional variations), or whether
it is another kind of character.
This example also adds a new computed instance property to
Character
, called
kind
, which returns the
appropriate
Kind
enumeration member for that character.
The nested enumeration can now be used with
Character
values:
1
func

printLetterKinds
(
word
:
String
) {
2

println
(
"'
\(
word
)
' is made up of the following kinds of letters:"
)
3

for

character

in

word
{
4

switch

character
.
kind
{
5

case
.
Vowel
:
6

print
(
"vowel "
)
7

case
.
Consonant
:
8

print
(
"consonant "
)
9

case
.
Other
:
10

print
(
"other "
)
11
}
12
}
13

print
(
"\n"
)
14
}
15
printLetterKinds
(
"Hello"
)
16
// 'Hello' is made up of the following kinds of letters:
17
// consonant vowel consonant consonant vowel

This function,
printLetterKinds
, takes an input
String
value and iterates over its characters. For each
character, it considers the
kind
computed property for that character, and prints an appropriate description of
that kind. The
printLetterKinds
function can then be called to print the kinds of letters in an entire word,
as shown here for the word
"Hello"
.
NOTE
character.kind
is already known to be of type
Character.Kind
. Because of this, all of the
Character.Kind
member values can be written in shorthand form inside the
switch
statement, such as
.Vowel
rather than
Character.Kind.Vowel
.

​
​
Protocols
A
protocol
defines a blueprint of methods, properties, and other requirements that suit a particular task or piece
of functionality. The protocol doesn’t actually provide an implementation for any of these requirements—it only
describes what an implementation will look like. The protocol can then be
adopted
by a class, structure, or
enumeration to provide an actual implementation of those requirements. Any type that satisfies the
requirements of a protocol is said to
conform
to that protocol.
Protocols can require that conforming types have specific instance properties, instance methods, type methods,
operators, and subscripts.
​
Protocol Syntax
You define protocols in a very similar way to classes, structures, and enumerations:
1
protocol

SomeProtocol
{
2

// protocol definition goes here
3
}
Custom types state that they adopt a particular protocol by placing the protocol’s name after the type’s name,
separated by a colon, as part of their definition. Multiple protocols can be listed, and are separated by commas:
1
struct

SomeStructure
:
FirstProtocol
,
AnotherProtocol
{
2

// structure definition goes here
3
}
If a class has a superclass, list the superclass name before any protocols it adopts, followed by a comma:
1
class

SomeClass
:
SomeSuperclass
,
FirstProtocol
,
AnotherProtocol
{
2

// class definition goes here
3
}

​
Property Requirements
A protocol can require any conforming type to provide an instance property or type property with a particular
name and type. The protocol doesn’t specify whether the property should be a stored property or a computed
property—it only specifies the required property name and type. The protocol also specifies whether each
property must be gettable or gettable
and
settable.
If a protocol requires a property to be gettable and settable, that property requirement cannot be fulfilled by a
constant stored property or a read-only computed property. If the protocol only requires a property to be
gettable, the requirement can be satisfied by any kind of property, and it is valid for it also to be settable if this is
useful for your own code.
Property requirements are always declared as variable properties, prefixed with the
var
keyword. Gettable
and settable properties are indicated by writing
{ get set }
after their type declaration, and gettable
properties are indicated by writing
{ get }
.
1
protocol

SomeProtocol
{
2

var

mustBeSettable
:
Int
{
get

set
}
3

var

doesNotNeedToBeSettable
:
Int
{
get
}
4
}
Always prefix type property requirements with the
class
keyword when you define them in a protocol. This is
true even though type property requirements are prefixed with the
static
keyword when implemented by a
structure or enumeration:
1
protocol

AnotherProtocol
{
2

class

var

someTypeProperty
:
Int
{
get

set
}
3
}
Here’s an example of a protocol with a single instance property requirement:
1
protocol

FullyNamed
{

2

var

fullName
:
String
{
get
}
3
}
The
FullyNamed
protocol defines any kind of thing that has a fully-qualified name. It doesn’t specify what
kind
of thing it must be—it only specifies that the thing must be able to provide a full name for itself. It specifies this
requirement by stating that any
FullyNamed
type must have a gettable instance property called
fullName
,
which is of type
String
.
Here’s an example of a simple structure that adopts and conforms to the
FullyNamed
protocol:
1
struct

Person
:
FullyNamed
{
2

var

fullName
:
String
3
}
4
let

john
=
Person
(
fullName
:
"John Appleseed"
)
5
// john.fullName is "John Appleseed"
This example defines a structure called
Person
, which represents a specific named person. It states that it
adopts the
FullyNamed
protocol as part of the first line of its definition.
Each instance of
Person
has a single stored property called
fullName
, which is of type
String
. This
matches the single requirement of the
FullyNamed
protocol, and means that
Person
has correctly
conformed to the protocol. (Swift reports an error at compile-time if a protocol requirement is not fulfilled.)
Here’s a more complex class, which also adopts and conforms to the
FullyNamed
protocol:
1
class

Starship
:
FullyNamed
{
2

var

prefix
:
String
?
3

var

name
:
String
4

init
(
name
:
String
,
prefix
:
String
? =
nil
) {
5

self
.
name
=
name
6

self
.
prefix
=
prefix
7
}
8

var

fullName
:
String
{
9

return
(
prefix
?
prefix
! +
" "
:
""
) +
name
10
}

11
}
12
var

ncc1701
=
Starship
(
name
:
"Enterprise"
,
prefix
:
"USS"
)
13
// ncc1701.fullName is "USS Enterprise"
This class implements the
fullName
property requirement as a computed read-only property for a starship.
Each
Starship
class instance stores a mandatory
name
and an optional
prefix
. The
fullName
property
uses the
prefix
value if it exists, and prepends it to the beginning of
name
to create a full name for the
starship.
​
Method Requirements
Protocols can require specific instance methods and type methods to be implemented by conforming types.
These methods are written as part of the protocol’s definition in exactly the same way as for normal instance
and type methods, but without curly braces or a method body. Variadic parameters are allowed, subject to the
same rules as for normal methods.
NOTE
Protocols use the same syntax as normal methods, but are not allowed to specify default values for
method parameters.
As with type property requirements, you always prefix type method requirements with the
class
keyword
when they are defined in a protocol. This is true even though type method requirements are prefixed with the
static
keyword when implemented by a structure or enumeration:
1
protocol

SomeProtocol
{
2

class

func

someTypeMethod
()
3
}
The following example defines a protocol with a single instance method requirement:

1
protocol

RandomNumberGenerator
{
2

func

random
() ->
Double
3
}
This protocol,
RandomNumberGenerator
, requires any conforming type to have an instance method called
random
, which returns a
Double
value whenever it is called. (Although it is not specified as part of the
protocol, it is assumed that this value will be a number between
0.0
and
1.0
inclusive.)
The
RandomNumberGenerator
protocol does not make any assumptions about how each random number
will be generated—it simply requires the generator to provide a standard way to generate a new random
number.
Here’s an implementation of a class that adopts and conforms to the
RandomNumberGenerator
protocol.
This class implements a pseudorandom number generator algorithm known as a
linear congruential generator
:
1
class

LinearCongruentialGenerator
:
RandomNumberGenerator
{
2

var

lastRandom
=
42.0
3

let

m
=
139968.0
4

let

a
=
3877.0
5

let

c
=
29573.0
6

func

random
() ->
Double
{
7

lastRandom
= ((
lastRandom
*
a
+
c
) %
m
)
8

return

lastRandom
/
m
9
}
10
}
11
let

generator
=
LinearCongruentialGenerator
()
12
println
(
"Here's a random number:
\(
generator
.
random
())
"
)
13
// prints "Here's a random number: 0.37464991998171"
14
println
(
"And another one:
\(
generator
.
random
())
"
)
15
// prints "And another one: 0.729023776863283"
​
Mutating Method Requirements

It is sometimes necessary for a method to modify (or
mutate
) the instance it belongs to. For instance methods
on value types (that is, structures and enumerations) you place the
mutating
keyword before a method’s
func
keyword to indicate that the method is allowed to modify the instance it belongs to and/or any properties
of that instance. This process is described in
Modifying Value Types from Within Instance Methods
.
If you define a protocol instance method requirement that is intended to mutate instances of any type that adopts
the protocol, mark the method with the
mutating
keyword as part of the protocol’s definition. This enables
structures and enumerations to adopt the protocol and satisfy that method requirement.
NOTE
If you mark a protocol instance method requirement as
mutating
, you do not need to write the
mutating
keyword when writing an implementation of that method for a class. The
mutating
keyword is only used by structures and enumerations.
The example below defines a protocol called
Togglable
, which defines a single instance method
requirement called
toggle
. As its name suggests, the
toggle
method is intended to toggle or invert the state
of any conforming type, typically by modifying a property of that type.
The
toggle
method is marked with the
mutating
keyword as part of the
Togglable
protocol definition, to
indicate that the method is expected to mutate the state of a conforming instance when it is called:
1
protocol

Togglable
{
2

mutating

func

toggle
()
3
}
If you implement the
Togglable
protocol for a structure or enumeration, that structure or enumeration can
conform to the protocol by providing an implementation of the
toggle
method that is also marked as
mutating
.
The example below defines an enumeration called
OnOffSwitch
. This enumeration toggles between two
states, indicated by the enumeration cases
On
and
Off
. The enumeration’s
toggle
implementation is

marked as
mutating
, to match the
Togglable
protocol’s requirements:
1
enum

OnOffSwitch
:
Togglable
{
2

case

Off
,
On
3

mutating

func

toggle
() {
4

switch

self
{
5

case

Off
:
6

self
=
On
7

case

On
:
8

self
=
Off
9
}
10
}
11
}
12
var

lightSwitch
=
OnOffSwitch
.
Off
13
lightSwitch
.
toggle
()
14
// lightSwitch is now equal to .On
​
Protocols as Types
Protocols do not actually implement any functionality themselves. Nonetheless, any protocol you create will
become a fully-fledged type for use in your code.
Because it is a type, you can use a protocol in many places where other types are allowed, including:
NOTE
Because protocols are types, begin their names with a capital letter (such as
FullyNamed
and
As a parameter type or return type in a function, method, or initializer
As the type of a constant, variable, or property
As the type of items in an array, dictionary, or other container

RandomNumberGenerator
) to match the names of other types in Swift (such as
Int
,
String
,
and
Double
).
Here’s an example of a protocol used as a type:
1
class

Dice
{
2

let

sides
:
Int
3

let

generator
:
RandomNumberGenerator
4

init
(
sides
:
Int
,
generator
:
RandomNumberGenerator
) {
5

self
.
sides
=
sides
6

self
.
generator
=
generator
7
}
8

func

roll
() ->
Int
{
9

return

Int
(
generator
.
random
() *
Double
(
sides
)) +
1
10
}
11
}
This example defines a new class called
Dice
, which represents an
n
-sided dice for use in a board game.
Dice
instances have an integer property called
sides
, which represents how many sides they have, and a
property called
generator
, which provides a random number generator from which to create dice roll
values.
The
generator
property is of type
RandomNumberGenerator
. Therefore, you can set it to an instance of
any
type that adopts the
RandomNumberGenerator
protocol. Nothing else is required of the instance you
assign to this property, except that the instance must adopt the
RandomNumberGenerator
protocol.
Dice
also has an initializer, to set up its initial state. This initializer has a parameter called
generator
,
which is also of type
RandomNumberGenerator
. You can pass a value of any conforming type in to this
parameter when initializing a new
Dice
instance.
Dice
provides one instance method,
roll
, which returns an integer value between 1 and the number of sides
on the dice. This method calls the generator’s
random
method to create a new random number between
0.0
and
1.0
, and uses this random number to create a dice roll value within the correct range. Because

generator
is known to adopt
RandomNumberGenerator
, it is guaranteed to have a
random
method to
call.
Here’s how the
Dice
class can be used to create a six-sided dice with a
LinearCongruentialGenerator
instance as its random number generator:
1
var

d6
=
Dice
(
sides
:
6
,
generator
:
LinearCongruentialGenerator
())
2
for

_

in

1
…
5
{
3

println
(
"Random dice roll is
\(
d6
.
roll
())
"
)
4
}
5
// Random dice roll is 3
6
// Random dice roll is 5
7
// Random dice roll is 4
8
// Random dice roll is 5
9
// Random dice roll is 4
​
Delegation
Delegation
is a design pattern that enables a class or structure to hand off (or
delegate
) some of its
responsibilities to an instance of another type. This design pattern is implemented by defining a protocol that
encapsulates the delegated responsibilities, such that a conforming type (known as a delegate) is guaranteed to
provide the functionality that has been delegated. Delegation can be used to respond to a particular action, or to
retrieve data from an external source without needing to know the underlying type of that source.
The example below defines two protocols for use with dice-based board games:
1
protocol

DiceGame
{
2

var

dice
:
Dice
{
get
}
3

func

play
()
4
}
5
protocol

DiceGameDelegate
{
6

func

gameDidStart
(
game
:
DiceGame
)
7

func

game
(
game
:
DiceGame
,
didStartNewTurnWithDiceRoll

diceRoll
:

Int
)

8

func

gameDidEnd
(
game
:
DiceGame
)
9
}
The
DiceGame
protocol is a protocol that can be adopted by any game that involves dice. The
DiceGameDelegate
protocol can be adopted by any type to track the progress of a
DiceGame
.
Here’s a version of the
Snakes and Ladders
game originally introduced in
Control Flow
. This version is adapted
to use a
Dice
instance for its dice-rolls; to adopt the
DiceGame
protocol; and to notify a
DiceGameDelegate
about its progress:
1
class

SnakesAndLadders
:
DiceGame
{
2

let

finalSquare
=
25
3

let

dice
=
Dice
(
sides
:
6
,
generator
:

LinearCongruentialGenerator
())
4

var

square
=
0
5

var

board
:
Int
[]
6

init
() {
7

board
=
Int
[](
count
:
finalSquare
+
1
,
repeatedValue
:
0
)
8

board
[
03
] = +
08
;
board
[
06
] = +
11
;
board
[
09
] = +
09
;
board
[
10
] =
+
02
9

board
[
14
] = –
10
;
board
[
19
] = –
11
;
board
[
22
] = –
02
;
board
[
24
] =
–
08
10
}
11

var

delegate
:
DiceGameDelegate
?
12

func

play
() {
13

square
=
0
14

delegate
?.
gameDidStart
(
self
)
15

gameLoop
:
while

square
!=
finalSquare
{
16

let

diceRoll
=
dice
.
roll
()
17

delegate
?.
game
(
self
,
didStartNewTurnWithDiceRoll
:

diceRoll
)
18

switch

square
+
diceRoll
{
19

case

finalSquare
:
20

break

gameLoop
21

case

let

newSquare

where

newSquare
>
finalSquare
:
22

continue

gameLoop

23

default
:
24

square
+=
diceRoll
25

square
+=
board
[
square
]
26
}
27
}
28

delegate
?.
gameDidEnd
(
self
)
29
}
30
}
For a description of the
Snakes and Ladders
gameplay, see the
Break
section of the
Control Flow
chapter.
This version of the game is wrapped up as a class called
SnakesAndLadders
, which adopts the
DiceGame
protocol. It provides a gettable
dice
property and a
play
method in order to conform to the
protocol. (The
dice
property is declared as a constant property because it does not need to change after
initialization, and the protocol only requires that it is gettable.)
The
Snakes and Ladders
game board setup takes place within the class’s
init()
initializer. All game logic is
moved into the protocol’s
play
method, which uses the protocol’s required
dice
property to provide its dice
roll values.
Note that the
delegate
property is defined as an
optional

DiceGameDelegate
, because a delegate isn’t
required in order to play the game. Because it is of an optional type, the
delegate
property is automatically
set to an initial value of
nil
. Thereafter, the game instantiator has the option to set the property to a suitable
delegate.
DiceGameDelegate
provides three methods for tracking the progress of a game. These three methods have
been incorporated into the game logic within the
play
method above, and are called when a new game starts, a
new turn begins, or the game ends.
Because the
delegate
property is an
optional

DiceGameDelegate
, the
play
method uses optional
chaining each time it calls a method on the delegate. If the
delegate
property is nil, these delegate calls fail
gracefully and without error. If the
delegate
property is non-nil, the delegate methods are called, and are
passed the
SnakesAndLadders
instance as a parameter.
This next example shows a class called
DiceGameTracker
, which adopts the
DiceGameDelegate
protocol:

1
class

DiceGameTracker
:
DiceGameDelegate
{
2

var

numberOfTurns
=
0
3

func

gameDidStart
(
game
:
DiceGame
) {
4

numberOfTurns
=
0
5

if

game

is

SnakesAndLadders
{
6

println
(
"Started a new game of Snakes and Ladders"
)
7
}
8

println
(
"The game is using a
\(
game
.
dice
.
sides
)
-sided dice"
)
9
}
10

func

game
(
game
:
DiceGame
,
didStartNewTurnWithDiceRoll

diceRoll
:
Int
) {
11
++
numberOfTurns
12

println
(
"Rolled a
\(
diceRoll
)
"
)
13
}
14

func

gameDidEnd
(
game
:
DiceGame
) {
15

println
(
"The game lasted for
\(
numberOfTurns
)
turns"
)
16
}
17
}
DiceGameTracker
implements all three methods required by
DiceGameDelegate
. It uses these
methods to keep track of the number of turns a game has taken. It resets a
numberOfTurns
property to zero
when the game starts; increments it each time a new turn begins; and prints out the total number of turns once
the game has ended.
The implementation of
gameDidStart
shown above uses the
game
parameter to print some introductory
information about the game that is about to be played. The
game
parameter has a type of
DiceGame
, not
SnakesAndLadders
, and so
gameDidStart
can access and use only methods and properties that are
implemented as part of the
DiceGame
protocol. However, the method is still able to use type casting to query
the type of the underlying instance. In this example, it checks whether
game
is actually an instance of
SnakesAndLadders
behind the scenes, and prints an appropriate message if so.
gameDidStart
also accesses the
dice
property of the passed
game
parameter. Because
game
is known
to conform to the
DiceGame
protocol, it is guaranteed to have a
dice
property, and so the
gameDidStart
method is able to access and print the dice’s
sides
property, regardless of what kind of game is being played.
Here’s how
DiceGameTracker
looks in action:

1
let

tracker
=
DiceGameTracker
()
2
let

game
=
SnakesAndLadders
()
3
game
.
delegate
=
tracker
4
game
.
play
()
5
// Started a new game of Snakes and Ladders
6
// The game is using a 6-sided dice
7
// Rolled a 3
8
// Rolled a 5
9
// Rolled a 4
10
// Rolled a 5
11
// The game lasted for 4 turns
​
Adding Protocol Conformance with an Extension
You can extend an existing type to adopt and conform to a new protocol, even if you do not have access to the
source code for the existing type. Extensions can add new properties, methods, and subscripts to an existing
type, and are therefore able to add any requirements that a protocol may demand. For more about extensions,
see
Extensions
.
NOTE
Existing instances of a type automatically adopt and conform to a protocol when that conformance is
added to the instance’s type in an extension.
For example, this protocol, called
TextRepresentable
, can be implemented by any type that has a way to
be represented as text. This might be a description of itself, or a text version of its current state:
1
protocol

TextRepresentable
{
2

func

asText
() ->
String
3
}

The
Dice
class from earlier can be extended to adopt and conform to
TextRepresentable
:
1
extension

Dice
:
TextRepresentable
{
2

func

asText
() ->
String
{
3

return

"A
\(
sides
)
-sided dice"
4
}
5
}
This extension adopts the new protocol in exactly the same way as if
Dice
had provided it in its original
implementation. The protocol name is provided after the type name, separated by a colon, and an
implementation of all requirements of the protocol is provided within the extension’s curly braces.
Any
Dice
instance can now be treated as
TextRepresentable
:
1
let

d12
=
Dice
(
sides
:
12
,
generator
:
LinearCongruentialGenerator
())
2
println
(
d12
.
asText
())
3
// prints "A 12-sided dice"
Similarly, the
SnakesAndLadders
game class can be extended to adopt and conform to the
TextRepresentable
protocol:
1
extension

SnakesAndLadders
:
TextRepresentable
{
2

func

asText
() ->
String
{
3

return

"A game of Snakes and Ladders with
\(
finalSquare
)

squares"
4
}
5
}
6
println
(
game
.
asText
())
7
// prints "A game of Snakes and Ladders with 25 squares"
​
Declaring Protocol Adoption with an Extension

If a type already conforms to all of the requirements of a protocol, but has not yet stated that it adopts that
protocol, you can make it adopt the protocol with an empty extension:
1
struct

Hamster
{
2

var

name
:
String
3

func

asText
() ->
String
{
4

return

"A hamster named
\(
name
)
"
5
}
6
}
7
extension

Hamster
:
TextRepresentable
{}
Instances of
Hamster
can now be used wherever
TextRepresentable
is the required type:
1
let

simonTheHamster
=
Hamster
(
name
:
"Simon"
)
2
let

somethingTextRepresentable
:
TextRepresentable
=
simonTheHamster
3
println
(
somethingTextRepresentable
.
asText
())
4
// prints "A hamster named Simon"
NOTE
Types do not automatically adopt a protocol just by satisfying its requirements. They must always
explicitly declare their adoption of the protocol.
​
Collections of Protocol Types
A protocol can be used as the type to be stored in a collection such as an array or a dictionary, as mentioned in
Protocols as Types
. This example creates an array of
TextRepresentable
things:
1
let

things
:
TextRepresentable
[] = [
game
,
d12
,
simonTheHamster
]

It is now possible to iterate over the items in the array, and print each item’s textual representation:
1
for

thing

in

things
{
2

println
(
thing
.
asText
())
3
}
4
// A game of Snakes and Ladders with 25 squares
5
// A 12-sided dice
6
// A hamster named Simon
Note that the
thing
constant is of type
TextRepresentable
. It is not of type
Dice
, or
DiceGame
, or
Hamster
, even if the actual instance behind the scenes is of one of those types. Nonetheless, because it is of
type
TextRepresentable
, and anything that is
TextRepresentable
is known to have an
asText
method, it is safe to call
thing.asText
each time through the loop.
​
Protocol Inheritance
A protocol can
inherit
one or more other protocols and can add further requirements on top of the requirements
it inherits. The syntax for protocol inheritance is similar to the syntax for class inheritance, but with the option to
list multiple inherited protocols, separated by commas:
1
protocol

InheritingProtocol
:
SomeProtocol
,
AnotherProtocol
{
2

// protocol definition goes here
3
}
Here’s an example of a protocol that inherits the
TextRepresentable
protocol from above:
1
protocol

PrettyTextRepresentable
:
TextRepresentable
{
2

func

asPrettyText
() ->
String
3
}
This example defines a new protocol,
PrettyTextRepresentable
, which inherits from
TextRepresentable
. Anything that adopts
PrettyTextRepresentable
must satisfy all of the

requirements enforced by
TextRepresentable
,
plus
the additional requirements enforced by
PrettyTextRepresentable
. In this example,
PrettyTextRepresentable
adds a single
requirement to provide an instance method called
asPrettyText
that returns a
String
.
The
SnakesAndLadders
class can be extended to adopt and conform to
PrettyTextRepresentable
:
1
extension

SnakesAndLadders
:
PrettyTextRepresentable
{
2

func

asPrettyText
() ->
String
{
3

var

output
=
asText
() +
":\n"
4

for

index

in

1
…
finalSquare
{
5

switch

board
[
index
] {
6

case

let

ladder

where

ladder
>
0
:
7

output
+=
"▲ "
8

case

let

snake

where

snake
<
0
:
9

output
+=
"▼ "
10

default
:
11

output
+=
"○ "
12
}
13
}
14

return

output
15
}
16
}
This extension states that it adopts the
PrettyTextRepresentable
protocol and provides an
implementation of the
asPrettyText
method for the
SnakesAndLadders
type. Anything that is
PrettyTextRepresentable
must also be
TextRepresentable
, and so the
asPrettyText
implementation starts by calling the
asText
method from the
TextRepresentable
protocol to begin an
output string. It appends a colon and a line break, and uses this as the start of its pretty text representation. It
then iterates through the array of board squares, and appends an emoji representation for each square:
The method implementation can now be used to print a pretty text description of any
SnakesAndLadders
If the square’s value is greater than
0
, it is the base of a ladder, and is represented by
▲
.
If the square’s value is less than
0
, it is the head of a snake, and is represented by
▼
.
Otherwise, the square’s value is
0
, and it is a “free” square, represented by
○
.

instance:
1
println
(
game
.
asPrettyText
())
2
// A game of Snakes and Ladders with 25 squares:
3
// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○
​
Protocol Composition
It can be useful to require a type to conform to multiple protocols at once. You can combine multiple protocols
into a single requirement with a
protocol composition
. Protocol compositions have the form
protocol<SomeProtocol, AnotherProtocol>
. You can list as many protocols within the pair of
angle brackets (
<>
) as you need, separated by commas.
Here’s an example that combines two protocols called
Named
and
Aged
into a single protocol composition
requirement on a function parameter:
1
protocol

Named
{
2

var

name
:
String
{
get
}
3
}
4
protocol

Aged
{
5

var

age
:
Int
{
get
}
6
}
7
struct

Person
:
Named
,
Aged
{
8

var

name
:
String
9

var

age
:
Int
10
}
11
func

wishHappyBirthday
(
celebrator
:
protocol
<
Named
,
Aged
>) {
12

println
(
"Happy birthday
\(
celebrator
.
name
)
– you're
\
(
celebrator
.
age
)
!"
)
13
}
14
let

birthdayPerson
=
Person
(
name
:
"Malcolm"
,
age
:
21
)
15
wishHappyBirthday
(
birthdayPerson
)
16
// prints "Happy birthday Malcolm – you're 21!"

This example defines a protocol called
Named
, with a single requirement for a gettable
String
property
called
name
. It also defines a protocol called
Aged
, with a single requirement for a gettable
Int
property
called
age
. Both of these protocols are adopted by a structure called
Person
.
The example also defines a function called
wishHappyBirthday
, which takes a single parameter called
celebrator
. The type of this parameter is
protocol<Named, Aged>
, which means “any type that
conforms to both the
Named
and
Aged
protocols.” It doesn’t matter what specific type is passed to the function,
as long as it conforms to both of the required protocols.
The example then creates a new
Person
instance called
birthdayPerson
and passes this new instance to
the
wishHappyBirthday
function. Because
Person
conforms to both protocols, this is a valid call, and the
wishHappyBirthday
function is able to print its birthday greeting.
NOTE
Protocol compositions do not define a new, permanent protocol type. Rather, they define a
temporary local protocol that has the combined requirements of all protocols in the composition.
​
Checking for Protocol Conformance
You can use the
is
and
as
operators described in
Type Casting
to check for protocol conformance, and to cast
to a specific protocol. Checking for and casting to a protocol follows exactly the same syntax as checking for
and casting to a type:
The
is
operator returns
true
if an instance conforms to a protocol and returns
false
if it does
not.
The
as?
version of the downcast operator returns an optional value of the protocol’s type, and
this value is
nil
if the instance does not conform to that protocol.
The
as
version of the downcast operator forces the downcast to the protocol type and triggers a
runtime error if the downcast does not succeed.

This example defines a protocol called
HasArea
, with a single property requirement of a gettable
Double
property called
area
:
1
@objc

protocol

HasArea
{
2

var

area
:
Double
{
get
}
3
}
NOTE
You can check for protocol conformance only if your protocol is marked with the
@objc
attribute, as
seen for the
HasArea
protocol above. This attribute indicates that the protocol should be exposed to
Objective-C code and is described in
Using Swift with Cocoa and Objective-C
. Even if you are not
interoperating with Objective-C, you need to mark your protocols with the
@objc
attribute if you
want to be able to check for protocol conformance.
Note also that
@objc
protocols can be adopted only by classes, and not by structures or
enumerations. If you mark your protocol as
@objc
in order to check for conformance, you will be
able to apply that protocol only to class types.
Here are two classes,
Circle
and
Country
, both of which conform to the
HasArea
protocol:
1
class

Circle
:
HasArea
{
2

let

pi
=
3.1415927
3

var

radius
:
Double
4

var

area
:
Double
{
return

pi
*
radius
*
radius
}
5

init
(
radius
:
Double
) {
self
.
radius
=
radius
}
6
}
7
class

Country
:
HasArea
{
8

var

area
:
Double
9

init
(
area
:
Double
) {
self
.
area
=
area
}
10
}

The
Circle
class implements the
area
property requirement as a computed property, based on a stored
radius
property. The
Country
class implements the
area
requirement directly as a stored property. Both
classes correctly conform to the
HasArea
protocol.
Here’s a class called
Animal
, which does not conform to the
HasArea
protocol:
1
class

Animal
{
2

var

legs
:
Int
3

init
(
legs
:
Int
) {
self
.
legs
=
legs
}
4
}
The
Circle
,
Country
and
Animal
classes do not have a shared base class. Nonetheless, they are all
classes, and so instances of all three types can be used to initialize an array that stores values of type
AnyObject
:
1
let

objects
:
AnyObject
[] = [
2

Circle
(
radius
:
2.0
),
3

Country
(
area
:
243_610
),
4

Animal
(
legs
:
4
)
5
]
The
objects
array is initialized with an array literal containing a
Circle
instance with a radius of 2 units; a
Country
instance initialized with the surface area of the United Kingdom in square kilometers; and an
Animal
instance with four legs.
The
objects
array can now be iterated, and each object in the array can be checked to see if it conforms to
the
HasArea
protocol:
1
for

object

in

objects
{
2

if

let

objectWithArea
=
object

as
?
HasArea
{
3

println
(
"Area is
\(
objectWithArea
.
area
)
"
)
4
}
else
{
5

println
(
"Something that doesn't have an area"
)
6
}

7
}
8
// Area is 12.5663708
9
// Area is 243610.0
10
// Something that doesn't have an area
Whenever an object in the array conforms to the
HasArea
protocol, the optional value returned by the
as?
operator is unwrapped with optional binding into a constant called
objectWithArea
. The
objectWithArea
constant is known to be of type
HasArea
, and so its
area
property can be accessed and
printed in a type-safe way.
Note that the underlying objects are not changed by the casting process. They continue to be a
Circle
, a
Country
and an
Animal
. However, at the point that they are stored in the
objectWithArea
constant, they
are only known to be of type
HasArea
, and so only their
area
property can be accessed.
​
Optional Protocol Requirements
You can define
optional requirements
for protocols, These requirements do not have to be implemented by
types that conform to the protocol. Optional requirements are prefixed by the
@optional
keyword as part of
the protocol’s definition.
An optional protocol requirement can be called with optional chaining, to account for the possibility that the
requirement was not implemented by a type that conforms to the protocol. For information on optional chaining,
see
Optional Chaining
.
You check for an implementation of an optional requirement by writing a question mark after the name of the
requirement when it is called, such as
someOptionalMethod?(someArgument)
. Optional property
requirements, and optional method requirements that return a value, will always return an optional value of the
appropriate type when they are accessed or called, to reflect the fact that the optional requirement may not have
been implemented.
NOTE
Optional protocol requirements can only be specified if your protocol is marked with the
@objc

attribute. Even if you are not interoperating with Objective-C, you need to mark your protocols with
the
@objc
attribute if you want to specify optional requirements.
Note also that
@objc
protocols can be adopted only by classes, and not by structures or
enumerations. If you mark your protocol as
@objc
in order to specify optional requirements, you
will only be able to apply that protocol to class types.
The following example defines an integer-counting class called
Counter
, which uses an external data source
to provide its increment amount. This data source is defined by the
CounterDataSource
protocol, which
has two optional requirements:
1
@objc

protocol

CounterDataSource
{
2

@optional

func

incrementForCount
(
count
:
Int
) ->
Int
3

@optional

var

fixedIncrement
:
Int
{
get
}
4
}
The
CounterDataSource
protocol defines an optional method requirement called
incrementForCount
and an optional property requirement called
fixedIncrement
. These requirements define two different
ways for data sources to provide an appropriate increment amount for a
Counter
instance.
NOTE
Strictly speaking, you can write a custom class that conforms to
CounterDataSource
without
implementing
either
protocol requirement. They are both optional, after all. Although technically
allowed, this wouldn’t make for a very good data source.
The
Counter
class, defined below, has an optional
dataSource
property of type
CounterDataSource?
:
1
@objc

class

Counter
{
2

var

count
=
0

3

var

dataSource
:
CounterDataSource
?
4

func

increment
() {
5

if

let

amount
=
dataSource
?.
incrementForCount
?(
count
) {
6

count
+=
amount
7
}
else

if

let

amount
=
dataSource
?.
fixedIncrement
? {
8

count
+=
amount
9
}
10
}
11
}
The
Counter
class stores its current value in a variable property called
count
. The
Counter
class also
defines a method called
increment
, which increments the
count
property every time the method is called.
The
increment
method first tries to retrieve an increment amount by looking for an implementation of the
incrementForCount
method on its data source. The
increment
method uses optional chaining to try to
call
incrementForCount
, and passes the current
count
value as the method’s single argument.
Note
two
levels of optional chaining at play here. Firstly, it is possible that
dataSource
may be
nil
, and so
dataSource
has a question mark after its name to indicate that
incrementForCount
should only be
called if
dataSource
is non-nil. Secondly, even if
dataSource

does
exist, there is no guarantee that it
implements
incrementForCount
, because it is an optional requirement. This is why
incrementForCount
is also written with a question mark after its name.
Because the call to
incrementForCount
can fail for either of these two reasons, the call returns an
optional
Int
value. This is true even though
incrementForCount
is defined as returning a non-optional
Int
value
in the definition of
CounterDataSource
.
After calling
incrementForCount
, the optional
Int
that it returns is unwrapped into a constant called
amount
, using optional binding. If the optional
Int
does contain a value—that is, if the delegate and method
both exist, and the method returned a value—the unwrapped
amount
is added onto the stored
count
property,
and incrementation is complete.
If it is
not
possible to retrieve a value from the
incrementForCount
method—either because
dataSource
is nil, or because the data source does not implement
incrementForCount
—then the
increment
method tries to retrieve a value from the data source’s
fixedIncrement
property instead. The
fixedIncrement
property is also an optional requirement, and so its name is also written using optional

chaining with a question mark on the end, to indicate that the attempt to access the property’s value can fail. As
before, the returned value is an optional
Int
value, even though
fixedIncrement
is defined as a non-
optional
Int
property as part of the
CounterDataSource
protocol definition.
Here’s a simple
CounterDataSource
implementation where the data source returns a constant value of
3
every time it is queried. It does this by implementing the optional
fixedIncrement
property requirement:
1
class

ThreeSource
:
CounterDataSource
{
2

let

fixedIncrement
=
3
3
}
You can use an instance of
ThreeSource
as the data source for a new
Counter
instance:
1
var

counter
=
Counter
()
2
counter
.
dataSource
=
ThreeSource
()
3
for

_

in

1
…
4
{
4

counter
.
increment
()
5

println
(
counter
.
count
)
6
}
7
// 3
8
// 6
9
// 9
10
// 12
The code above creates a new
Counter
instance; sets its data source to be a new
ThreeSource
instance;
and calls the counter’s
increment
method four times. As expected, the counter’s
count
property increases
by three each time
increment
is called.
Here’s a more complex data source called
TowardsZeroSource
, which makes a
Counter
instance count
up or down towards zero from its current
count
value:
1
class

TowardsZeroSource
:
CounterDataSource
{
2

func

incrementForCount
(
count
:
Int
) ->
Int
{
3

if

count
==
0
{

4

return

0
5
}
else

if

count
<
0
{
6

return

1
7
}
else
{
8

return
–
1
9
}
10
}
11
}
The
TowardsZeroSource
class implements the optional
incrementForCount
method from the
CounterDataSource
protocol and uses the
count
argument value to work out which direction to count in.
If
count
is already zero, the method returns
0
to indicate that no further counting should take place.
You can use an instance of
TowardsZeroSource
with the existing
Counter
instance to count from
-4
to
zero. Once the counter reaches zero, no more counting takes place:
1
counter
.
count
= –
4
2
counter
.
dataSource
=
TowardsZeroSource
()
3
for

_

in

1
…
5
{
4

counter
.
increment
()
5

println
(
counter
.
count
)
6
}
7
// -3
8
// -2
9
// -1
10
// 0
11
// 0

​
​
Generics
Generic code
enables you to write flexible, reusable functions and types that can work with any type, subject to
requirements that you define. You can write code that avoids duplication and expresses its intent in a clear,
abstracted manner.
Generics are one of the most powerful features of Swift, and much of the Swift standard library is built with
generic code. In fact, you’ve been using generics throughout this Language Guide, even if you didn’t realize it.
For example, Swift’s
Array
and
Dictionary
types are both generic collections. You can create an array
that holds
Int
values, or an array that holds
String
values, or indeed an array for any other type that can be
created in Swift. Similarly, you can create a dictionary to store values of any specified type, and there are no
limitations on what that type can be.
​
The Problem That Generics Solve
Here’s a standard, non-generic function called
swapTwoInts
, which swaps two
Int
values:
1
func

swapTwoInts
(
inout

a
:
Int
,
inout

b
:
Int
) {
2

let

temporaryA
=
a
3

a
=
b
4

b
=
temporaryA
5
}
This function makes use of in-out parameters to swap the values of
a
and
b
, as described in
In-Out
Parameters
.
The
swapTwoInts
function swaps the original value of
b
into
a
, and the original value of
a
into
b
. You can call
this function to swap the values in two
Int
variables:
1
var

someInt
=
3
2
var

anotherInt
=
107

3
swapTwoInts
(&
someInt
, &
anotherInt
)
4
println
(
"someInt is now
\(
someInt
)
, and anotherInt is now
\
(
anotherInt
)
"
)
5
// prints "someInt is now 107, and anotherInt is now 3"
The
swapTwoInts
function is useful, but it can only be used with
Int
values. If you want to swap two
String
values, or two
Double
values, you have to write more functions, such as the
swapTwoStrings
and
swapTwoDoubles
functions shown below:
1
func

swapTwoStrings
(
inout

a
:
String
,
inout

b
:
String
) {
2

let

temporaryA
=
a
3

a
=
b
4

b
=
temporaryA
5
}
6

7
func

swapTwoDoubles
(
inout

a
:
Double
,
inout

b
:
Double
) {
8

let

temporaryA
=
a
9

a
=
b
10

b
=
temporaryA
11
}
You may have noticed that the bodies of the
swapTwoInts
,
swapTwoStrings
, and
swapTwoDoubles
functions are identical. The only difference is the type of the values that they accept (
Int
,
String
, and
Double
).
It would be much more useful, and considerably more flexible, to write a single function that could swap two
values of
any
type. This is the kind of problem that generic code can solve. (A generic version of these functions
is defined below.)
NOTE
In all three functions, it is important that the types of
a
and
b
are defined to be the same as each
other. If
a
and
b
were not of the same type, it would not be possible to swap their values. Swift is a
type-safe language, and does not allow (for example) a variable of type
String
and a variable of

type
Double
to swap values with each other. Attempting to do so would be reported as a compile-
time error.
​
Generic Functions
Generic functions
can work with any type. Here’s a generic version of the
swapTwoInts
function from above,
called
swapTwoValues
:
1
func

swapTwoValues
<
T
>(
inout

a
:
T
,
inout

b
:
T
) {
2

let

temporaryA
=
a
3

a
=
b
4

b
=
temporaryA
5
}
The body of the
swapTwoValues
function is identical to the body of the
swapTwoInts
function. However,
the first line of
swapTwoValues
is slightly different from
swapTwoInts
. Here’s how the first lines
compare:
1
func

swapTwoInts
(
inout

a
:
Int
,
inout

b
:
Int
)
2
func

swapTwoValues
<
T
>(
inout

a
:
T
,
inout

b
:
T
)
The generic version of the function uses a
placeholder
type name (called
T
, in this case) instead of an
actual
type name (such as
Int
,
String
, or
Double
). The placeholder type name doesn’t say anything about what
T
must be, but it
does
say that both
a
and
b
must be of the same type
T
, whatever
T
represents. The actual type to
use in place of
T
will be determined each time the
swapTwoValues
function is called.
The other difference is that the generic function’s name (
swapTwoValues
) is followed by the placeholder type
name (
T
) inside angle brackets (
<T>
). The brackets tell Swift that
T
is a placeholder type name within the
swapTwoValues
function definition. Because
T
is a placeholder, Swift does not look for an actual type called
T
.

The
swapTwoValues
function can now be called in the same way as
swapTwoInts
, except that it can be
passed two values of
any
type, as long as both of those values are of the same type as each other. Each time
swapTwoValues
is called, the type to use for
T
is inferred from the types of values passed to the function.
In the two examples below,
T
is inferred to be
Int
and
String
respectively:
1
var

someInt
=
3
2
var

anotherInt
=
107
3
swapTwoValues
(&
someInt
, &
anotherInt
)
4
// someInt is now 107, and anotherInt is now 3
5

6
var

someString
=
"hello"
7
var

anotherString
=
"world"
8
swapTwoValues
(&
someString
, &
anotherString
)
9
// someString is now "world", and anotherString is now "hello"
NOTE
The
swapTwoValues
function defined above is inspired by a generic function called
swap
, which
is part of the Swift standard library, and is automatically made available for you to use in your apps.
If you need the behavior of the
swapTwoValues
function in your own code, you can use Swift’s
existing
swap
function rather than providing your own implementation.
​
Type Parameters
In the
swapTwoValues
example above, the placeholder type
T
is an example of a
type parameter
. Type
parameters specify and name a placeholder type, and are written immediately after the function’s name,
between a pair of matching angle brackets (such as
<T>
).
Once specified, a type parameter can be used to define the type of a function’s parameters (such as the
a
and
b
parameters of the
swapTwoValues
function); or as the function’s return type; or as a type annotation within

the body of the function. In each case, the placeholder type represented by the type parameter is replaced with
an
actual
type whenever the function is called. (In the
swapTwoValues
example above,
T
was replaced with
Int
the first time the function was called, and was replaced with
String
the second time it was called.)
You can provide more than one type parameter by writing multiple type parameter names within the angle
brackets, separated by commas.
​
Naming Type Parameters
In simple cases where a generic function or generic type needs to refer to a single placeholder type (such as the
swapTwoValues
generic function above, or a generic collection that stores a single type, such as
Array
), it
is traditional to use the single-character name
T
for the type parameter. However, you are can use any valid
identifier as the type parameter name.
If you are defining more complex generic functions, or generic types with multiple parameters, it can be useful
to provide more descriptive type parameter names. For example, Swift’s
Dictionary
type has two type
parameters—one for its keys and one for its values. If you were writing
Dictionary
yourself, you might
name these two type parameters
KeyType
and
ValueType
to remind you of their purpose as you use them
within your generic code.
NOTE
Always give type parameters
UpperCamelCase
names (such as
T
and
KeyType
) to indicate
that they are a placeholder for a
type
, not a value.
​
Generic Types
In addition to generic functions, Swift enables you to define your own
generic types
. These are custom classes,
structures, and enumerations that can work with
any
type, in a similar way to
Array
and
Dictionary
.

This section shows you how to write a generic collection type called
Stack
. A stack is an ordered set of
values, similar to an array, but with a more restricted set of operations than Swift’s
Array
type. An array
allows new items to be inserted and removed at any location in the array. A stack, however, allows new items
to be appended only to the end of the collection (known as
pushing
a new value on to the stack). Similarly, a
stack allows items to be removed only from the end of the collection (known as
popping
a value off the stack).
NOTE
The concept of a stack is used by the
UINavigationController
class to model the view
controllers in its navigation hierarchy. You call the
UINavigationController
class
pushViewController:animated:
method to add (or push) a view controller on to the
navigation stack, and its
popViewControllerAnimated:
method to remove (or pop) a view
controller from the navigation stack. A stack is a useful collection model whenever you need a strict
“last in, first out” approach to managing a collection.
The illustration below shows the push / pop behavior for a stack:

1
.
There are currently three values on the stack.
2
.
A fourth value is “pushed” on to the top of the stack.
3
.
The stack now holds four values, with the most recent one at the top.
4
.
The top item in the stack is removed, or “popped”.
5
.
After popping a value, the stack once again holds three values.
Here’s how to write a non-generic version of a stack, in this case for a stack of
Int
values:
1
struct

IntStack
{
2

var

items
=
Int
[]()
3

mutating

func

push
(
item
:
Int
) {
4

items
.
append
(
item
)
5
}

6

mutating

func

pop
() ->
Int
{
7

return

items
.
removeLast
()
8
}
9
}
This structure uses an
Array
property called
items
to store the values in the stack.
Stack
provides two
methods,
push
and
pop
, to push and pop values on and off the stack. These methods are marked as
mutating
, because they need to modify (or
mutate
) the structure’s
items
array.
The
IntStack
type shown above can only be used with
Int
values, however. It would be much more useful
to define a
generic

Stack
class, that can manage a stack of
any
type of value.
Here’s a generic version of the same code:
1
struct

Stack
<
T
> {
2

var

items
=
T
[]()
3

mutating

func

push
(
item
:
T
) {
4

items
.
append
(
item
)
5
}
6

mutating

func

pop
() ->
T
{
7

return

items
.
removeLast
()
8
}
9
}
Note how the generic version of
Stack
is essentially the same as the non-generic version, but with a
placeholder type parameter called
T
instead of an actual type of
Int
. This type parameter is written within a
pair of angle brackets (
<T>
) immediately after the structure’s name.
T
defines a placeholder name for “some type
T
” to be provided later on. This future type can be referred to as
“
T
” anywhere within the structure’s definition. In this case,
T
is used as a placeholder in three places:
To create a property called
items
, which is initialized with an empty array of values of type
T
To specify that the
push
method has a single parameter called
item
, which must be of type
T
To specify that the value returned by the
pop
method will be a value of type
T

You create instances of
Stack
in a similar way to
Array
and
Dictionary
, by writing the actual type to be
used for this specific stack within angle brackets after the type name when creating a new instance with
initializer syntax:
1
var

stackOfStrings
=
Stack
<
String
>()
2
stackOfStrings
.
push
(
"uno"
)
3
stackOfStrings
.
push
(
"dos"
)
4
stackOfStrings
.
push
(
"tres"
)
5
stackOfStrings
.
push
(
"cuatro"
)
6
// the stack now contains 4 strings
Here’s how
stackOfStrings
looks after pushing these four values on to the stack:
Popping a value from the stack returns and removes the top value,
"cuatro"
:
1
let

fromTheTop
=
stackOfStrings
.
pop
()

2
// fromTheTop is equal to "cuatro", and the stack now contains 3

strings
Here’s how the stack looks after popping its top value:
Because it is a generic type,
Stack
can be used to create a stack of
any
valid type in Swift, in a similar manner
to
Array
and
Dictionary
.
​
Type Constraints
The
swapTwoValues
function and the
Stack
type can work with any type. However, it is sometimes useful
to enforce certain
type constraints
on the types that can be used with generic functions and generic types. Type
constraints specify that a type parameter must inherit from a specific class, or conform to a particular protocol
or protocol composition.
For example, Swift’s
Dictionary
type places a limitation on the types that can be used as keys for a
dictionary. As described in
Dictionaries
, the type of a dictionary’s keys must be
hashable
. That is, it must

provide a way to make itself uniquely representable.
Dictionary
needs its keys to be hashable so that it can
check whether it already contains a value for a particular key. Without this requirement,
Dictionary
could
not tell whether it should insert or replace a value for a particular key, nor would it be able to find a value for a
given key that is already in the dictionary.
This requirement is enforced by a type constraint on the key type for
Dictionary
, which specifies that the
key type must conform to the
Hashable
protocol, a special protocol defined in the Swift standard library. All of
Swift’s basic types (such as
String
,
Int
,
Double
, and
Bool
) are hashable by default.
You can define your own type constraints when creating custom generic types, and these constraints provide
much of the power of generic programming. Abstract concepts like
Hashable
characterize types in terms of
their conceptual characteristics, rather than their explicit type.
​
Type Constraint Syntax
You write type constraints by placing a single class or protocol constraint after a type parameter’s name,
separated by a colon, as part of the type parameter list. The basic syntax for type constraints on a generic
function is shown below (although the syntax is the same for generic types):
1
func

someFunction
<
T
:
SomeClass
,
U
:
SomeProtocol
>(
someT
:
T
,
someU
:
U
) {
2

// function body goes here
3
}
The hypothetical function above has two type parameters. The first type parameter,
T
, has a type constraint that
requires
T
to be a subclass of
SomeClass
. The second type parameter,
U
, has a type constraint that requires
U
to conform to the protocol
SomeProtocol
.
​
Type Constraints in Action
Here’s a non-generic function called
findStringIndex
, which is given a
String
value to find and an
array of
String
values within which to find it. The
findStringIndex
function returns an optional
Int
value, which will be the index of the first matching string in the array if it is found, or
nil
if the string cannot be
found:

1
func

findStringIndex
(
array
:
String
[],
valueToFind
:
String
) ->
Int
? {
2

for
(
index
,
value
)
in

enumerate
(
array
) {
3

if

value
==
valueToFind
{
4

return

index
5
}
6
}
7

return

nil
8
}
The
findStringIndex
function can be used to find a string value in an array of strings:
1
let

strings
= [
"cat"
,
"dog"
,
"llama"
,
"parakeet"
,
"terrapin"
]
2
if

let

foundIndex
=
findStringIndex
(
strings
,
"llama"
) {
3

println
(
"The index of llama is
\(
foundIndex
)
"
)
4
}
5
// prints "The index of llama is 2"
The principle of finding the index of a value in an array isn’t useful only for strings, however. You can write the
same functionality as a generic function called
findIndex
, by replacing any mention of strings with values of
some type
T
instead.
Here’s how you might expect a generic version of
findStringIndex
, called
findIndex
, to be written.
Note that the return type of this function is still
Int?
, because the function returns an optional index number, not
an optional value from the array. Be warned, though—this function does not compile, for reasons explained after
the example:
1
func

findIndex
<
T
>(
array
:
T
[],
valueToFind
:
T
) ->
Int
? {
2

for
(
index
,
value
)
in

enumerate
(
array
) {
3

if

value
==
valueToFind
{
4

return

index
5
}
6
}
7

return

nil

8
}
This function does not compile as written above. The problem lies with the equality check, “
if value ==
valueToFind
”. Not every type in Swift can be compared with the equal to operator (
==
). If you create your
own class or structure to represent a complex data model, for example, then the meaning of “equal to” for that
class or structure is not something that Swift can guess for you. Because of this, it is not possible to guarantee
that this code will work for
every
possible type
T
, and an appropriate error is reported when you try to compile
the code.
All is not lost, however. The Swift standard library defines a protocol called
Equatable
, which requires any
conforming type to implement the equal to operator (
==
) and the not equal to operator (
!=
) to compare any two
values of that type. All of Swift’s standard types automatically support the
Equatable
protocol.
Any type that is
Equatable
can be used safely with the
findIndex
function, because it is guaranteed to
support the equal to operator. To express this fact, you write a type constraint of
Equatable
as part of the type
parameter’s definition when you define the function:
1
func

findIndex
<
T
:
Equatable
>(
array
:
T
[],
valueToFind
:
T
) ->
Int
? {
2

for
(
index
,
value
)
in

enumerate
(
array
) {
3

if

value
==
valueToFind
{
4

return

index
5
}
6
}
7

return

nil
8
}
The single type parameter for
findIndex
is written as
T: Equatable
, which means “any type
T
that
conforms to the
Equatable
protocol.”
The
findIndex
function now compiles successfully and can be used with any type that is
Equatable
, such
as
Double
or
String
:
1
let

doubleIndex
=
findIndex
([
3.14159
,
0.1
,
0.25
],
9.3
)
2
// doubleIndex is an optional Int with no value, because 9.3 is not in
the array

3
let

stringIndex
=
findIndex
([
"Mike"
,
"Malcolm"
,
"Andrea"
],
"Andrea"
)
4
// stringIndex is an optional Int containing a value of 2
​
Associated Types
When defining a protocol, it is sometimes useful to declare one or more
associated types
as part of the
protocol’s definition. An associated type gives a placeholder name (or
alias
) to a type that is used as part of the
protocol. The actual type to use for that associated type is not specified until the protocol is adopted. Associated
types are specified with the
typealias
keyword.
​
Associated Types in Action
Here’s an example of a protocol called
Container
, which declares an associated type called
ItemType
:
1
protocol

Container
{
2

typealias

ItemType
3

mutating

func

append
(
item
:
ItemType
)
4

var

count
:
Int
{
get
}
5

subscript
(
i
:
Int
) ->
ItemType
{
get
}
6
}
The
Container
protocol defines three required capabilities that any container must provide:
This protocol doesn’t specify how the items in the container should be stored or what type they are allowed to
be. The protocol only specifies the three bits of functionality that any type must provide in order to be considered
It must be possible to add a new item to the container with an
append
method.
It must be possible to access a count of the items in the container through a
count
property that
returns an
Int
value.
It must be possible to retrieve each item in the container with a subscript that takes an
Int
index
value.

a
Container
. A conforming type can provide additional functionality, as long as it satisfies these three
requirements.
Any type that conforms to the
Container
protocol must be able to specify the type of values it stores.
Specifically, it must ensure that only items of the right type are added to the container, and it must be clear about
the type of the items returned by its subscript.
To define these requirements, the
Container
protocol needs a way to refer to the type of the elements that a
container will hold, without knowing what that type is for a specific container. The
Container
protocol needs
to specify that any value passed to the
append
method must have the same type as the container’s element
type, and that the value returned by the container’s subscript will be of the same type as the container’s element
type.
To achieve this, the
Container
protocol declares an associated type called
ItemType
, written as
typealias ItemType
. The protocol does not define what
ItemType
is an alias
for
—that information is
left for any conforming type to provide. Nonetheless, the
ItemType
alias provides a way to refer to the type of
the items in a
Container
, and to define a type for use with the
append
method and subscript, to ensure that
the expected behavior of any
Container
is enforced.
Here’s a version of the non-generic
IntStack
type from earlier, adapted to conform to the
Container
protocol:
1
struct

IntStack
:
Container
{
2

// original IntStack implementation
3

var

items
=
Int
[]()
4

mutating

func

push
(
item
:
Int
) {
5

items
.
append
(
item
)
6
}
7

mutating

func

pop
() ->
Int
{
8

return

items
.
removeLast
()
9
}
10

// conformance to the Container protocol
11

typealias

ItemType
=
Int
12

mutating

func

append
(
item
:
Int
) {
13

self
.
push
(
item
)
14
}

15

var

count
:
Int
{
16

return

items
.
count
17
}
18

subscript
(
i
:
Int
) ->
Int
{
19

return

items
[
i
]
20
}
21
}
The
IntStack
type implements all three of the
Container
protocol’s requirements, and in each case
wraps part of the
IntStack
type’s existing functionality to satisfy these requirements.
Moreover,
IntStack
specifies that for this implementation of
Container
, the appropriate
ItemType
to
use is a type of
Int
. The definition of
typealias ItemType = Int
turns the abstract type of
ItemType
into a concrete type of
Int
for this implementation of the
Container
protocol.
Thanks to Swift’s type inference, you don’t actually need to declare a concrete
ItemType
of
Int
as part of the
definition of
IntStack
. Because
IntStack
conforms to all of the requirements of the
Container
protocol,
Swift can infer the appropriate
ItemType
to use, simply by looking at the type of the
append
method’s
item
parameter and the return type of the subscript. Indeed, if you delete the
typealias ItemType = Int
line
from the code above, everything still works, because it is clear what type should be used for
ItemType
.
You can also make the generic
Stack
type conform to the
Container
protocol:
1
struct

Stack
<
T
>:
Container
{
2

// original Stack<T> implementation
3

var

items
=
T
[]()
4

mutating

func

push
(
item
:
T
) {
5

items
.
append
(
item
)
6
}
7

mutating

func

pop
() ->
T
{
8

return

items
.
removeLast
()
9
}
10

// conformance to the Container protocol
11

mutating

func

append
(
item
:
T
) {
12

self
.
push
(
item
)

13
}
14

var

count
:
Int
{
15

return

items
.
count
16
}
17

subscript
(
i
:
Int
) ->
T
{
18

return

items
[
i
]
19
}
20
}
This time, the placeholder type parameter
T
is used as the type of the
append
method’s
item
parameter and
the return type of the subscript. Swift can therefore infer that
T
is the appropriate type to use as the
ItemType
for this particular container.
​
Extending an Existing Type to Specify an Associated Type
You can extend an existing type to add conformance to a protocol, as described in
Adding Protocol
Conformance with an Extension
. This includes a protocol with an associated type.
Swift’s
Array
type already provides an
append
method, a
count
property, and a subscript with an
Int
index to retrieve its elements. These three capabilities match the requirements of the
Container
protocol.
This means that you can extend
Array
to conform to the
Container
protocol simply by declaring that
Array
adopts the protocol. You do this with an empty extension, as described in
Declaring Protocol Adoption
with an Extension
:
1
extension

Array
:
Container
{}
Array’s existing
append
method and subscript enable Swift to infer the appropriate type to use for
ItemType
,
just as for the generic
Stack
type above. After defining this extension, you can use any
Array
as a
Container
.
​
Where Clauses

Type constraints, as described in
Type Constraints
, enable you to define requirements on the type parameters
associated with a generic function or type.
It can also be useful to define requirements for associated types. You do this by defining
where clauses
as part
of a type parameter list. A where clause enables you to require that an associated type conforms to a certain
protocol, and/or that certain type parameters and associated types be the same. You write a where clause by
placing the
where
keyword immediately after the list of type parameters, followed by one or more constraints
for associated types, and/or one or more equality relationships between types and associated types.
The example below defines a generic function called
allItemsMatch
, which checks to see if two
Container
instances contain the same items in the same order. The function returns a Boolean value of
true
if all items match and a value of
false
if they do not.
The two containers to be checked do not have to be the same type of container (although they can be), but they
do have to hold the same type of items. This requirement is expressed through a combination of type
constraints and where clauses:
1
func

allItemsMatch
<
2

C1
:
Container
,
C2
:
Container
3

where

C1
.
ItemType
==
C2
.
ItemType
,
C1
.
ItemType
:
Equatable
>
4
(
someContainer
:
C1
,
anotherContainer
:
C2
) ->
Bool
{
5

6

// check that both containers contain the same number of items
7

if

someContainer
.
count
!=
anotherContainer
.
count
{
8

return

false
9
}
10

11

// check each pair of items to see if they are

equivalent
12

for

i

in

0
..
someContainer
.
count
{
13

if

someContainer
[
i
] !=
anotherContainer
[
i
] {
14

return

false
15
}
16
}
17

18

// all items match, so return true

19

return

true
20

21
}
This function takes two arguments called
someContainer
and
anotherContainer
. The
someContainer
argument is of type
C1
, and the
anotherContainer
argument is of type
C2
. Both
C1
and
C2
are placeholder type parameters for two container types to be determined when the function is called.
The function’s type parameter list places the following requirements on the two type parameters:
The third and fourth requirements are defined as part of a where clause, and are written after the
where
keyword as part of the function’s type parameter list.
These requirements mean:
The third and fourth requirements combine to mean that the items in
anotherContainer
can
also
be
checked with the
!=
operator, because they are exactly the same type as the items in
someContainer
.
These requirements enable the
allItemsMatch
function to compare the two containers, even if they are of a
different container type.
C1
must conform to the
Container
protocol (written as
C1: Container
).
C2
must also conform to the
Container
protocol (written as
C2: Container
).
The
ItemType
for
C1
must be the same as the
ItemType
for
C2
(written as
C1.ItemType
== C2.ItemType
).
The
ItemType
for
C1
must conform to the
Equatable
protocol (written as
C1.ItemType:
Equatable
).
someContainer
is a container of type
C1
.
anotherContainer
is a container of type
C2
.
someContainer
and
anotherContainer
contain the same type of items.
The items in
someContainer
can be checked with the not equal operator (
!=
) to see if they
are different from each other.

The
allItemsMatch
function starts by checking that both containers contain the same number of items. If
they contain a different number of items, there is no way that they can match, and the function returns
false
.
After making this check, the function iterates over all of the items in
someContainer
with a
for
–
in
loop and
the half-closed range operator (
..
). For each item, the function checks whether the item from
someContainer
is not equal to the corresponding item in
anotherContainer
. If the two items are not
equal, then the two containers do not match, and the function returns
false
.
If the loop finishes without finding a mismatch, the two containers match, and the function returns
true
.
Here’s how the
allItemsMatch
function looks in action:
1
var

stackOfStrings
=
Stack
<
String
>()
2
stackOfStrings
.
push
(
"uno"
)
3
stackOfStrings
.
push
(
"dos"
)
4
stackOfStrings
.
push
(
"tres"
)
5

6
var

arrayOfStrings
= [
"uno"
,
"dos"
,
"tres"
]
7

8
if

allItemsMatch
(
stackOfStrings
,
arrayOfStrings
) {
9

println
(
"All items match."
)
10
}
else
{
11

println
(
"Not all items match."
)
12
}
13
// prints "All items match."
The example above creates a
Stack
instance to store
String
values, and pushes three strings onto the
stack. The example also creates an
Array
instance initialized with an array literal containing the same three
strings as the stack. Even though the stack and the array are of a different type, they both conform to the
Container
protocol, and both contain the same type of values. You can therefore call the
allItemsMatch
function with these two containers as its arguments. In the example above, the
allItemsMatch
function
correctly reports that all of the items in the two containers match.

​
​
Advanced Operators
In addition to the operators described in
Basic Operators
, Swift provides several advanced operators that
perform more complex value manipulation. These include all of the bitwise and bit shifting operators you will be
familiar with from C and Objective-C.
Unlike arithmetic operators in C, arithmetic operators in Swift do not overflow by default. Overflow behavior is
trapped and reported as an error. To opt in to overflow behavior, use Swift’s second set of arithmetic operators
that overflow by default, such as the overflow addition operator (
&+
). All of these overflow operators begin with
an ampersand (
&
).
When you define your own structures, classes, and enumerations, it can be useful to provide your own
implementations of the standard Swift operators for these custom types. Swift makes it easy to provide tailored
implementations of these operators and to determine exactly what their behavior should be for each type you
create.
You’re not just limited to the predefined operators. Swift gives you the freedom to define your own custom infix,
prefix, postfix, and assignment operators, with custom precedence and associativity values. These operators
can be used and adopted in your code just like any of the predefined operators, and you can even extend existing
types to support the custom operators you define.
​
Bitwise Operators
Bitwise operators
enable you to manipulate the individual raw data bits within a data structure. They are often
used in low-level programming, such as graphics programming and device driver creation. Bitwise operators
can also be useful when you work with raw data from external sources, such as encoding and decoding data for
communication over a custom protocol.
Swift supports all of the bitwise operators found in C, as described below.
​
Bitwise NOT Operator

The
bitwise NOT operator
(
~
) inverts all bits in a number:
The bitwise NOT operator is a prefix operator, and appears immediately before the value it operates on, without
any white space:
1
let

initialBits
:
UInt8
=
0b00001111
2
let

invertedBits
= ~
initialBits

// equals 11110000
UInt8
integers have eight bits and can store any value between
0
and
255
. This example initializes a
UInt8
integer with the binary value
00001111
, which has its first four bits set to
0
, and its second four bits set to
1
.
This is equivalent to a decimal value of
15
.
The bitwise NOT operator is then used to create a new constant called
invertedBits
, which is equal to
initialBits
, but with all of the bits inverted. Zeroes become ones, and ones become zeroes. The value of
invertedBits
is
11110000
, which is equal to an unsigned decimal value of
240
.
​
Bitwise AND Operator
The
bitwise AND operator
(
&
) combines the bits of two numbers. It returns a new number whose bits are set to
1
only if the bits were equal to
1
in
both
input numbers:

In the example below, the values of
firstSixBits
and
lastSixBits
both have four middle bits equal to
1
. The bitwise AND operator combines them to make the number
00111100
, which is equal to an unsigned
decimal value of
60
:
1
let

firstSixBits
:
UInt8
=
0b11111100
2
let

lastSixBits
:
UInt8
=
0b00111111
3
let

middleFourBits
=
firstSixBits
&
lastSixBits

// equals 00111100
​
Bitwise OR Operator
The
bitwise OR operator
(
|
) compares the bits of two numbers. The operator returns a new number whose bits
are set to
1
if the bits are equal to
1
in
either
input number:

In the example below, the values of
someBits
and
moreBits
have different bits set to
1
. The bitwise OR
operator combines them to make the number
11111110
, which equals an unsigned decimal of
254
:
1
let

someBits
:
UInt8
=
0b10110010
2
let

moreBits
:
UInt8
=
0b01011110
3
let

combinedbits
=
someBits
|
moreBits

// equals 11111110
​
Bitwise XOR Operator
The
bitwise XOR operator
, or “exclusive OR operator” (
^
), compares the bits of two numbers. The operator
returns a new number whose bits are set to
1
where the input bits are different and are set to
0
where the input
bits are the same:

In the example below, the values of
firstBits
and
otherBits
each have a bit set to
1
in a location that the
other does not. The bitwise XOR operator sets both of these bits to
1
in its output value. All of the other bits in
firstBits
and
otherBits
match and are set to
0
in the output value:
1
let

firstBits
:
UInt8
=
0b00010100
2
let

otherBits
:
UInt8
=
0b00000101
3
let

outputBits
=
firstBits
^
otherBits

// equals 00010001
​
Bitwise Left and Right Shift Operators
The
bitwise left shift operator
(
<<
) and
bitwise right shift operator
(
>>
) move all bits in a number to the left or
the right by a certain number of places, according to the rules defined below.
Bitwise left and right shifts have the effect of multiplying or dividing an integer number by a factor of two. Shifting
an integer’s bits to the left by one position doubles its value, whereas shifting it to the right by one position halves
its value.

​
Shifting Behavior for Unsigned Integers
The bit-shifting behavior for unsigned integers is as follows:
1
.
Existing bits are moved to the left or right by the requested number of places.
2
.
Any bits that are moved beyond the bounds of the integer’s storage are discarded.
3
.
Zeroes are inserted in the spaces left behind after the original bits are moved to the left or right.
This approach is known as a
logical shift
.
The illustration below shows the results of
11111111 << 1
(which is
11111111
shifted to the left by
1
place), and
11111111 >> 1
(which is
11111111
shifted to the right by
1
place). Blue numbers are shifted,
gray numbers are discarded, and orange zeroes are inserted:
Here’s how bit shifting looks in Swift code:
1
let

shiftBits
:
UInt8
=
4

// 00000100 in binary
2
shiftBits
<<
1

// 00001000
3
shiftBits
<<
2

// 00010000
4
shiftBits
<<
5

// 10000000
5
shiftBits
<<
6

// 00000000
6
shiftBits
>>
2

// 00000001

You can use bit shifting to encode and decode values within other data types:
1
let

pink
:
UInt32
=
0xCC6699
2
let

redComponent
= (
pink
&
0xFF0000
) >>
16

// redComponent is 0xCC,
or 204
3
let

greenComponent
= (
pink
&
0x00FF00
) >>
8

// greenComponent is

0x66, or 102
4
let

blueComponent
=
pink
&
0x0000FF

// blueComponent is

0x99, or 153
This example uses a
UInt32
constant called
pink
to store a Cascading Style Sheets color value for the color
pink. The CSS color value
#CC6699
is written as
0xCC6699
in Swift’s hexadecimal number representation.
This color is then decomposed into its red (
CC
), green (
66
), and blue (
99
) components by the bitwise AND
operator (
&
) and the bitwise right shift operator (
>>
).
The red component is obtained by performing a bitwise AND between the numbers
0xCC6699
and
0xFF0000
. The zeroes in
0xFF0000
effectively “mask” the second and third bytes of
0xCC6699
, causing
the
6699
to be ignored and leaving
0xCC0000
as the result.
This number is then shifted 16 places to the right (
>> 16
). Each pair of characters in a hexadecimal number
uses 8 bits, so a move 16 places to the right will convert
0xCC0000
into
0x0000CC
. This is the same as
0xCC
, which has a decimal value of
204
.
Similarly, the green component is obtained by performing a bitwise AND between the numbers
0xCC6699
and
0x00FF00
, which gives an output value of
0x006600
. This output value is then shifted eight places to the
right, giving a a value of
0x66
, which has a decimal value of
102
.
Finally, the blue component is obtained by performing a bitwise AND between the numbers
0xCC6699
and
0x0000FF
, which gives an output value of
0x000099
. There’s no need to shift this to the right, as
0x000099
already equals
0x99
, which has a decimal value of
153
.
​
Shifting Behavior for Signed Integers
The shifting behavior is more complex for signed integers than for unsigned integers, because of the way signed

integers are represented in binary. (The examples below are based on 8-bit signed integers for simplicity, but
the same principles apply for signed integers of any size.)
Signed integers use their first bit (known as the
sign bit
) to indicate whether the integer is positive or negative. A
sign bit of
0
means positive, and a sign bit of
1
means negative.
The remaining bits (known as the
value bits
) store the actual value. Positive numbers are stored in exactly the
same way as for unsigned integers, counting upwards from
0
. Here’s how the bits inside an
Int8
look for the
number
4
:
The sign bit is
0
(meaning “positive”), and the seven value bits are just the number
4
, written in binary notation.
Negative numbers, however, are stored differently. They are stored by subtracting their absolute value from
2
to the power of
n
, where
n
is the number of value bits. An eight-bit number has seven value bits, so this means
2
to the power of
7
, or
128
.
Here’s how the bits inside an
Int8
look for the number
-4
:

This time, the sign bit is
1
(meaning “negative”), and the seven value bits have a binary value of
124
(which is
128 – 4
):
The encoding for negative numbers is known as a
two’s complement
representation. It may seem an unusual
way to represent negative numbers, but it has several advantages.
First, you can add
-1
to
-4
, simply by performing a standard binary addition of all eight bits (including the sign
bit), and discarding anything that doesn’t fit in the eight bits once you’re done:
Second, the two’s complement representation also lets you shift the bits of negative numbers to the left and right
like positive numbers, and still end up doubling them for every shift you make to the left, or halving them for

every shift you make to the right. To achieve this, an extra rule is used when signed integers are shifted to the
right:
This action ensures that signed integers have the same sign after they are shifted to the right, and is known as
an
arithmetic shift
.
Because of the special way that positive and negative numbers are stored, shifting either of them to the right
moves them closer to zero. Keeping the sign bit the same during this shift means that negative integers remain
negative as their value moves closer to zero.
​
Overflow Operators
If you try to insert a number into an integer constant or variable that cannot hold that value, by default Swift
reports an error rather than allowing an invalid value to be created. This behavior gives extra safety when you
work with numbers that are too large or too small.
For example, the
Int16
integer type can hold any signed integer number between
-32768
and
32767
.
Trying to set a
UInt16
constant or variable to a number outside of this range causes an error:
1
var

potentialOverflow
=
Int16
.
max
When you shift signed integers to the right, apply the same rules as for unsigned integers, but fill
any empty bits on the left with the
sign bit
, rather than with a zero.

2
// potentialOverflow equals 32767, which is the largest value an Int16
can hold
3
potentialOverflow
+=
1
4
// this causes an error
Providing error handling when values get too large or too small gives you much more flexibility when coding for
boundary value conditions.
However, when you specifically want an overflow condition to truncate the number of available bits, you can opt
in to this behavior rather than triggering an error. Swift provides five arithmetic
overflow operators
that opt in to
the overflow behavior for integer calculations. These operators all begin with an ampersand (
&
):
​
Value Overflow
Here’s an example of what happens when an unsigned value is allowed to overflow, using the overflow addition
operator (
&+
):
1
var

willOverflow
=
UInt8
.
max
2
// willOverflow equals 255, which is the largest value a UInt8 can

hold
3
willOverflow
=
willOverflow
&+
1
4
// willOverflow is now equal to 0
The variable
willOverflow
is initialized with the largest value a
UInt8
can hold (
255
, or
11111111
in
binary). It is then incremented by
1
using the overflow addition operator (
&+
). This pushes its binary
representation just over the size that a
UInt8
can hold, causing it to overflow beyond its bounds, as shown in
Overflow addition (
&+
)
Overflow subtraction (
&-
)
Overflow multiplication (
&*
)
Overflow division (
&/
)
Overflow remainder (
&%
)

the diagram below. The value that remains within the bounds of the
UInt8
after the overflow addition is
00000000
, or zero:
​
Value Underflow
Numbers can also become too small to fit in their type’s maximum bounds. Here’s an example.
The
smallest
value that a UInt8 can hold is
0
(which is
00000000
in eight-bit binary form). If you subtract
1
from
00000000
using the overflow subtraction operator, the number will overflow back round to
11111111
,
or
255
in decimal:

Here’s how that looks in Swift code:
1
var

willUnderflow
=
UInt8
.
min
2
// willUnderflow equals 0, which is the smallest value a UInt8 can

hold
3
willUnderflow
=
willUnderflow
&-
1
4
// willUnderflow is now equal to 255
A similar underflow occurs for signed integers. All subtraction for signed integers is performed as straight
binary subtraction, with the sign bit included as part of the numbers being subtracted, as described in
Bitwise
Left and Right Shift Operators
. The smallest number that an
Int8
can hold is
-128
, which is
10000000
in
binary. Subtracting
1
from this binary number with the overflow operator gives a binary value of
01111111
,
which toggles the sign bit and gives positive
127
, the largest positive value that an
Int8
can hold:

Here’s the same thing in Swift code:
1
var

signedUnderflow
=
Int8
.
min
2
// signedUnderflow equals -128, which is the smallest value an Int8

can hold
3
signedUnderflow
=
signedUnderflow
&-
1
4
// signedUnderflow is now equal to 127
The end result of the overflow and underflow behavior described above is that for both signed and unsigned
integers, overflow always wraps around from the largest valid integer value back to the smallest, and underflow
always wraps around from the smallest value to the largest.
​
Division by Zero
Dividing a number by zero (
i / 0
), or trying to calculate remainder by zero (
i % 0
), causes an error:
1
let

x
=
1
2
let

y
=
x
/
0

However, the overflow versions of these operators (
&/
and
&%
) return a value of zero if you divide by zero:
1
let

x
=
1
2
let

y
=
x
&/
0
3
// y is equal to 0
​
Precedence and Associativity
Operator
precedence
gives some operators higher priority than others; these operators are calculated first.
Operator
associativity
defines how operators of the same precedence are grouped together (or
associated
)—
either grouped from the left, or grouped from the right. Think of it as meaning “they associate with the
expression to their left,” or “they associate with the expression to their right.”
It is important to consider each operator’s precedence and associativity when working out the order in which a
compound expression will be calculated. Here’s an example. Why does the following expression equal
4
?
1
2
+
3
*
4
%
5
2
// this equals 4
Taken strictly from left to right, you might expect this to read as follows:
However, the actual answer is
4
, not
0
. Higher-precedence operators are evaluated before lower-precedence
ones. In Swift, as in C, the multiplication operator (
*
) and the remainder operator (
%
) have a higher precedence
than the addition operator (
+
). As a result, they are both evaluated before the addition is considered.
However, multiplication and remainder have the
same
precedence as each other. To work out the exact
2 plus 3 equals 5;
5 times 4 equals 20;
20 remainder 5 equals 0

evaluation order to use, you also need to consider their associativity. Multiplication and remainder both
associate with the expression to their left. Think of this as adding implicit parentheses around these parts of the
expression, starting from their left:
1
2
+ ((
3
*
4
) %
5
)
(3 * 4)
is
12
, so this is equivalent to:
1
2
+ (
12
%
5
)
(12 % 5)
is
2
, so this is equivalent to:
1
2
+
2
This calculation yields the final answer of
4
.
For a complete list of Swift operator precedences and associativity rules, see
Expressions
.
NOTE
Swift’s operator precedences and associativity rules are simpler and more predictable than those
found in C and Objective-C. However, this means that they are not the same as in C-based
languages. Be careful to ensure that operator interactions still behave in the way you intend when
porting existing code to Swift.
​
Operator Functions
Classes and structures can provide their own implementations of existing operators. This is known as
overloading
the existing operators.

The example below shows how to implement the arithmetic addition operator (
+
) for a custom structure. The
arithmetic addition operator is a
binary operator
because it operates on two targets and is said to be
infix
because it appears in between those two targets.
The example defines a
Vector2D
structure for a two-dimensional position vector
(x, y)
, followed by a
definition of an
operator function
to add together instances of the
Vector2D
structure:
1
struct

Vector2D
{
2

var

x
=
0.0
,
y
=
0.0
3
}
4
@infix

func
+ (
left
:
Vector2D
,
right
:
Vector2D
) ->
Vector2D
{
5

return

Vector2D
(
x
:
left
.
x
+
right
.
x
,
y
:
left
.
y
+
right
.
y
)
6
}
The operator function is defined as a global function called
+
, which takes two input parameters of type
Vector2D
and returns a single output value, also of type
Vector2D
. You implement an infix operator by
writing the
@infix
attribute before the
func
keyword when declaring the operator function.
In this implementation, the input parameters are named
left
and
right
to represent the
Vector2D
instances that will be on the left side and right side of the
+
operator. The function returns a new
Vector2D
instance, whose
x
and
y
properties are initialized with the sum of the
x
and
y
properties from the two
Vector2D
instances that are added together.
The function is defined globally, rather than as a method on the
Vector2D
structure, so that it can be used as
an infix operator between existing
Vector2D
instances:
1
let

vector
=
Vector2D
(
x
:
3.0
,
y
:
1.0
)
2
let

anotherVector
=
Vector2D
(
x
:
2.0
,
y
:
4.0
)
3
let

combinedVector
=
vector
+
anotherVector
4
// combinedVector is a Vector2D instance with values of (5.0, 5.0)
This example adds together the vectors
(3.0, 1.0)
and
(2.0, 4.0)
to make the vector
(5.0, 5.0)
,
as illustrated below.

​
Prefix and Postfix Operators
The example shown above demonstrates a custom implementation of a binary infix operator. Classes and
structures can also provide implementations of the standard
unary operators
. Unary operators operate on a
single target. They are
prefix
if they precede their target (such as
-a
) and
postfix
operators if they follow their

target (such as
i++
).
You implement a prefix or postfix unary operator by writing the
@prefix
or
@postfix
attribute before the
func
keyword when declaring the operator function:
1
@prefix

func
– (
vector
:
Vector2D
) ->
Vector2D
{
2

return

Vector2D
(
x
: –
vector
.
x
,
y
: –
vector
.
y
)
3
}
The example above implements the unary minus operator (
-a
) for
Vector2D
instances. The unary minus
operator is a prefix operator, and so this function has to be qualified with the
@prefix
attribute.
For simple numeric values, the unary minus operator converts positive numbers into their negative equivalent
and vice versa. The corresponding implementation for
Vector2D
instances performs this operation on both
the
x
and
y
properties:
1
let

positive
=
Vector2D
(
x
:
3.0
,
y
:
4.0
)
2
let

negative
= –
positive
3
// negative is a Vector2D instance with values of (-3.0, -4.0)
4
let

alsoPositive
= –
negative
5
// alsoPositive is a Vector2D instance with values of (3.0, 4.0)
​
Compound Assignment Operators
Compound assignment operators
combine assignment (
=
) with another operation. For example, the addition
assignment operator (
+=
) combines addition and assignment into a single operation. Operator functions that
implement compound assignment must be qualified with the
@assignment
attribute. You must also mark a
compound assignment operator’s left input parameter as
inout
, because the parameter’s value will be
modified directly from within the operator function.
The example below implements an addition assignment operator function for
Vector2D
instances:
1
@assignment

func
+= (
inout

left
:
Vector2D
,
right
:
Vector2D
) {

2

left
=
left
+
right
3
}
Because an addition operator was defined earlier, you don’t need to reimplement the addition process here.
Instead, the addition assignment operator function takes advantage of the existing addition operator function, and
uses it to set the left value to be the left value plus the right value:
1
var

original
=
Vector2D
(
x
:
1.0
,
y
:
2.0
)
2
let

vectorToAdd
=
Vector2D
(
x
:
3.0
,
y
:
4.0
)
3
original
+=
vectorToAdd
4
// original now has values of (4.0, 6.0)
You can combine the
@assignment
attribute with either the
@prefix
or
@postfix
attribute, as in this
implementation of the prefix increment operator (
++a
) for
Vector2D
instances:
1
@prefix

@assignment

func
++ (
inout

vector
:
Vector2D
) ->
Vector2D
{
2

vector
+=
Vector2D
(
x
:
1.0
,
y
:
1.0
)
3

return

vector
4
}
The prefix increment operator function above takes advantage of the addition assignment operator defined
earlier. It adds a
Vector2D
with
x
and
y
values of
1.0
to the
Vector2D
on which it is called, and returns the
result:
1
var

toIncrement
=
Vector2D
(
x
:
3.0
,
y
:
4.0
)
2
let

afterIncrement
= ++
toIncrement
3
// toIncrement now has values of (4.0, 5.0)
4
// afterIncrement also has values of (4.0, 5.0)
NOTE
It is not possible to overload the default assignment operator (
=
). Only the compound assignment

operators can be overloaded. Similarly, the ternary conditional operator (
a ? b : c
) cannot be
overloaded.
​
Equivalence Operators
Custom classes and structures do not receive a default implementation of the
equivalence operators
, known as
the “equal to” operator (
==
) and “not equal to” operator (
!=
). It is not possible for Swift to guess what would
qualify as “equal” for your own custom types, because the meaning of “equal” depends on the roles that those
types play in your code.
To use the equivalence operators to check for equivalence of your own custom type, provide an implementation
of the operators in the same way as for other infix operators:
1
@infix

func
== (
left
:
Vector2D
,
right
:
Vector2D
) ->
Bool
{
2

return
(
left
.
x
==
right
.
x
) && (
left
.
y
==
right
.
y
)
3
}
4
@infix

func
!= (
left
:
Vector2D
,
right
:
Vector2D
) ->
Bool
{
5

return
!(
left
==
right
)
6
}
The above example implements an “equal to” operator (
==
) to check if two
Vector2D
instances have
equivalent values. In the context of
Vector2D
, it makes sense to consider “equal” as meaning “both instances
have the same
x
values and
y
values”, and so this is the logic used by the operator implementation. The
example also implements the “not equal to” operator (
!=
), which simply returns the inverse of the result of the
“equal to” operator.
You can now use these operators to check whether two
Vector2D
instances are equivalent:
1
let

twoThree
=
Vector2D
(
x
:
2.0
,
y
:
3.0
)
2
let

anotherTwoThree
=
Vector2D
(
x
:
2.0
,
y
:
3.0
)
3
if

twoThree
==
anotherTwoThree
{
4

println
(
"These two vectors are equivalent."
)

5
}
6
// prints "These two vectors are equivalent."
​
Custom Operators
You can declare and implement your own
custom operators
in addition to the standard operators provided by
Swift. Custom operators can be defined only with the characters
/ = – + * % < > ! & | ^ . ~
.
New operators are declared at a global level using the
operator
keyword, and can be declared as
prefix
,
infix
or
postfix
:
1
operator

prefix
+++ {}
The example above defines a new prefix operator called
+++
. This operator does not have an existing meaning
in Swift, and so it is given its own custom meaning below in the specific context of working with
Vector2D
instances. For the purposes of this example,
+++
is treated as a new “prefix doubling incrementer” operator. It
doubles the
x
and
y
values of a
Vector2D
instance, by adding the vector to itself with the addition assignment
operator defined earlier:
1
@prefix

@assignment

func
+++ (
inout

vector
:
Vector2D
) ->
Vector2D
{
2

vector
+=
vector
3

return

vector
4
}
This implementation of
+++
is very similar to the implementation of
++
for
Vector2D
, except that this
operator function adds the vector to itself, rather than adding
Vector2D(1.0, 1.0)
:
1
var

toBeDoubled
=
Vector2D
(
x
:
1.0
,
y
:
4.0
)
2
let

afterDoubling
= +++
toBeDoubled
3
// toBeDoubled now has values of (2.0, 8.0)
4
// afterDoubling also has values of (2.0, 8.0)

​
Precedence and Associativity for Custom Infix Operators
Custom
infix
operators can also specify a
precedence
and an
associativity
. See
Precedence and
Associativity
for an explanation of how these two characteristics affect an infix operator’s interaction with other
infix operators.
The possible values for
associativity
are
left
,
right
, and
none
. Left-associative operators associate
to the left if written next to other left-associative operators of the same precedence. Similarly, right-associative
operators associate to the right if written next to other right-associative operators of the same precedence. Non-
associative operators cannot be written next to other operators with the same precedence.
The
associativity
value defaults to
none
if it is not specified. The
precedence
value defaults to
100
if
it is not specified.
The following example defines a new custom
infix
operator called
+-
, with
left
associativity and a
precedence of
140
:
1
operator

infix
+- {
associativity

left

precedence

140
}
2
func
+- (
left
:
Vector2D
,
right
:
Vector2D
) ->
Vector2D
{
3

return

Vector2D
(
x
:
left
.
x
+
right
.
x
,
y
:
left
.
y
–
right
.
y
)
4
}
5
let

firstVector
=
Vector2D
(
x
:
1.0
,
y
:
2.0
)
6
let

secondVector
=
Vector2D
(
x
:
3.0
,
y
:
4.0
)
7
let

plusMinusVector
=
firstVector
+-
secondVector
8
// plusMinusVector is a Vector2D instance with values of (4.0, -2.0)
This operator adds together the
x
values of two vectors, and subtracts the
y
value of the second vector from the
first. Because it is in essence an “additive” operator, it has been given the same associativity and precedence
values (
left
and
140
) as default additive infix operators such as
+
and
–
. For a complete list of the default
Swift operator precedence and associativity settings, see
Expressions
.

​
​
Language Reference

​
​
About the Language Reference
This part of the book describes the formal grammar of the Swift programming language. The grammar
described here is intended to help you understand the language in more detail, rather than to allow you to
directly implement a parser or compiler.
The Swift language is relatively small, because many common types, functions, and operators that appear
virtually everywhere in Swift code are actually defined in the Swift standard library. Although these types,
functions, and operators are not part of the Swift language itself, they are used extensively in the discussions
and code examples in this part of the book.
​
How to Read the Grammar
The notation used to describe the formal grammar of the Swift programming language follows a few
conventions:
As an example, the grammar of a getter-setter block is defined as follows:
GRAMMAR OF A GETTER-SETTER BLOCK
An arrow (
→
) is used to mark grammar productions and can be read as “can consist of.“
Syntactic categories are indicated by
italic
text and appear on both sides of a grammar
production rule.
Literal words and punctuation are indicated by boldface
constant width
text and appear
only on the right-hand side of a grammar production rule.
Alternative grammar productions are separated by vertical bars (|). When alternative
productions are too long to read easily, they are broken into multiple grammar production rules
on new lines.
In a few cases, regular font text is used to describe the right-hand side of a grammar production
rule.
Optional syntactic categories and literals are marked by a trailing subscript,
opt
.

​

getter-setter-block
→

{
getter-clause
setter-clause
opt
}

{
setter-clause
getter-
clause
}
This definition indicates that a getter-setter block can consist of a getter clause followed by an optional setter
clause, enclosed in braces,
or
a setter clause followed by a getter clause, enclosed in braces. The grammar
production above is equivalent to the following two productions, where the alternatives are spelled out explicitly:
GRAMMAR OF A GETTER SETTER BLOCK
​

getter-setter-block
→

{
getter-clause
setter-clause
opt
}
​

getter-setter-block
→

{
setter-clause
getter-clause
}

​
​
Lexical Structure
The
lexical structure
of Swift describes what sequence of characters form valid tokens of the language. These
valid tokens form the lowest-level building blocks of the language and are used to describe the rest of the
language in subsequent chapters.
In most cases, tokens are generated from the characters of a Swift source file by considering the longest
possible substring from the input text, within the constraints of the grammar that are specified below. This
behavior is referred to as
longest match
or
maximal munch
.
​
Whitespace and Comments
Whitespace has two uses: to separate tokens in the source file and to help determine whether an operator is a
prefix or postfix (see
Operators
), but is otherwise ignored. The following characters are considered whitespace:
space (U+0020), line feed (U+000A), carriage return (U+000D), horizontal tab (U+0009), vertical tab
(U+000B), form feed (U+000C) and null (U+0000).
Comments are treated as whitespace by the compiler. Single line comments begin with
//
and continue until the
end of the line. Multiline comments begin with
/*
and end with
*/
. Nesting is allowed, but the comment
markers must be balanced.
​
Identifiers
Identifiers
begin with an upper case or lower case letter A through Z, an underscore (
_
), a noncombining
alphanumeric Unicode character in the Basic Multilingual Plane, or a character outside the Basic Multilingual
Plan that isn’t in a Private Use Area. After the first character, digits and combining Unicode characters are also
allowed.
To use a reserved word as an identifier, put a backtick (
`
) before and after it. For example,
class
is not a
valid identifier, but
`class`
is valid. The backticks are not considered part of the identifier;
`x`
and
x
have
the same meaning.

Inside a closure with no explicit parameter names, the parameters are implicitly named
$0
,
$1
,
$2
, and so on.
These names are valid identifiers within the scope of the closure.
GRAMMAR OF AN IDENTIFIER
​

identifier
→

identifier-head
identifier-characters
opt
​

identifier
→

`
identifier-head
identifier-characters
opt
`
​

identifier
→

implicit-parameter-name
​

identifier-list
→

identifier

identifier
,
identifier-list
​

identifier-head
→

Upper- or lowercase letter A through Z
​

identifier-head
→

U+00A8, U+00AA, U+00AD, U+00AF, U+00B2–U+00B5, or U+00B7–U+00BA
​

identifier-head
→

U+00BC–U+00BE, U+00C0–U+00D6, U+00D8–U+00F6, or U+00F8–U+00FF
​

identifier-head
→

U+0100–U+02FF, U+0370–U+167F, U+1681–U+180D, or U+180F–U+1DBF
​

identifier-head
→

U+1E00–U+1FFF
​

identifier-head
→

U+200B–U+200D, U+202A–U+202E, U+203F–U+2040, U+2054, or U+2060–
U+206F
​

identifier-head
→

U+2070–U+20CF, U+2100–U+218F, U+2460–U+24FF, or U+2776–U+2793
​

identifier-head
→

U+2C00–U+2DFF or U+2E80–U+2FFF
​

identifier-head
→

U+3004–U+3007, U+3021–U+302F, U+3031–U+303F, or U+3040–U+D7FF
​

identifier-head
→

U+F900–U+FD3D, U+FD40–U+FDCF, U+FDF0–U+FE1F, or U+FE30–U+FE44
​

identifier-head
→

U+FE47–U+FFFD
​

identifier-head
→

U+10000–U+1FFFD, U+20000–U+2FFFD, U+30000–U+3FFFD, or U+40000–
U+4FFFD
​

identifier-head
→

U+50000–U+5FFFD, U+60000–U+6FFFD, U+70000–U+7FFFD, or U+80000–
U+8FFFD
​

identifier-head
→

U+90000–U+9FFFD, U+A0000–U+AFFFD, U+B0000–U+BFFFD, or U+C0000–
U+CFFFD
​

identifier-head
→

U+D0000–U+DFFFD or U+E0000–U+EFFFD
​

identifier-character
→

Digit 0 through 9
​

identifier-character
→

U+0300–U+036F, U+1DC0–U+1DFF, U+20D0–U+20FF, or U+FE20–
U+FE2F
​

identifier-character
→

identifier-head
​

identifier-characters
→

identifier-character
identifier-characters
opt
​

implicit-parameter-name
→

$
decimal-digits
​
Keywords
The following keywords are reserved and may not be used as identifiers, unless they’re escaped with
backticks, as described above in
Identifiers
.

​
Literals
A
literal
is the source code representation of a value of an integer, floating-point number, or string type. The
following are examples of literals:
1
42

// Integer literal
2
3.14159

// Floating-point literal
3
"Hello, world!"

// String literal
GRAMMAR OF A LITERAL
​

literal
→

integer-literal

floating-point-literal

string-literal
​
Integer Literals
Integer literals
represent integer values of unspecified precision. By default, integer literals are expressed in
decimal; you can specify an alternate base using a prefix. Binary literals begin with
0b
, octal literals begin with
0o
, and hexadecimal literals begin with
0x
.
Keywords used in declarations:
class
,
deinit
,
enum
,
extension
,
func
,
import
,
init
,
let
,
protocol
,
static
,
struct
,
subscript
,
typealias
, and
var
.
Keywords used in statements:
break
,
case
,
continue
,
default
,
do
,
else
,
fallthrough
,
if
,
in
,
for
,
return
,
switch
,
where
, and
while
.
Keywords used in expressions and types:
as
,
dynamicType
,
is
,
new
,
super
,
self
,
Self
,
Type
,
__COLUMN__
,
__FILE__
,
__FUNCTION__
, and
__LINE__
.
Keywords reserved in particular contexts:
associativity
,
didSet
,
get
,
infix
,
inout
,
left
,
mutating
,
none
,
nonmutating
,
operator
,
override
,
postfix
,
precedence
,
prefix
,
right
,
set
,
unowned
,
unowned(safe)
,
unowned(unsafe)
,
weak
and
willSet
. Outside the context in which they appear in the grammar, they can be
used as identifiers.

Decimal literals contain the digits
0
through
9
. Binary literals contain
0
and
1
, octal literals contain
0
through
7
,
and hexadecimal literals contain
0
through
9
as well as
A
through
F
in upper- or lowercase.
Negative integers literals are expressed by prepending a minus sign (
–
) to an integer literal, as in
-42
.
Underscores (
_
) are allowed between digits for readability, but are ignored and therefore don’t affect the value
of the literal. Integer literals can begin with leading zeros (
0
), but are likewise ignored and don’t affect the base
or value of the literal.
Unless otherwise specified, the default type of an integer literal is the Swift standard library type
Int
. The Swift
standard library also defines types for various sizes of signed and unsigned integers, as described in
Integers
.
GRAMMAR OF AN INTEGER LITERAL
​

integer-literal
→

binary-literal
​

integer-literal
→

octal-literal
​

integer-literal
→

decimal-literal
​

integer-literal
→

hexadecimal-literal
​

binary-literal
→

0b
binary-digit
binary-literal-characters
opt
​

binary-digit
→

Digit 0 or 1
​

binary-literal-character
→

binary-digit

_
​

binary-literal-characters
→

binary-literal-character
binary-literal-characters
opt
​

octal-literal
→

0o
octal-digit
octal-literal-characters
opt
​

octal-digit
→

Digit 0 through 7
​

octal-literal-character
→

octal-digit

_
​

octal-literal-characters
→

octal-literal-character
octal-literal-characters
opt
​

decimal-literal
→

decimal-digit
decimal-literal-characters
opt
​

decimal-digit
→

Digit 0 through 9
​

decimal-digits
→

decimal-digit
decimal-digits
opt
​

decimal-literal-character
→

decimal-digit

_
​

decimal-literal-characters
→

decimal-literal-character
decimal-literal-characters
opt
​

hexadecimal-literal
→

0x
hexadecimal-digit
hexadecimal-literal-characters
opt
​

hexadecimal-digit
→

Digit 0 through 9, a through f, or A through F
​

hexadecimal-literal-character
→

hexadecimal-digit

_
​

hexadecimal-literal-characters
→

hexadecimal-literal-character
hexadecimal-literal-
characters
opt
​

Floating-Point Literals
Floating-point literals
represent floating-point values of unspecified precision.
By default, floating-point literals are expressed in decimal (with no prefix), but they can also be expressed in
hexadecimal (with a
0x
prefix).
Decimal floating-point literals consist of a sequence of decimal digits followed by either a decimal fraction, a
decimal exponent, or both. The decimal fraction consists of a decimal point (
.
) followed by a sequence of
decimal digits. The exponent consists of an upper- or lowercase
e
prefix followed by sequence of decimal digits
that indicates what power of 10 the value preceding the
e
is multiplied by. For example,
1.25e2
represents
1.25
⨉
10
2
, which evaluates to
125.0
. Similarly,
1.25e-2
represents 1.25
⨉
10
-2
, which evaluates to
0.0125
.
Hexadecimal floating-point literals consist of a
0x
prefix, followed by an optional hexadecimal fraction, followed
by a hexadecimal exponent. The hexadecimal fraction consists of a decimal point followed by a sequence of
hexadecimal digits. The exponent consists of an upper- or lowercase
p
prefix followed by sequence of decimal
digits that indicates what power of 2 the value preceding the
p
is multiplied by. For example,
0xFp2
represents
15
⨉
2
2
, which evaluates to
60
. Similarly,
0xFp-2
represents 15
⨉
2
-2
, which evaluates to
3.75
.
Unlike with integer literals, negative floating-point numbers are expressed by applying the unary minus operator
(
–
) to a floating-point literal, as in
-42.0
. The result is an expression, not a floating-point integer literal.
Underscores (
_
) are allowed between digits for readability, but are ignored and therefore don’t affect the value
of the literal. Floating-point literals can begin with leading zeros (
0
), but are likewise ignored and don’t affect the
base or value of the literal.
Unless otherwise specified, the default type of a floating-point literal is the Swift standard library type
Double
,
which represents a 64-bit floating-point number. The Swift standard library also defines a
Float
type, which
represents a 32-bit floating-point number.
GRAMMAR OF A FLOATING-POINT LITERAL
​

floating-point-literal
→

decimal-literal
decimal-fraction
opt
decimal-exponent
opt
​

floating-point-literal
→

hexadecimal-literal
hexadecimal-fraction
opt
hexadecimal-exponent
​

decimal-fraction
→

.
decimal-literal
​

decimal-exponent
→

floating-point-e
sign
opt
decimal-literal

​

hexadecimal-fraction
→

.
hexadecimal-literal
opt
​

hexadecimal-exponent
→

floating-point-p
sign
opt
hexadecimal-literal
​

floating-point-e
→

e

E
​

floating-point-p
→

p

P
​

sign
→

+

–
​
String Literals
A string literal is a sequence of characters surrounded by double quotes, with the following form:
"
characters
"
String literals cannot contain an unescaped double quote (
"
), an unescaped backslash (
\
), a carriage return, or
a line feed.
Special characters can be included in string literals using the following escape sequences:
Characters can also be expressed by
\x
followed by two hexadecimal digits,
\u
followed by four hexadecimal
digits, or
\U
followed by eight hexadecimal digits. The digits in these escape sequences identify a Unicode
codepoint.
The value of an expression can be inserted into a string literal by placing the expression in parentheses after a
backslash (
\
). The interpolated expression must not contain an unescaped double quote (
"
), an unescaped
Null Character (
\0
)
Backslash (
\\
)
Horizontal Tab (
\t
)
Line Feed (
\n
)
Carriage Return (
\r
)
Double Quote (
\"
)
Single Quote (
\'
)

backslash (
\
), a carriage return, or a line feed. The expression must evaluate to a value of a type that the
String
class has an initializer for.
For example, all the following string literals have the same value:
1
"1 2 3"
2
"1 2
\(
3
)
"
3
"1 2
\(
1
+
2
)
"
4
var

x
=
3
;
"1 2
\(
x
)
"
The default type of a string literal is
String
. The characters that make up a string are of type
Character
.
For more information about the
String
and
Character
types, see
Strings and Characters
.
GRAMMAR OF A STRING LITERAL
​

string-literal
→

"
quoted-text
"
​

quoted-text
→

quoted-text-item
quoted-text
opt
​

quoted-text-item
→

escaped-character
​

quoted-text-item
→

\(
expression
)
​

quoted-text-item
→

Any Unicode extended grapheme cluster except
"
,
\
, U+000A, or U+000D
​

escaped-character
→

\0

\\

\t

\n

\r

\"

\'
​

escaped-character
→

\x
hexadecimal-digit
hexadecimal-digit
​

escaped-character
→

\u
hexadecimal-digit
hexadecimal-digit
hexadecimal-
digit
hexadecimal-digit
​

escaped-character
→

\U
hexadecimal-digit
hexadecimal-digit
hexadecimal-
digit
hexadecimal-digit
hexadecimal-digit
hexadecimal-digit
hexadecimal-
digit
hexadecimal-digit
​
Operators
The Swift standard library defines a number of operators for your use, many of which are discussed in
Basic
Operators
and
Advanced Operators
. The present section describes which characters can be used as operators.
Operators are made up of one or more of the following characters:
/
,
=
,
–
,
+
,
!
,
*
,
%
,
<
,
>
,
&
,
|
,
^
,
~
, and
.
.
That said, the tokens
=
,
->
,
//
,
/*
,
*/
,
.
, and the unary prefix operator
&
are reserved. These tokens can’t be
overloaded, nor can they be used to define custom operators.

The whitespace around an operator is used to determine whether an operator is used as a prefix operator, a
postfix operator, or a binary operator. This behavior is summarized in the following rules:
For the purposes of these rules, the characters
(
,
[
, and
{
before an operator, the characters
)
,
]
, and
}
after
an operator, and the characters
,
,
;
, and
:
are also considered whitespace.
There is one caveat to the rules above. If the
!
or
?
operator has no whitespace on the left, it is treated as a
postfix operator, regardless of whether it has whitespace on the right. To use the
?
operator as syntactic sugar
for the
Optional
type, it must not have whitespace on the left. To use it in the conditional (
? :
) operator, it
must have whitespace around both sides.
In certain constructs, operators with a leading
<
or
>
may be split into two or more tokens. The remainder is
treated the same way and may be split again. As a result, there is no need to use whitespace to disambiguate
between the closing
>
characters in constructs like
Dictionary<String, Array<Int>>
. In this
example, the closing
>
characters are not treated as a single token that may then be misinterpreted as a bit shift
>>
operator.
To learn how to define new, custom operators, see
Custom Operators
and
Operator Declaration
. To learn how
to overload existing operators, see
Operator Functions
.
GRAMMAR OF OPERATORS
​

operator
→

operator-character
operator
opt
​

operator-character
→

/

=

–

+

!

*

%

<

>

&

|

^

~

.
​

binary-operator
→

operator
​

prefix-operator
→

operator
If an operator has whitespace around both sides or around neither side, it is treated as a binary
operator. As an example, the
+
operator in
a+b
and
a + b
is treated as a binary operator.
If an operator has whitespace on the left side only, it is treated as a prefix unary operator. As an
example, the
++
operator in
a ++b
is treated as a prefix unary operator.
If an operator has whitespace on the right side only, it is treated as a postfix unary operator. As
an example, the
++
operator in
a++ b
is treated as a postfix unary operator.
If an operator has no whitespace on the left but is followed immediately by a dot (
.
), it is treated
as a postfix unary operator. As an example, the
++
operator in
a++.b
is treated as a postfix
unary operator (
a++ . b
rather than
a ++ .b
).

​

postfix-operator
→

operator

​
​
Types
In Swift, there are two kinds of types: named types and compound types. A
named type
is a type that can be
given a particular name when it is defined. Named types include classes, structures, enumerations, and
protocols. For example, instances of a user-defined class named
MyClass
have the type
MyClass
. In
addition to user-defined named types, the Swift standard library defines many commonly used named types,
including those that represent arrays, dictionaries, and optional values.
Data types that are normally considered basic or primitive in other languages—such as types that represent
numbers, characters, and strings—are actually named types, defined and implemented in the Swift standard
library using structures. Because they are named types, you can extend their behavior to suit the needs of your
program, using an extension declaration, discussed in
Extensions
and
Extension Declaration
.
A
compound type
is a type without a name, defined in the Swift language itself. There are two compound types:
function types and tuple types. A compound type may contain named types and other compound types. For
instance, the tuple type
(Int, (Int, Int))
contains two elements: The first is the named type
Int
, and
the second is another compound type
(Int, Int)
.
This chapter discusses the types defined in the Swift language itself and describes the type inference behavior
of Swift.
GRAMMAR OF A TYPE
​

type
→

array-type

function-type

type-identifier

tuple-type

optional-type

implicitly-
unwrapped-optional-type

protocol-composition-type

metatype-type
​
Type Annotation
A
type annotation
explicitly specifies the type of a variable or expression. Type annotations begin with a colon
(
:
) and end with a type, as the following examples show:
1
let

someTuple
: (
Double
,
Double
) = (
3.14159
,
2.71828
)
2
func

someFunction
(
a
:
Int
) {
/* … */
}

In the first example, the expression
someTuple
is specified to have the tuple type
(Double, Double)
. In
the second example, the parameter
a
to the function
someFunction
is specified to have the type
Int
.
Type annotations can contain an optional list of type attributes before the type.
GRAMMAR OF A TYPE ANNOTATION
​

type-annotation
→

:
attributes
opt
type
​
Type Identifier
A type identifier refers to either a named type or a type alias of a named or compound type.
Most of the time, a type identifier directly refers to a named type with the same name as the identifier. For
example,
Int
is a type identifier that directly refers to the named type
Int
, and the type identifier
Dictionary<String, Int>
directly refers to the named type
Dictionary<String, Int>
.
There are two cases in which a type identifier does not refer to a type with the same name. In the first case, a
type identifier refers to a type alias of a named or compound type. For instance, in the example below, the use of
Point
in the type annotation refers to the tuple type
(Int, Int)
.
1
typealias

Point
= (
Int
,
Int
)
2
let

origin
:
Point
= (
0
,
0
)
In the second case, a type identifier uses dot (
.
) syntax to refer to named types declared in other modules or
nested within other types. For example, the type identifier in the following code references the named type
MyType
that is declared in the
ExampleModule
module.
1
var

someValue
:
ExampleModule
.
MyType
GRAMMAR OF A TYPE IDENTIFIER
​

type-identifier
→

type-name
generic-argument-clause
opt

type-name
generic-argument-
clause
opt
.
type-identifier
​

type-name
→

identifier

​
Tuple Type
A tuple type is a comma-separated list of zero or more types, enclosed in parentheses.
You can use a tuple type as the return type of a function to enable the function to return a single tuple containing
multiple values. You can also name the elements of a tuple type and use those names to refer to the values of
the individual elements. An element name consists of an identifier followed immediately by a colon (:). For an
example that demonstrates both of these features, see
Functions with Multiple Return Values
.
Void
is a typealias for the the empty tuple type,
()
. If there is only one element inside the parentheses, the type
is simply the type of that element. For example, the type of
(Int)
is
Int
, not
(Int)
. As a result, you can
label a tuple element only when the tuple type has two or more elements.
GRAMMAR OF A TUPLE TYPE
​

tuple-type
→

(
tuple-type-body
opt
)
​

tuple-type-body
→

tuple-type-element-list
…
opt
​

tuple-type-element-list
→

tuple-type-element

tuple-type-element
,
tuple-type-element-list
​

tuple-type-element
→

attributes
opt
inout
opt
type

inout
opt
element-name
type-
annotation
​

element-name
→

identifier
​
Function Type
A function type represents the type of a function, method, or closure and consists of a parameter and return type
separated by an arrow (
->
):
parameter type
->
return type
Because the
parameter type
and the
return type
can be a tuple type, function types support functions and
methods that take multiple paramaters and return multiple values.
You can apply the
auto_closure
attribute to a function type that has a parameter type of
()
and that returns
the type of an expression (see
Type Attributes
). An autoclosure function captures an implicit closure over the

specified expression, instead of the expression itself. The following example uses the
auto_closure
attribute in defining a very simple assert function:
1
func

simpleAssert
(
condition
:
@auto_closure
() ->
Bool
,
message
:

String
) {
2

if
!
condition
() {
3

println
(
message
)
4
}
5
}
6
let

testNumber
=
5
7
simpleAssert
(
testNumber
%
2
==
0
,
"testNumber isn't an even number."
)
8
// prints "testNumber isn't an even number."
A function type can have a variadic parameter as the last parameter in its
parameter type
. Syntactically, a
variadic parameter consists of a base type name followed immediately by three dots (
…
), as in
Int…
. A
variadic parameter is treated as an array that contains elements of the base type name. For instance, the
variadic parameter
Int…
is treated as
Int[]
. For an example that uses a variadic parameter, see
Variadic
Parameters
.
To specify an in-out parameter, prefix the parameter type with the
inout
keyword. You can’t mark a variadic
parameter or a return type with the
inout
keyword. In-out parameters are discussed in
In-Out Parameters
.
The type of a curried function is equivalent to a nested function type. For example, the type of the curried
function
addTwoNumbers()()
below is
Int -> Int -> Int
:
1
func

addTwoNumbers
(
a
:
Int
)(
b
:
Int
) ->
Int
{
2

return

a
+
b
3
}
4
addTwoNumbers
(
4
)(
5
)
// Returns 9
The function types of a curried function are grouped from right to left. For instance, the function type
Int ->
Int -> Int
is understood as
Int -> (Int -> Int)
—that is, a function that takes an
Int
and returns
another function that takes and return an
Int
. For example, you can rewrite the curried function
addTwoNumbers()()
as the following nested function:

1
func

addTwoNumbers
(
a
:
Int
) -> (
Int
->
Int
) {
2

func

addTheSecondNumber
(
b
:
Int
) ->
Int
{
3

return

a
+
b
4
}
5

return

addTheSecondNumber
6
}
7
addTwoNumbers
(
4
)(
5
)
// Returns 9
GRAMMAR OF A FUNCTION TYPE
​

function-type
→

type
->
type
​
Array Type
The Swift language uses square brackets (
[]
) immediately after the name of a type as syntactic sugar for the
named type
Array<T>
, which is defined in the Swift standard library. In other words, the following two
declarations are equivalent:
1
let

someArray
:
String
[] = [
"Alex"
,
"Brian"
,
"Dave"
]
2
let

someArray
:
Array
<
String
> = [
"Alex"
,
"Brian"
,
"Dave"
]
In both cases, the constant
someArray
is declared as an array of strings. The elements of an array can be
accessed using square brackets as well:
someArray[0]
refers to the element at index 0,
"Alex"
.
As the above example also shows, you can use square brackets to create an array using an array literal. Empty
array literals are written using an empty pair of square brackets and can be used to create an empty array of a
specified type.
1
var

emptyArray
:
Double
[] = []
You can create multidimensional arrays by chaining multiple sets of square brackets to the name of the base
type of the elements. For example, you can create a three-dimensional array of integers using three sets of
square brackets:

1
var

array3D
:
Int
[][][] = [[[
1
,
2
], [
3
,
4
]], [[
5
,
6
], [
7
,
8
]]]
When accessing the elements in a multidimensional array, the left-most subscript index refers to the element at
that index in the outermost array. The next subscript index to the right refers to the element at that index in the
array that’s nested one level in. And so on. This means that in the example above,
array3D[0]
refers to
[[1, 2], [3, 4]]
,
array3D[0][1]
refers to
[3, 4]
, and
array3D[0][1][1]
refers to the value
4.
For a detailed discussion of the Swift standard library
Array
type, see
Arrays
.
GRAMMAR OF AN ARRAY TYPE
​

array-type
→

type
[
]

array-type
[
]
​
Optional Type
The Swift language defines the postfix
?
as syntactic sugar for the named type
Optional<T>
, which is
defined in the Swift standard library. In other words, the following two declarations are equivalent:
1
var

optionalInteger
:
Int
?
2
var

optionalInteger
:
Optional
<
Int
>
In both cases, the variable
optionalInteger
is declared to have the type of an optional integer. Note that no
whitespace may appear between the type and the
?
.
The type
Optional<T>
is an enumeration with two cases,
None
and
Some(T)
, which are used to represent
values that may or may not be present. Any type can be explicitly declared to be (or implicitly converted to) an
optional type. When declaring an optional type, be sure to use parentheses to properly scope the
?
operator. As
an example, to declare an optional array of integers, write the type annotation as
(Int[])?
; writing
Int[]?
produces an error.
If you don’t provide an initial value when you declare an optional variable or property, its value automatically
defaults to
nil
.

Optionals conform to the
LogicValue
protocol and therefore may occur in a Boolean context. In that context,
if an instance of an optional type
T?
contains any value of type
T
(that is, it’s value is
Optional.Some(T)
),
the optional type evaluates to
true
. Otherwise, it evaluates to
false
.
If an instance of an optional type contains a value, you can access that value using the postfix operator
!
, as
shown below:
1
optionalInteger
=
42
2
optionalInteger
!
// 42
Using the
!
operator to unwrap an optional that has a value of
nil
results in a runtime error.
You can also use optional chaining and optional binding to conditionally perform an operation on an optional
expression. If the value is
nil
, no operation is performed and therefore no runtime error is produced.
For more information and to see examples that show how to use optional types, see
Optionals
.
GRAMMAR OF AN OPTIONAL TYPE
​

optional-type
→

type
?
​
Implicitly Unwrapped Optional Type
The Swift language defines the postfix
!
as syntactic sugar for the named type
ImplicitlyUnwrappedOptional<T>
, which is defined in the Swift standard library. In other words, the
following two declarations are equivalent:
1
var

implicitlyUnwrappedString
:
String
!
2
var

implicitlyUnwrappedString
:
ImplicitlyUnwrappedOptional
<
String
>
In both cases, the variable
implicitlyUnwrappedString
is declared to have the type of an implicitly
unwrapped optional string. Note that no whitespace may appear between the type and the
!
.

You can use implicitly unwrapped optionals in all the same places in your code that you can use optionals. For
instance, you can assign values of implicitly unwrapped optionals to variables, constants, and properties of
optionals, and vice versa.
As with optionals, if you don’t provide an initial value when you declare an implicitly unwrapped optional
variable or property, it’s value automatically defaults to
nil
.
Because the value of an implicitly unwrapped optional is automatically unwrapped when you use it, there’s no
need to use the
!
operator to unwrap it. That said, if you try to use an implicitly unwrapped optional that has a
value of
nil
, you’ll get a runtime error.
Use optional chaining to conditionally perform an operation on an implicitly unwrapped optional expression. If
the value is
nil
, no operation is performed and therefore no runtime error is produced.
For more information about implicitly unwrapped optional types, see
Implicitly Unwrapped Optionals
.
GRAMMAR OF AN IMPLICITLY UNWRAPPED OPTIONAL TYPE
​

implicitly-unwrapped-optional-type
→

type
!
​
Protocol Composition Type
A protocol composition type describes a type that conforms to each protocol in a list of specified protocols.
Protocol composition types may be used in type annotations and in generic parameters.
Protocol composition types have the following form:
protocol
<
Protocol 1
,
Protocol 2
>
A protocol composition type allows you to specify a value whose type conforms to the requirements of multiple
protocols without having to explicitly define a new, named protocol that inherits from each protocol you want the
type to conform to. For example, specifying a protocol composition type
protocol<ProtocolA,
ProtocolB, ProtocolC>
is effectively the same as defining a new protocol
ProtocolD
that inherits
from
ProtocolA
,
ProtocolB
, and
ProtocolC
, but without having to introduce a new name.

Each item in a protocol composition list must be either the name of protocol or a type alias of a protocol
composition type. If the list is empty, it specifies the empty protocol composition type, which every type
conforms to.
GRAMMAR OF A PROTOCOL COMPOSITION TYPE
​

protocol-composition-type
→

protocol
<
protocol-identifier-list
opt
>
​

protocol-identifier-list
→

protocol-identifier

protocol-identifier
,
protocol-identifier-list
​

protocol-identifier
→

type-identifier
​
Metatype Type
A metatype type refers to the type of any type, including class types, structure types, enumeration types, and
protocol types.
The metatype of a class, structure, or enumeration type is the name of that type followed by
.Type
. The
metatype of a protocol type—not the concrete type that conforms to the protocol at runtime—is the name of that
protocol followed by
.Protocol
. For example, the metatype of the class type
SomeClass
is
SomeClass.Type
and the metatype of the protocol
SomeProtocol
is
SomeProtocol.Protocol
.
You can use the postfix
self
expression to access a type as a value. For example,
SomeClass.self
returns
SomeClass
itself, not an instance of
SomeClass
. And
SomeProtocol.self
returns
SomeProtocol
itself, not an instance of a type that conforms to
SomeProtocol
at runtime. You can use a
dynamicType
expression with an instance of a type to access that instance’s runtime type as a value, as the
following example shows:
1
class

SomeBaseClass
{
2

class

func

printClassName
() {
3

println
(
"SomeBaseClass"
)
4
}
5
}
6
class

SomeSubClass
:
SomeBaseClass
{
7

override

class

func

printClassName
() {
8

println
(
"SomeSubClass"
)
9
}

10
}
11
let

someInstance
:
SomeBaseClass
=
SomeSubClass
()
12
// someInstance is of type SomeBaseClass at compile time, but
13
// someInstance is of type SomeSubClass at runtime
14
someInstance
.
dynamicType
.
printClassName
()
15
// prints "SomeSubClass"
GRAMMAR OF A METATYPE TYPE
​

metatype-type
→

type
.
Type

type
.
Protocol
​
Type Inheritance Clause
A type inheritance clause is used to specify which class a named type inherits from and which protocols a
named type conforms to. A type inheritance clause begins with a colon (
:
), followed by a comma-separated list
of type identifiers.
Class types may inherit from a single superclass and conform to any number of protocols. When defining a
class, the name of the superclass must appear first in the list of type identifiers, followed by any number of
protocols the class must conform to. If the class does not inherit from another class, the list may begin with a
protocol instead. For an extended discussion and several examples of class inheritance, see
Inheritance
.
Other named types may only inherit from or conform to a list of protocols. Protocol types may inherit from any
number of other protocols. When a protocol type inherits from other protocols, the set of requirements from
those other protocols are aggregated together, and any type that inherits from the current protocol must conform
to all of those requirements.
A type inheritance clause in an enumeration definition may be either a list of protocols, or in the case of an
enumeration that assigns raw values to its cases, a single, named type that specifies the type of those raw
values. For an example of an enumeration definition that uses a type inheritance clause to specify the type of its
raw values, see
Raw Values
.
GRAMMAR OF A TYPE INHERITANCE CLAUSE
​

type-inheritance-clause
→

:
type-inheritance-list
​

type-inheritance-list
→

type-identifier

type-identifier
,
type-inheritance-list

​
Type Inference
Swift uses type inference extensively, allowing you to omit the type or part of the type of many variables and
expressions in your code. For example, instead of writing
var x: Int = 0
, you can omit the type
completely and simply write
var x = 0
—the compiler correctly infers that
x
names a value of type
Int
.
Similarly, you can omit part of a type when the full type can be inferred from context. For instance, if you write
let dict: Dictionary = ["A": 1]
, the compiler infers that
dict
has the type
Dictionary<String, Int>
.
In both of the examples above, the type information is passed up from the leaves of the expression tree to its
root. That is, the type of
x
in
var x: Int = 0
is inferred by first checking the type of
0
and then passing this
type information up to the root (the variable
x
).
In Swift, type information can also flow in the opposite direction—from the root down to the leaves. In the
following example, for instance, the explicit type annotation (
: Float
) on the constant
eFloat
causes the
numeric literal
2.71828
to have type
Float
instead of type
Double
.
1
let

e
=
2.71828

// The type of e is inferred to be Double.
2
let

eFloat
:
Float
=
2.71828

// The type of eFloat is Float.
Type inference in Swift operates at the level of a single expression or statement. This means that all of the
information needed to infer an omitted type or part of a type in an expression must be accessible from type-
checking the expression or one of its subexpressions.

​
​
Expressions
In Swift, there are four kinds of expressions: prefix expressions, binary expressions, primary expressions, and
postfix expressions. Evaluating an expression returns a value, causes a side effect, or both.
Prefix and binary expressions let you apply operators to smaller expressions. Primary expressions are
conceptually the simplest kind of expression, and they provide a way to access values. Postfix expressions, like
prefix and binary expressions, let you build up more complex expressions using postfixes such as function calls
and member access. Each kind of expression is described in detail in the sections below.
GRAMMAR OF AN EXPRESSION
​

expression
→

prefix-expression
binary-expressions
opt
​

expression-list
→

expression

expression
,
expression-list
​
Prefix Expressions
Prefix expressions
combine an optional prefix operator with an expression. Prefix operators take one argument,
the expression that follows them.
The Swift standard library provides the following prefix operators:
For information about the behavior of these operators, see
Basic Operators
and
Advanced Operators
.
++
Increment
–
Decrement
!
Logical NOT
~
Bitwise NOT
+
Unary plus
–
Unary minus

In addition to the standard library operators listed above, you use
&
immediately before the name of a variable
that’s being passed as an in-out argument to a function call expression. For more information and to see an
example, see
In-Out Parameters
.
GRAMMAR OF A PREFIX EXPRESSION
​

prefix-expression
→

prefix-operator
opt
postfix-expression
​

prefix-expression
→

in-out-expression
​

in-out-expression
→

&
identifier
​
Binary Expressions
Binary expressions
combine an infix binary operator with the expression that it takes as its left-hand and right-
hand arguments. It has the following form:
left-hand argument

operator

right-hand argument
The Swift standard library provides the following binary operators:
Exponentiative (No associativity, precedence level 160)
<<
Bitwise left shift
>>
Bitwise right shift
Multiplicative (Left associative, precedence level 150)
*
Multiply
/
Divide
%
Remainder
&*
Multiply, ignoring overflow
&/
Divide, ignoring overflow
&%
Remainder, ignoring overflow

&
Bitwise AND
Additive (Left associative, precedence level 140)
+
Add
–
Subtract
&+
Add with overflow
&-
Subtract with overflow
|
Bitwise OR
^
Bitwise XOR
Range (No associativity, precedence level 135)
..
Half-closed range
…
Closed range
Cast (No associativity, precedence level 132)
is
Type check
as
Type cast
Comparative (No associativity, precedence level 130)
<
Less than
<=
Less than or equal
>
Greater than
>=
Greater than or equal
==
Equal
!=
Not equal
===
Identical

!==
Not identical
~=
Pattern match
Conjunctive (Left associative, precedence level 120)
&&
Logical AND
Disjunctive (Left associative, precedence level 110)
||
Logical OR
Ternary Conditional (Right associative, precedence level 100)
?
:
Ternary conditional
Assignment (Right associative, precedence level 90)
=
Assign
*=
Multiply and assign
/=
Divide and assign
%=
Remainder and assign
+=
Add and assign
-=
Subtract and assign
<<=
Left bit shift and assign
>>=
Right bit shift and assign
&=
Bitwise AND and assign
^=
Bitwise XOR and assign
|=
Bitwise OR and assign
&&=
Logical AND and assign
||=
Logical OR and assign

For information about the behavior of these operators, see
Basic Operators
and
Advanced Operators
.
NOTE
At parse time, an expression made up of binary operators is represented as a flat list. This list is
transformed into a tree by applying operator precedence For example, the expression
2 + 3 * 5
is initially understood as a flat list of five items,
2
,
+
, „ 3„,
*
, and
5
. This process transforms it into
the tree (2 + (3 * 5)).
GRAMMAR OF A BINARY EXPRESSION
​

binary-expression
→

binary-operator
prefix-expression
​

binary-expression
→

assignment-operator
prefix-expression
​

binary-expression
→

conditional-operator
prefix-expression
​

binary-expression
→

type-casting-operator
​

binary-expressions
→

binary-expression
binary-expressions
opt
​
Assignment Operator
The
assigment operator
sets a new value for a given expression. It has the following form:
expression
=
value
The value of the
expression
is set to the value obtained by evaluating the
value
. If the
expression
is a tuple, the
value
must be a tuple with the same number of elements. (Nested tuples are allowed.) Assignment is
performed from each part of the
value
to the corresponding part of the
expression
. For example:
1
(
a
,
_
, (
b
,
c
)) = (
"test"
,
9.45
, (
12
,
3
))
2
// a is "test", b is 12, c is 3, and 9.45 is ignored
The assignment operator does not return any value.

GRAMMAR OF AN ASSIGNMENT OPERATOR
​

assignment-operator
→

=
​
Ternary Conditional Operator
The
ternary conditional operator
evaluates to one of two given values based on the value of a condition. It has the
following form:
condition
?
expression used if true
:
expression used if false
If the
condition
evaluates to
true
, the conditional operator evaluates the first expression and returns its value.
Otherwise, it evaluates the second expression and returns its value. The unused expression is not evaluated.
For an example that uses the ternary conditional operator, see
Ternary Conditional Operator
.
GRAMMAR OF A CONDITIONAL OPERATOR
​

conditional-operator
→

?
expression
:
​
Type-Casting Operators
There are two type-casting operators, the
as
operator and the
is
operator. They have the following form:
expression

as

type
expression

as
?
type
expression

is

type
The
as
operator performs a cast of the
expression
to the specified
type
. It behaves as follows:

1
class

SomeSuperType
{}
2
class

SomeType
:
SomeSuperType
{}
3
class

SomeChildType
:
SomeType
{}
4
let

s
=
SomeType
()
5

6
let

x
=
s

as

SomeSuperType

// known to succeed; type is SomeSuperType
7
let

y
=
s

as

Int

// known to fail; compile-time error
8
let

z
=
s

as

SomeChildType

// might fail at runtime; type is

SomeChildType?
Specifying a type with
as
provides the same information to the compiler as a type annotation, as shown in the
following example:
1
let

y1
=
x

as

SomeType

// Type information from 'as'
2
let

y2
:
SomeType
=
x

// Type information from an annotation
The
is
operator checks at runtime to see whether the
expression
is of the specified
type
. If so, it returns
true
;
otherwise, it returns
false
.
The check must not be known to be true or false at compile time. The following are invalid:
1
"hello"

is

String
2
"hello"

is

Int
If conversion to the specified
type
is guaranteed to succeed, the value of the
expression
is
returned as an instance of the specified
type
. An example is casting from a subclass to a
superclass.
If conversion to the specified
type
is guaranteed to fail, a compile-time error is raised.
Otherwise, if it’s not known at compile time whether the conversion will succeed, the type of the
cast expresion is an optional of the specified
type
. At runtime, if the cast succeeds, the value of
expression
is wrapped in an optional and returned; otherwise, the value returned is
nil
. An
example is casting from a superclass to a subclass.

For more information about type casting and to see more examples that use the type-casting operators, see
Type Casting
.
GRAMMAR OF A TYPE-CASTING OPERATOR
​

type-casting-operator
→

is
type

as
?
opt
type
​
Primary Expressions
Primary expressions
are the most basic kind of expression. They can be used as expressions on their own, and
they can be combined with other tokens to make prefix expressions, binary expressions, and postfix
expressions.
GRAMMAR OF A PRIMARY EXPRESSION
​

primary-expression
→

identifier
generic-argument-clause
opt
​

primary-expression
→

literal-expression
​

primary-expression
→

self-expression
​

primary-expression
→

superclass-expression
​

primary-expression
→

closure-expression
​

primary-expression
→

parenthesized-expression
​

primary-expression
→

implicit-member-expression
​

primary-expression
→

wildcard-expression
​
Literal Expression
A
literal expression
consists of either an ordinary literal (such as a string or a number), an array or dictionary
literal, or one of the following special literals:
Literal
Type
Value
The name

__FILE__
String
of the file
in which it
appears.
__LINE__
Int
The line
number on
which it
appears.
__COLUMN__
Int
The
column
number in
which it
begins.
__FUNCTION__
String
The name
of the
declaration
in which it
appears.
Inside a function, the value of
__FUNCTION__
is the name of that function, inside a method it is the name of
that method, inside a property getter or setter it is the name of that property, inside special members like
init
or
subscript
it is the name of that keyword, and at the top level of a file it is the name of the current module.
An
array literal
is an ordered collection of values. It has the following form:

[
value 1
,
value 2
,
…
]
The last expression in the array can be followed by an optional comma. An empty array literal is written as an
empty pair of brackets (
[]
). The value of an array literal has type
T[]
, where
T
is the type of the expressions
inside it. If there are expressions of multiple types,
T
is their closest common supertype.
A
dictionary literal
is an unordered collection of key-value pairs. It has the following form:
[
key 1
:
value 1
,
key 2
:
value 2
,
…
]
The last expression in the dictionary can be followed by an optional comma. An empty dictionary literal is
written as a colon inside a pair of brackets (
[:]
) to distinguish it from an empty array literal. The value of a
dictionary literal has type
Dictionary<KeyType, ValueType>
, where
KeyType
is the type of its key
expressions and
ValueType
is the type of its value expressions. If there are expressions of multiple types,
KeyType
and
ValueType
are the closest common supertype for their respective values.
GRAMMAR OF A LITERAL EXPRESSION
​

literal-expression
→

literal
​

literal-expression
→

array-literal

dictionary-literal
​

literal-expression
→

__FILE__

__LINE__

__COLUMN__

__FUNCTION__
​

array-literal
→

[
array-literal-items
opt
]
​

array-literal-items
→

array-literal-item
,
opt

array-literal-item
,
array-literal-items
​

array-literal-item
→

expression
​

dictionary-literal
→

[
dictionary-literal-items
]

[
:
]
​

dictionary-literal-items
→

dictionary-literal-item
,
opt

dictionary-literal-item
,
dictionary-
literal-items
​

dictionary-literal-item
→

expression
:
expression
​
Self Expression
The
self
expression is an explicit reference to the current type or instance of the type in which it occurs. It has
the following forms:

self
self
.
member name
self
[
subscript index
]
self
(
initializer arguments
)
self
.
init
(
initializer arguments
)
In an initializer, subscript, or instance method,
self
refers to the current instance of the type in which it occurs.
In a static or class method,
self
refers to the current type in which it occurs.
The
self
expression is used to specify scope when accessing members, providing disambiguation when
there is another variable of the same name in scope, such as a function parameter. For example:
1
class

SomeClass
{
2

var

greeting
:
String
3

init
(
greeting
:
String
) {
4

self
.
greeting
=
greeting
5
}
6
}
In a mutating method of value type, you can assign a new instance of that value type to
self
. For example:
1
struct

Point
{
2

var

x
=
0.0
,
y
=
0.0
3

mutating

func

moveByX
(
deltaX
:
Double
,
y

deltaY
:
Double
) {
4

self
=
Point
(
x
:
x
+
deltaX
,
y
:
y
+
deltaY
)
5
}
6
}
GRAMMAR OF A SELF EXPRESSION
​

self-expression
→

self
​

self-expression
→

self
.
identifier
​

self-expression
→

self
[
expression
]
​

self-expression
→

self
.
init

​
Superclass Expression
A
superclass expression
lets a class interact with its superclass. It has one of the following forms:
super
.
member name
super
[
subscript index
]
super
.
init
(
initializer arguments
)
The first form is used to access a member of the superclass. The second form is used to access the
superclass’s subscript implementation. The third form is used to access an initializer of the superclass.
Subclasses can use a superclass expression in their implementation of members, subscripting, and initializers
to make use of the implementation in their superclass.
GRAMMAR OF A SUPERCLASS EXPRESSION
​

superclass-expression
→

superclass-method-expression

superclass-subscript-expression
superclass-initializer-expression
​

superclass-method-expression
→

super
.
identifier
​

superclass-subscript-expression
→

super
[
expression
]
​

superclass-initializer-expression
→

super
.
init
​
Closure Expression
A
closure expression
creates a closure, also known as a
lambda
or an
anonymous function
in other
programming languages. Like function declarations, closures contain statements which they execute, and they
capture values from their enclosing scope. It has the following form:
{ (
parameters
) ->
return type

in

statements

}
The
parameters
have the same form as the parameters in a function declaration, as described in
Function
Declaration
.
There are several special forms that allow closures to be written more concisely:
The following closure expressions are equivalent:
1
myFunction
{
2
(
x
:
Int
,
y
:
Int
) ->
Int

in
3

return

x
+
y
4
}
5

6
myFunction
{
7
(
x
,
y
)
in
8

return

x
+
y
9
}
10

11
myFunction
{
return

$0
+
$1
}
12

13
myFunction
{
$0
+
$1
}
For information about passing a closure as an argument to a function, see
Function Call Expression
.
A closure can omit the types of its parameters, its return type, or both. If you omit the parameter
names and both types, omit the
in
keyword before the statements. If the omitted types can’t be
inferred, a compile-time error is raised.
A closure may omit names for its parameters. Its parameters are then implicitly named
$
followed by their position:
$0
,
$1
,
$2
, and so on.
A closure that consists of only a single expression is understood to return the value of that
expression. The contents of this expression is also considered when performing type inference
on the surrounding expression.

A closure expression can explicitly specify the values that it captures from the surrounding scope using a
capture list
. A capture list is written as a comma separated list surrounded by square brackets, before the list of
parameters. If you use a capture list, you must also use the
in
keyword, even if you omit the parameter names,
parameter types, and return type.
Each entry in the capture list can be marked as
weak
or
unowned
to capture a weak or unowned reference to
the value.
1
myFunction
{
print
(
self
.
title
) }
// strong capture
2
myFunction
{ [
weak

self
]
in

print
(
self
!.
title
) }
// weak capture
3
myFunction
{ [
unowned

self
]
in

print
(
self
.
title
) }
// unowned capture
You can also bind arbitrary expression to named values in the capture list. The expression is evaluated when
the closure is formed, and captured with the specified strength. For example:
1
// Weak capture of "self.parent" as "parent"
2
myFunction
{ [
weak

parent
=
self
.
parent
]
in

print
(
parent
!.
title
) }
For more information and examples of closure expressions, see
Closure Expressions
.
GRAMMAR OF A CLOSURE EXPRESSION
​

closure-expression
→

{
closure-signature
opt
statements
}
​

closure-signature
→

parameter-clause
function-result
opt
in
​

closure-signature
→

identifier-list
function-result
opt
in
​

closure-signature
→

capture-list
parameter-clause
function-result
opt
in
​

closure-signature
→

capture-list
identifier-list
function-result
opt
in
​

closure-signature
→

capture-list
in
​

capture-list
→

[
capture-specifier
expression
]
​

capture-specifier
→

weak

unowned

unowned(safe)

unowned(unsafe)
​
Implicit Member Expression
An
implicit member expression
is an abbreviated way to access a member of a type, such as an enumeration

case or a class method, in a context where type inference can determine the implied type. It has the following
form:
.
member name
For example:
1
var

x
=
MyEnumeration
.
SomeValue
2
x
= .
AnotherValue
GRAMMAR OF A IMPLICIT MEMBER EXPRESSION
​

implicit-member-expression
→

.
identifier
​
Parenthesized Expression
A
parenthesized expression
consists of a comma-separated list of expressions surrounded by parentheses.
Each expression can have an optional identifier before it, separated by a colon (
:
). It has the following form:
(
identifier 1
:
expression 1
,
identifier 2
:
expression 2
,
…
)
Use parenthesized expressions to create tuples and to pass arguments to a function call. If there is only one
value inside the parenthesized expression, the type of the parenthesized expression is the type of that value. For
example, the type of the parenthesized expression
(1)
is
Int
, not
(Int)
.
GRAMMAR OF A PARENTHESIZED EXPRESSION
​

parenthesized-expression
→

(
expression-element-list
opt
)
​

expression-element-list
→

expression-element

expression-element
,
expression-element-list
​

expression-element
→

expression

identifier
:
expression

​
Wildcard Expression
A
wildcard expression
is used to explicitly ignore a value during an assignment. For example, in the following
assignment 10 is assigned to
x
and 20 is ignored:
1
(
x
,
_
) = (
10
,
20
)
2
// x is 10, 20 is ignored
GRAMMAR OF A WILDCARD EXPRESSION
​

wildcard-expression
→

_
​
Postfix Expressions
Postfix expressions
are formed by applying a postfix operator or other postfix syntax to an expression.
Syntactically, every primary expression is also a postfix expression.
The Swift standard library provides the following postfix operators:
For information about the behavior of these operators, see
Basic Operators
and
Advanced Operators
.
GRAMMAR OF A POSTFIX EXPRESSION
​

postfix-expression
→

primary-expression
​

postfix-expression
→

postfix-expression
postfix-operator
​

postfix-expression
→

function-call-expression
​

postfix-expression
→

initializer-expression
​

postfix-expression
→

explicit-member-expression
​

postfix-expression
→

postfix-self-expression
​

postfix-expression
→

dynamic-type-expression
​

postfix-expression
→

subscript-expression
​

postfix-expression
→

forced-value-expression
++
Increment
–
Decrement

​

postfix-expression
→

optional-chaining-expression
​
Function Call Expression
A
function call expression
consists of a function name followed by a comma-separated list of the function’s
arguments in parentheses. Function call expressions have the following form:
function name
(
argument value 1
,
argument value 2
)
The
function name
can be any expression whose value is of a function type.
If the function definition includes names for its parameters, the function call must include names before its
argument values separated by a colon (
:
). This kind of function call expression has the following form:
function name
(
argument name 1
:
argument value 1
,
argument name 2
:
argument value 2
)
A function call expression can include a trailing closure in the form of a closure expression immediately after
the closing parenthesis. The trailing closure is understood as an argument to the function, added after the last
parenthesized argument. The following function calls are equivalent:
1
// someFunction takes an integer and a closure as its arguments
2
someFunction
(
x
, {
$0
==
13
})
3
someFunction
(
x
) {
$0
==
13
}
If the trailing closure is the function’s only argument, the parentheses can be omitted.
1
// someFunction takes a closure as its only argument
2
myData
.
someMethod
() {
$0
==
13
}
3
myData
.
someMethod
{
$0
==
13
}

GRAMMAR OF A FUNCTION CALL EXPRESSION
​

function-call-expression
→

postfix-expression
parenthesized-expression
​

function-call-expression
→

postfix-expression
parenthesized-expression
opt
trailing-closure
​

trailing-closure
→

closure-expression
​
Initializer Expression
An
initializer expression
provides access to a type’s initializer. It has the following form:
expression
.
init
(
initializer arguments
)
You use the initializer expression in a function call expression to initialize a new instance of a type. Unlike
functions, an initializer can’t be used as a value. For example:
1
var

x
=
SomeClass
.
someClassFunction

// ok
2
var

y
=
SomeClass
.
init

// error
You also use an initializer expression to delegate to the initializer of a superclass.
1
class

SomeSubClass
:
SomeSuperClass
{
2

init
() {
3

// subclass initialization goes here
4

super
.
init
()
5
}
6
}
GRAMMAR OF AN INITIALIZER EXPRESSION
​

initializer-expression
→

postfix-expression
.
init
​
Explicit Member Expression

A
explicit member expression
allows access to the members of a named type, a tuple, or a module. It consists
of a period (
.
) between the item and the identifier of its member.
expression
.
member name
The members of a named type are named as part of the type’s declaration or extension. For example:
1
class

SomeClass
{
2

var

someProperty
=
42
3
}
4
let

c
=
SomeClass
()
5
let

y
=
c
.
someProperty

// Member access
The members of a tuple are implicitly named using integers in the order they appear, starting from zero. For
example:
1
var

t
= (
10
,
20
,
30
)
2
t
.
0
=
t
.
1
3
// Now t is (20, 20, 30)
The members of a module access the top-level declarations of that module.
GRAMMAR OF AN EXPLICIT MEMBER EXPRESSION
​

explicit-member-expression
→

postfix-expression
.
decimal-digit
​

explicit-member-expression
→

postfix-expression
.
identifier
generic-argument-clause
opt
​
Postfix Self Expression
A postfix
self
expression consists of an expression or the name of a type, immediately followed by
.self
. It
has the following forms:

expression
.
self
type
.
self
The first form evaluates to the value of the
expression
. For example,
x.self
evaluates to
x
.
The second form evaluates to the value of the
type
. Use this form to access a type as a value. For example,
because
SomeClass.self
evaluates to the
SomeClass
type itself, you can pass it to a function or method
that accepts a type-level argument.
GRAMMAR OF A SELF EXPRESSION
​

postfix-self-expression
→

postfix-expression
.
self
​
Dynamic Type Expression
A
dynamicType
expression consists of an expression, immediately followed by
.dynamicType
. It has the
following form:
expression
.
dynamicType
The
expression
can’t be the name of a type. The entire
dynamicType
expression evaluates to the value of the
runtime type of the
expression
, as the following example shows:
1
class

SomeBaseClass
{
2

class

func

printClassName
() {
3

println
(
"SomeBaseClass"
)
4
}
5
}
6
class

SomeSubClass
:
SomeBaseClass
{
7

override

class

func

printClassName
() {
8

println
(
"SomeSubClass"
)
9
}

10
}
11
let

someInstance
:
SomeBaseClass
=
SomeSubClass
()
12
// someInstance is of type SomeBaseClass at compile time, but
13
// someInstance is of type SomeSubClass at runtime
14
someInstance
.
dynamicType
.
printClassName
()
15
// prints "SomeSubClass"
GRAMMAR OF A DYNAMIC TYPE EXPRESSION
​

dynamic-type-expression
→

postfix-expression
.
dynamicType
​
Subscript Expression
A
subscript expression
provides subscript access using the getter and setter of the corresponding subscript
declaration. It has the following form:
expression
[
index expressions
]
To evaluate the value of a subscript expression, the subscript getter for the
expression
’s type is called with the
index expressions
passed as the subscript parameters. To set its value, the subscript setter is called in the
same way.
For information about subscript declarations, see
Protocol Subscript Declaration
.
GRAMMAR OF A SUBSCRIPT EXPRESSION
​

subscript-expression
→

postfix-expression
[
expression-list
]
​
Forced-Value Expression
A
forced-value expression
unwraps an optional value that you are certain is not
nil
. It has the following form:

expression
!
If the value of the
expression
is not
nil
, the optional value is unwrapped and returned with the corresponding
nonoptional type. Otherwise, a runtime error is raised.
GRAMMAR OF A FORCED-VALUE EXPRESSION
​

forced-value-expression
→

postfix-expression
!
​
Optional-Chaining Expression
An
optional-chaining expression
provides a simplified syntax for using optional values in postfix expressions. It
has the following form:
expression
?
On its own, the postfix
?
operator simply returns the value of its argument as an optional.
Postfix expressions that contain an optional-chaining expression are evaluated in a special way. If the optional-
chaining expression is
nil
, all of the other operations in the postfix expression are ignored and the entire
postfix expression evaluates to
nil
. If the optional-chaining expression is not
nil
, the value of the optional-
chaining expression is unwrapped and used to evaluate the rest of the postfix expression. In either case, the
value of the postfix expression is still of an optional type.
If a postfix expression that contains an optional-chaining expression is nested inside other postfix expressions,
only the outermost expression returns an optional type. In the example below, when
c
is not
nil
, its value is
unwrapped and used to evaluate both
.property
and
.performAction()
, and the entire expression
c?.property.performAction()
has a value of an optional type.
1
var

c
:
SomeClass
?
2
var

result
:
Bool
? =
c
?.
property
.
performAction
()

The following example shows the behavior of the example above without using optional chaining.
1
if

let

unwrappedC
=
c
{
2

result
=
unwrappedC
.
property
.
performAction
()
3
}
GRAMMAR OF AN OPTIONAL-CHAINING EXPRESSION
​

optional-chaining-expression
→

postfix-expression
?

​
​
Statements
In Swift, there are two kinds of statements: simple statements and control flow statements. Simple statements
are the most common and consist of either an expression or a declaration. Control flow statements are used to
control the flow of execution in a program. There are three types of control flow statements in Swift: loop
statements, branch statements, and control transfer statements.
Loop statements allow a block of code to be executed repeatedly, branch statements allow a certain block of
code to be executed only when certain conditions are met, and control transfer statements provide a way to alter
the order in which code is executed. Each type of control flow statement is described in detail below.
A semicolon (
;
) can optionally appear after any statement and is used to separate multiple statements if they
appear on the same line.
GRAMMAR OF A STATEMENT
​

statement
→

expression
;
opt
​

statement
→

declaration
;
opt
​

statement
→

loop-statement
;
opt
​

statement
→

branch-statement
;
opt
​

statement
→

labeled-statement
​

statement
→

control-transfer-statement
;
opt
​

statements
→

statement
statements
opt
​
Loop Statements
Loop statements allow a block of code to be executed repeatedly, depending on the conditions specified in the
loop. Swift has four loop statements: a
for
statement, a
for
–
in
statement, a
while
statement, and a
do
–
while
statement.
Control flow in a loop statement can be changed by a
break
statement and a
continue
statement and is
discussed in
Break Statement
and
Continue Statement
below.
GRAMMAR OF A LOOP STATEMENT

​

loop-statement
→

for-statement
​

loop-statement
→

for-in-statement
​

loop-statement
→

while-statement
​

loop-statement
→

do-while-statement
​
For Statement
A
for
statement allows a block of code to be executed repeatedly while incrementing a counter, as long as a
condition remains true.
A
for
statement has the following form:
for

initialization
;
condition
;
increment
{

statements
}
The semicolons between the
initialization
,
condition
, and
increment
are required. The braces around the
statements
in the body of the loop are also required.
A
for
statement is executed as follows:
1
.
The
initialization
is evaluated only once. It is typically used to declare and initialize any variables
that are needed for the remainder of the loop.
2
.
The
condition
expression is evaluated.
If
true
, the program executes the
statements
, and execution continues to step 3. If
false
, the
program does not execute the
statements
or the
increment
expression, and the program is finished
executing the
for
statement.
3
.
The
increment
expression is evaluated, and execution returns to step 2.
Variables defined within the
initialization
are valid only within the scope of the
for
statement itself.
The value of the
condition
expression must have a type that conforms to the
LogicValue
protocol.

GRAMMAR OF A FOR STATEMENT
​

for-statement
→

for
for-init
opt
;
expression
opt
;
expression
opt
code-block
​

for-statement
→

for
(
for-init
opt
;
expression
opt
;
expression
opt
)
code-block
​

for-init
→

variable-declaration

expression-list
​
For-In Statement
A
for
–
in
statement allows a block of code to be executed once for each item in a collection (or any type) that
conforms to the
Sequence
protocol.
A
for
–
in
statement has the following form:
for

item

in

collection
{

statements
}
The
generate
method is called on the
collection
expression to obtain a value of a generator type—that is, a
type that conforms to the
Generator
protocol. The program begins executing a loop by calling the
next
method on the stream. If the value returned is not
None
, it is assigned to the
item
pattern, the program executes
the
statements
, and then continues execution at the beginning of the loop. Otherwise, the program does not
perform assignment or execute the
statements
, and it is finished executing the
for
–
in
statement.
GRAMMAR OF A FOR-IN STATEMENT
​

for-in-statement
→

for
pattern
in
expression
code-block
​
While Statement
A
while
statement allows a block of code to be executed repeatedly, as long as a condition remains true.

A
while
statement has the following form:
while

condition
{

statements
}
A
while
statement is executed as follows:
1
.
The
condition
is evaluated.
If
true
, execution continues to step 2. If
false
, the program is finished executing the
while
statement.
2
.
The program executes the
statements
, and execution returns to step 1.
Because the value of the
condition
is evaluated before the
statements
are executed, the
statements
in a
while
statement can be executed zero or more times.
The value of the
condition
must have a type that conforms to the
LogicValue
protocol. The condition can also
be an optional binding declaration, as discussed in
Optional Binding
.
GRAMMAR OF A WHILE STATEMENT
​

while-statement
→

while
while-condition
code-block
​

while-condition
→

expression

declaration
​
Do-While Statement
A
do
–
while
statement allows a block of code to be executed one or more times, as long as a condition
remains true.
A
do
–
while
statement has the following form:
do
{

statements
}
while

condition
A
do
–
while
statement is executed as follows:
1
.
The program executes the
statements
, and execution continues to step 2.
2
.
The
condition
is evaluated.
If
true
, execution returns to step 1. If
false
, the program is finished executing the
do
–
while
statement.
Because the value of the
condition
is evaluated after the
statements
are executed, the
statements
in a
do
–
while
statement are executed at least once.
The value of the
condition
must have a type that conforms to the
LogicValue
protocol. The condition can also
be an optional binding declaration, as discussed in
Optional Binding
.
GRAMMAR OF A DO-WHILE STATEMENT
​

do-while-statement
→

do
code-block
while
while-condition
​
Branch Statements
Branch statements allow the program to execute certain parts of code depending on the value of one or more
conditions. The values of the conditions specified in a branch statement control how the program branches and,
therefore, what block of code is executed. Swift has two branch statements: an
if
statement and a
switch
statement.
Control flow in a
switch
statement can be changed by a
break
statement and is discussed in
Break
Statement
below.
GRAMMAR OF A BRANCH STATEMENT
​

branch-statement
→

if-statement
​

branch-statement
→

switch-statement

​
If Statement
An
if
statement is used for executing code based on the evaluation of one or more conditions.
There are two basic forms of an
if
statement. In each form, the opening and closing braces are required.
The first form allows code to be executed only when a condition is true and has the following form:
if

condition
{

statements
}
The second form of an
if
statement provides an additional
else clause
(introduced by the
else
keyword) and
is used for executing one part of code when the condition is true and another part code when the same condition
is false. When a single else clause is present, an
if
statement has the following form:
if

condition
{

statements to execute if condition is true
}
else
{

statements to execute if condition is false
}
The else clause of an
if
statement can contain another
if
statement to test more than one condition. An
if
statement chained together in this way has the following form:
if

condition 1
{

statements to execute if condition 1 is true
}
else

if

condition 2
{

statements to execute if condition 2 is true
}
else
{

statements to execute if both conditions are false
}
The value of any condition in an
if
statement must have a type that conforms to the
LogicValue
protocol.
The condition can also be an optional binding declaration, as discussed in
Optional Binding
.
GRAMMAR OF AN IF STATEMENT
​

if-statement
→

if
if-condition
code-block
else-clause
opt
​

if-condition
→

expression

declaration
​

else-clause
→

else
code-block

else
if-statement
​
Switch Statement
A
switch
statement allows certain blocks of code to be executed depending on the value of a control
expression.
A switch statement has the following form:
switch

control expression
{
case

pattern 1
:

statements
case

pattern 2

where

condition
:

statements
case

pattern 3

where

condition
,
pattern 4

where

condition
:

statements

default
:

statements
}
The
control expression
of the
switch
statement is evaluated and then compared with the patterns specified in
each case. If a match is found, the program executes the
statements
listed within the scope of that case. The
scope of each case can’t be empty. As a result, you must include at least one statement following the colon (
:
)
of each case label. Use a single
break
statement if you don’t intend to execute any code in the body of a
matched case.
The values of expressions your code can branch on is very flexible. For instance, in addition to the values of
scalar types, such as integers and characters, your code can branch on the values of any type, including
floating-point numbers, strings, tuples, instances of custom classes, and optionals. The value of the
control
expression
can even be matched to the value of a case in an enumeration and checked for inclusion in a
specified range of values. For examples of how to use these various types of values in
switch
statements,
see
Switch
in the
Control Flow
chapter.
A
switch
case can optionally contain a guard expression after each pattern. A
guard expression
is introduced
by the keyword
where
followed by an expression, and is used to provide an additional condition before a
pattern in a case is considered matched to the
control expression
. If a guard expression is present, the
statements
within the relevant case are executed only if the value of the
control expression
matches one of the
patterns of the case and the guard expression evaluates to
true
. For instance, a
control expression
matches
the case in the example below only if it is a tuple that contains two elements of the same value, such as
(1,
1)
.
1
case

let
(
x
,
y
)
where

x
==
y
:
As the above example shows, patterns in a case can also bind constants using the keyword
let
(they can also
bind variables using the keyword
var
). These constants (or variables) can then be referenced in a
corresponding guard expression and throughout the rest of the code within the scope of the case. That said, if the
case contains multiple patterns that match the control expression, none of those patterns can contain constant or
variable bindings.
A
switch
statement can also include a default case, introduced by the keyword
default
. The code within a

default case is executed only if no other cases match the control expression. A
switch
statement can include
only one default case, which must appear at the end of the
switch
statement.
Although the actual execution order of pattern-matching operations, and in particular the evaluation order of
patterns in cases, is unspecified, pattern matching in a
switch
statement behaves as if the evaluation is
performed in source order—that is, the order in which they appear in source code. As a result, if multiple cases
contain patterns that evaluate to the same value, and thus can match the value of the control expression, the
program executes only the code within the first matching case in source order.
​
Switch Statements Must Be Exhaustive
In Swift, every possible value of the control expression’s type must match the value of at least one pattern of a
case. When this simply isn’t feasible (for instance, when the control expression’s type is
Int
), you can include
a default case to satisfy the requirement.
​
Execution Does Not Fall Through Cases Implicitly
After the code within a matched case has finished executing, the program exits from the
switch
statement.
Program execution does not continue or “fall through” to the next case or default case. That said, if you want
execution to continue from one case to the next, explicitly include a
fallthrough
statement, which simply
consists of the keyword
fallthrough
, in the case from which you want execution to continue. For more
information about the
fallthrough
statement, see
Fallthrough Statement
below.
GRAMMAR OF A SWITCH STATEMENT
​

switch-statement
→

switch
expression
{
switch-cases
opt
}
​

switch-cases
→

switch-case
switch-cases
opt
​

switch-case
→

case-label
statements

default-label
statements
​

switch-case
→

case-label
;

default-label
;
​

case-label
→

case
case-item-list
:
​

case-item-list
→

pattern
guard-clause
opt

pattern
guard-clause
opt
,
case-item-list
​

default-label
→

default
:
​

guard-clause
→

where
guard-expression
​

guard-expression
→

expression

​
Labeled Statement
You can prefix a loop statement or a
switch
statement with a
statement label
, which consists of the name of
the label followed immediately by a colon (:). Use statement labels with
break
and
continue
statements to
be explicit about how you want to change control flow in a loop statement or a
switch
statement, as discussed
in
Break Statement
and
Continue Statement
below.
The scope of a labeled statement is the entire statement following the statement label. You can nest labeled
statements, but the name of each statement label must be unique.
For more information and to see examples of how to use statement labels, see
Labeled Statements
in the
Control Flow
chapter.
GRAMMAR OF A LABELED STATEMENT
​

labeled-statement
→

statement-label
loop-statement

statement-label
switch-statement
​

statement-label
→

label-name
:
​

label-name
→

identifier
​
Control Transfer Statements
Control transfer statements can change the order in which code in your program is executed by unconditionally
transferring program control from one piece of code to another. Swift has four control transfer statements: a
break
statement, a
continue
statement, a
fallthrough
statement, and a
return
statement.
GRAMMAR OF A CONTROL TRANSFER STATEMENT
​

control-transfer-statement
→

break-statement
​

control-transfer-statement
→

continue-statement
​

control-transfer-statement
→

fallthrough-statement
​

control-transfer-statement
→

return-statement
​
Break Statement

A
break
statement ends program execution of a loop or a
switch
statement. A
break
statement can consist
of only the keyword
break
, or it can consist of the keyword
break
followed by the name of a statement label,
as shown below.
break
break

label name
When a
break
statement is followed by the name of a statement label, it ends program execution of the loop or
switch
statement named by that label.
When a
break
statement is not followed by the name of a statement label, it ends program execution of the
switch
statement or the innermost enclosing loop statement in which it occurs.
In both cases, program control is then transferred to the first line of code following the enclosing loop or
switch
statement, if any.
For examples of how to use a
break
statement, see
Break
and
Labeled Statements
in the
Control Flow
chapter.
GRAMMAR OF A BREAK STATEMENT
​

break-statement
→

break
label-name
opt
​
Continue Statement
A
continue
statement ends program execution of the current iteration of a loop statement but does not stop
execution of the loop statement. A
continue
statement can consist of only the keyword
continue
, or it can
consist of the keyword
continue
followed by the name of a statement label, as shown below.
continue
continue

label name

When a
continue
statement is followed by the name of a statement label, it ends program execution of the
current iteration of the loop statement named by that label.
When a
continue
statement is not followed by the name of a statement label, it ends program execution of
the current iteration of the innermost enclosing loop statement in which it occurs.
In both cases, program control is then transferred to the condition of the enclosing loop statement.
In a
for
statement, the increment expression is still evaluated after the
continue
statement is executed,
because the increment expression is evaluated after the execution of the loop’s body.
For examples of how to use a
continue
statement, see
Continue
and
Labeled Statements
in the
Control Flow
chapter.
GRAMMAR OF A CONTINUE STATEMENT
​

continue-statement
→

continue
label-name
opt
​
Fallthrough Statement
A
fallthrough
statement consists of the
fallthrough
keyword and occurs only in a case block of a
switch
statement. A
fallthrough
statement causes program execution to continue from one case in a
switch
statement to the next case. Program execution continues to the next case even if the patterns of the
case label do not match the value of the
switch
statement’s control expression.
A
fallthrough
statement can appear anywhere inside a
switch
statement, not just as the last statement of
a case block, but it can’t be used in the final case block. It also cannot transfer control into a case block whose
pattern contains value binding patterns.
For an example of how to use a
fallthrough
statement in a
switch
statement, see
Control Transfer
Statements
in the
Control Flow
chapter.
GRAMMAR OF A FALLTHROUGH STATEMENT
​

fallthrough-statement
→

fallthrough

​
Return Statement
A
return
statement occurs only in the body of a function or method definition and causes program execution
to return to the calling function or method. Program execution continues at the point immediately following the
function or method call.
A
return
statement can consist of only the keyword
return
, or it can consist of the keyword
return
followed by an expression, as shown below.
return
return

expression
When a
return
statement is followed by an expression, the value of the expression is returned to the calling
function or method. If the value of the expression does not match the value of the return type declared in the
function or method declaration, the expression’s value is converted to the return type before it is returned to the
calling function or method.
When a
return
statement is not followed by an expression, it can be used only to return from a function or
method that does not return a value (that is, when the return type of the function or method is
Void
or
()
).
GRAMMAR OF A RETURN STATEMENT
​

return-statement
→

return
expression
opt

​
​
Declarations
A
declaration
introduces a new name or construct into your program. For example, you use declarations to
introduce functions and methods, variables and constants, and to define new, named enumeration, structure,
class, and protocol types. You can also use a declaration to extend the the behavior of an existing named type
and to import symbols into your program that are declared elsewhere.
In Swift, most declarations are also definitions in the sense that they are implemented or initialized at the same
time they are declared. That said, because protocols don’t implement their members, most protocol members
are declarations only. For convenience and because the distinction isn’t that important in Swift, the term
declaration
covers both declarations and definitions.
GRAMMAR OF A DECLARATION
​

declaration
→

import-declaration
​

declaration
→

constant-declaration
​

declaration
→

variable-declaration
​

declaration
→

typealias-declaration
​

declaration
→

function-declaration
​

declaration
→

enum-declaration
​

declaration
→

struct-declaration
​

declaration
→

class-declaration
​

declaration
→

protocol-declaration
​

declaration
→

initializer-declaration
​

declaration
→

deinitializer-declaration
​

declaration
→

extension-declaration
​

declaration
→

subscript-declaration
​

declaration
→

operator-declaration
​

declarations
→

declaration
declarations
opt
​

declaration-specifiers
→

declaration-specifier
declaration-specifiers
opt
​

declaration-specifier
→

class

mutating

nonmutating

override

static
unowned

unowned(safe)

unowned(unsafe)

weak
​
Module Scope
The module scope defines the code that’s visible to other code in Swift source files that are part of the same
module. The top-level code in a Swift source file consists of zero or more statements, declarations, and

expressions. Variables, constants, and other named declarations that are declared at the top-level of a source
file are visible to code in every source file that is part of the same module.
GRAMMAR OF A TOP-LEVEL DECLARATION
​

top-level-declaration
→

statements
opt
​
Code Blocks
A
code block
is used by a variety of declarations and control structures to group statements together. It has the
following form:
{

statements
}
The
statements
inside a code block include declarations, expressions, and other kinds of statements and are
executed in order of their appearance in source code.
GRAMMAR OF A CODE BLOCK
​

code-block
→

{
statements
opt
}
​
Import Declaration
An
import declaration
lets you access symbols that are declared outside the current file. The basic form
imports the entire module; it consists of the
import
keyword followed by a module name:
import

module

Providing more detail limits which symbols are imported—you can specify a specific submodule or a specific
declaration within a module or submodule. When this detailed form is used, only the imported symbol (and not
the module that declares it) is made available in the current scope.
import

import kind

module
.
symbol name
import

module
.
submodule
GRAMMAR OF AN IMPORT DECLARATION
​

import-declaration
→

attributes
opt
import
import-kind
opt
import-path
​

import-kind
→

typealias

struct

class

enum

protocol

var

func
​

import-path
→

import-path-identifier

import-path-identifier
.
import-path
​

import-path-identifier
→

identifier

operator
​
Constant Declaration
A
constant declaration
introduces a constant named value into your program. Constant declarations are
declared using the keyword
let
and have the following form:
let

constant name
:
type
=
expression
A constant declaration defines an immutable binding between the
constant name
and the value of the initializer
expression
; after the value of a constant is set, it cannot be changed. That said, if a constant is initialized with a
class object, the object itself can change, but the binding between the constant name and the object it refers to
can’t.
When a constant is declared at global scope, it must be initialized with a value. When a constant declaration
occurs in the context of a class or structure declaration, it is considered a
constant property
. Constant
declarations are not computed properties and therefore do not have getters or setters.
If the
constant name
of a constant declaration is a tuple pattern, the name of each item in the tuple is bound to the
corresponding value in the initializer
expression
.

1
let
(
firstNumber
,
secondNumber
) = (
10
,
42
)
In this example,
firstNumber
is a named constant for the value
10
, and
secondNumber
is a named
constant for the value
42
. Both constants can now be used independently:
1
println
(
"The first number is
\(
firstNumber
)
."
)
2
// prints "The first number is 10."
3
println
(
"The second number is
\(
secondNumber
)
."
)
4
// prints "The second number is 42."
The type annotation (
:

type
) is optional in a constant declaration when the type of the
constant name
can be
inferred, as described in
Type Inference
.
To declare a static constant property, mark the declaration with the
static
keyword. Static properties are
discussed in
Type Properties
.
For more information about constants and for guidance about when to use them, see
Constants and Variables
and
Stored Properties
.
GRAMMAR OF A CONSTANT DECLARATION
​

constant-declaration
→

attributes
opt
declaration-specifiers
opt
let
pattern-initializer-list
​

pattern-initializer-list
→

pattern-initializer

pattern-initializer
,
pattern-initializer-list
​

pattern-initializer
→

pattern
initializer
opt
​

initializer
→

=
expression
​
Variable Declaration
A
variable declaration
introduces a variable named value into your program and is declared using the keyword
var
.
Variable declarations have several forms that declare different kinds of named, mutable values, including stored
and computed variables and properties, stored variable and property observers, and static variable properties.
The appropriate form to use depends on the scope at which the variable is declared and the kind of variable you

intend to declare.
NOTE
You can also declare properties in the context of a protocol declaration, as described in
Protocol
Property Declaration
.
You can override a property in a subclass by prefixing the subclass’s property declaration with the
override
keyword, as described in
Overriding
.
​
Stored Variables and Stored Variable Properties
The following form declares a stored variable or stored variable property:
var

variable name
:
type
=
expression
You define this form of a variable declaration at global scope, the local scope of a function, or in the context of a
class or structure declaration. When a variable declaration of this form is declared at global scope or the local
scope of a function, it is referred to as a
stored variable
. When it is declared in the context of a class or structure
declaration, it is referred to as a
stored variable property
.
The initializer
expression
can’t be present in a protocol declaration, but in all other contexts, the initializer
expression
is optional. That said, if no initializer
expression
is present, the variable declaration must include an
explicit type annotation (
:

type
).
As with constant declarations, if the
variable name
is a tuple pattern, the name of each item in the tuple is bound
to the corresponding value in the initializer
expression
.
As their names suggest, the value of a stored variable or a stored variable property is stored in memory.

​
Computed Variables and Computed Properties
The following form declares a computed variable or computed property:
var

variable name
:
type
{
get
{

statements
}
set
(
setter name
) {

statements
}
}
You define this form of a variable declaration at global scope, the local scope of a function, or in the context of a
class, structure, enumeration, or extension declaration. When a variable declaration of this form is declared at
global scope or the local scope of a function, it is referred to as a
computed variable
. When it is declared in the
context of a class, structure, or extension declaration, it is referred to as a
computed property
.
The getter is used to read the value, and the setter is used to write the value. The setter clause is optional, and
when only a getter is needed, you can omit both clauses and simply return the requested value directly, as
described in
Read-Only Computed Properties
. But if you provide a setter clause, you must also provide a getter
clause.
The
setter name
and enclosing parentheses is optional. If you provide a setter name, it is used as the name of
the parameter to the setter. If you do not provide a setter name, the default parameter name to the setter is
newValue
, as described in
Shorthand Setter Declaration
.
Unlike stored named values and stored variable properties, the value of a computed named value or a
computed property is not stored in memory.
For more information and to see examples of computed properties, see
Computed Properties
.

​
Stored Variable Observers and Property Observers
You can also declare a stored variable or property with
willSet
and
didSet
observers. A stored variable or
property declared with observers has the following form:
var

variable name
:
type
=
expression
{
willSet
(
setter name
) {

statements
}
didSet
(
setter name
{

statements
}
}
You define this form of a variable declaration at global scope, the local scope of a function, or in the context of a
class or structure declaration. When a variable declaration of this form is declared at global scope or the local
scope of a function, the observers are referred to as
stored variable observers
. When it is declared in the
context of a class or structure declaration, the observers are referred to as
property observers
.
You can add property observers to any stored property. You can also add property observers to any inherited
property (whether stored or computed) by overriding the property within a subclass, as described in
Overriding
Property Observers
.
The initializer
expression
is optional in the context of a class or structure declaration, but required elsewhere.
The type annotation is required in all variable declarations that include observers, regardless of the context in
which they are declared.
The
willSet
and
didSet
observers provide a way to observe (and to respond appropriately) when the value
of a variable or property is being set. The observers are not called when the variable or property is first
initialized. Instead, they are called only when the value is set outside of an initialization context.

A
willSet
observer is called just before the value of the variable or property is set. The new value is passed
to the
willSet
observer as a constant, and therefore it can’t be changed in the implementation of the
willSet
clause. The
didSet
observer is called immediately after the new value is set. In contrast to the
willSet
observer, the old value of the variable or property is passed to the
didSet
observer in case you still
need access to it. That said, if you assign a value to a variable or property within its own
didSet
observer
clause, that new value that you assign will replace the one that was just set and passed to the
willSet
observer.
The
setter name
and enclosing parentheses in the
willSet
and
didSet
clauses are optional. If you provide
setter names, they are used as the parameter names to the
willSet
and
didSet
observers. If you do not
provide setter names, the default parameter name to the
willSet
observer is
newValue
and the default
parameter name to the
didSet
observer is
oldValue
.
The
didSet
clause is optional when you provide a
willSet
clause. Likewise, the
willSet
clause is
optional when you provide a
didSet
clause.
For more information and to see an example of how to use property observers, see
Property Observers
.
​
Class and Static Variable Properties
To declare a class computed property, mark the declaration with the
class
keyword. To declare a static
variable property, mark the declaration with the
static
keyword. Class and static properties are discussed in
Type Properties
.
GRAMMAR OF A VARIABLE DECLARATION
​

variable-declaration
→

variable-declaration-head
pattern-initializer-list
​

variable-declaration
→

variable-declaration-head
variable-name
type-annotation
code-block
​

variable-declaration
→

variable-declaration-head
variable-name
type-annotation
getter-setter-
block
​

variable-declaration
→

variable-declaration-head
variable-name
type-annotation
getter-setter-
keyword-block
​

variable-declaration
→

variable-declaration-head
variable-name
type-
annotation
initializer
opt
willSet-didSet-block
​

variable-declaration-head
→

attributes
opt
declaration-specifiers
opt
var
​

variable-name
→

identifier

​

getter-setter-block
→

{
getter-clause
setter-clause
opt
}
​

getter-setter-block
→

{
setter-clause
getter-clause
}
​

getter-clause
→

attributes
opt
get
code-block
​

setter-clause
→

attributes
opt
set
setter-name
opt
code-block
​

setter-name
→

(
identifier
)
​

getter-setter-keyword-block
→

{
getter-keyword-clause
setter-keyword-clause
opt
}
​

getter-setter-keyword-block
→

{
setter-keyword-clause
getter-keyword-clause
}
​

getter-keyword-clause
→

attributes
opt
get
​

setter-keyword-clause
→

attributes
opt
set
​

willSet-didSet-block
→

{
willSet-clause
didSet-clause
opt
}
​

willSet-didSet-block
→

{
didSet-clause
willSet-clause
}
​

willSet-clause
→

attributes
opt
willSet
setter-name
opt
code-block
​

didSet-clause
→

attributes
opt
didSet
setter-name
opt
code-block
​
Type Alias Declaration
A
type alias declaration
introduces a named alias of an existing type into your program. Type alias declarations
begin with the keyword
typealias
and have the following form:
typealias

name
=
existing type
After a type alias is declared, the aliased
name
can be used instead of the
existing type
everywhere in your
program. The
existing type
can be a named type or a compound type. Type aliases do not create new types;
they simply allow a name to refer to an existing type.
See also
Protocol Associated Type Declaration
.
GRAMMAR OF A TYPE ALIAS DECLARATION
​

typealias-declaration
→

typealias-head
typealias-assignment
​

typealias-head
→

typealias
typealias-name
​

typealias-name
→

identifier
​

typealias-assignment
→

=
type
​

Function Declaration
A :newTerm`function declaration` introduces a function or method into your program. A function declared in the
context of class, structure, enumeration, or protocol is referred to as a
method
. Function declarations are
declared using the keyword
func
and have the following form:
func

function name
(
parameters
) ->
return type
{

statements
}
If the function has a return type of
Void
, the return type can be omitted as follows:
func

function name
(
parameters
) {

statements
}
The type of each parameter must be included—it can’t be inferred. By default, the parameters to a function are
constants. Write
var
in front of a parameter’s name to make it a variable, scoping any changes made to the
variable just to the function body, or write
inout
to make those changes also apply to the argument that was
passed in the caller’s scope. For a discussion of in-out parameters, see
In-Out Parameters
.
Functions can return multiple values using a tuple type as the return type of the function.
A function definition can appear inside another function declaration. This kind of function is known as a
nested
function
. For a discussion of nested functions, see
Nested Functions
.
​
Parameter Names
Function parameters are a comma separated list where each parameter has one of several forms. The order of
arguments in a function call must match the order of parameters in the function’s declaration. The simplest
entry in a parameter list has the following form:

parameter name
:
parameter type
For function parameters, the parameter name is used within the function body, but is not used when calling the
function. For method parameters, the parameter name is used as within the function body, and is also used as a
label for the argument when calling the method. The name of a method’s first parameter is used only within the
function body, like the parameter of a function. For example:
1
func

f
(
x
:
Int
,
y
:
String
) ->
String
{
2

return

y
+
String
(
x
)
3
}
4
f
(
7
,
"hello"
)
// x and y have no name
5

6
class

C
{
7

func

f
(
x
:
Int
,
y
:
String
) ->
String
{
8

return

y
+
String
(
x
)
9
}
10
}
11
let

c
=
C
()
12
c
.
f
(
7
,
y
:
"hello"
)
// x has no name, y has a name
You can override the default behavior for how parameter names are used with one of the following forms:
external parameter name

local parameter name
:
parameter type
#
parameter name
:
parameter type
_

local parameter name
:
parameter type
A second name before the local parameter name gives the parameter an external name, which can be different
than the local parameter name. The external parameter name must be used when the function is called. The
corresponding argument must have the external name in function or method calls.
A hash symbol (
#
) before a parameter name indicates that the name should be used as both an external and a
local parameter name. It has the same meaning as writing the local parameter name twice. The corresponding
argument must have this name in function or method calls.

An underscore (
_
) before a local parameter name gives that parameter no name to be used in function calls.
The corresponding argument must have no name in function or method calls.
​
Special Kinds of Parameters
Parameters can be ignored, take a variable number of values, and provide default values using the following
forms:
_
: <#
parameter

type
#.
parameter name
:
parameter type
…
parameter name
:
parameter type
=
default argument value
A parameter named with an underscore (
_
) is explicitly ignored an can’t be accessed within the body of the
function.
A parameter with a base type name followed immediately by three dots (
…
) is understood as a variadic
parameter. A function can have at most one variadic parameter, which must be its last parameter. A variadic
parameter is treated as an array that contains elements of the base type name. For instance, the variadic
parameter
Int…
is treated as
Int[]
. For an example that uses a variadic parameter, see
Variadic
Parameters
.
A parameter with an equals sign (
=
) and an expression after its type is understood to have a default value of the
given expression. If the parameter is omitted when calling the function, the default value is used instead. If the
parameter is not omitted, it must have its name in the function call. For example,
f()
and
f(x: 7)
are both
valid calls to a function with a single default parameter named
x
, but
f(7)
is invalid because it provides a
value without a name.
​
Special Kinds of Methods
Methods on an enumeration or a structure that modify
self
must be marked with the
mutating
keyword at
the start of the function declaration.

Methods that override a superclass method must be marked with the
override
keyword at the start of the
function declaration. It is an error to override a method without the
override
keyword or to use the
override
keyword on a method that doesn’t override a superclass method.
Methods associated with a type rather than an instance of a type must be marked with the
static
attribute for
enumerations and structures or the
class
attribute for classes.
​
Curried Functions and Methods
Curried functions and methods have the following form:
func

function name
(
parameters
)(
parameters
) ->
return type

{

statements
}
A function declared this way is understood as a function whose return type is another function. For example, the
following two declarations are equivalent:
1
func

addTwoNumbers
(
a
:
Int
)(
b
:
Int
) ->
Int
{
2

return

a
+
b
3
}
4
func

addTwoNumbers
(
a
:
Int
) -> (
Int
->
Int
) {
5

func

addTheSecondNumber
(
b
:
Int
) ->
Int
{
6

return

a
+
b
7
}
8

return

addTheSecondNumber
9
}
10

11
addTwoNumbers
(
4
)(
5
)
// Returns 9

Multiple levels of currying are allowed.
GRAMMAR OF A FUNCTION DECLARATION
​

function-declaration
→

function-head
function-name
generic-parameter-clause
opt
function-
signature
function-body
​

function-head
→

attributes
opt
declaration-specifiers
opt
func
​

function-name
→

identifier

operator
​

function-signature
→

parameter-clauses
function-result
opt
​

function-result
→

->
attributes
opt
type
​

function-body
→

code-block
​

parameter-clauses
→

parameter-clause
parameter-clauses
opt
​

parameter-clause
→

(
)

(
parameter-list
…
opt
)
​

parameter-list
→

parameter

parameter
,
parameter-list
​

parameter
→

inout
opt
let
opt
#
opt
parameter-name
local-parameter-name
opt
type-
annotation
default-argument-clause
opt
​

parameter
→

inout
opt
var
#
opt
parameter-name
local-parameter-name
opt
type-
annotation
default-argument-clause
opt
​

parameter
→

attributes
opt
type
​

parameter-name
→

identifier

_
​

local-parameter-name
→

identifier

_
​

default-argument-clause
→

=
expression
​
Enumeration Declaration
An
enumeration declaration
introduces a named enumeration type into your program.
Enumeration declarations have two basic forms and are declared using the keyword
enum
. The body of an
enumeration declared using either form contains zero or more values—called
enumeration cases
—and any
number of declarations, including computed properties, instance methods, static methods, initializers, type
aliases, and even other enumeration, structure, and class declarations. Enumeration declarations can’t contain
destructor or protocol declarations.
Unlike classes and structures, enumeration types do not have an implicitly provided default initializer; all
initializers must be declared explicitly. Initializers can delegate to other initializers in the enumeration, but the
initialization process is complete only after an initializer assigns one of the enumeration cases to
self
.

Like structures but unlike classes, enumerations are value types; instances of an enumeration are copied when
assigned to variables or constants, or when passed as arguments to a function call. For information about value
types, see
Structures and Enumerations Are Value Types
.
You can extend the behavior of an enumeration type with an extension declaration, as discussed in
Extension
Declaration
.
​
Enumerations with Cases of Any Type
The following form declares an enumeration type that contains enumeration cases of any type:
enum

enumeration name
{

case

enumeration case 1

case

enumeration case 2
(
associated value types
)
}
Enumerations declared in this form are sometimes called
discriminated unions
in other programming
languages.
In this form, each case block consists of the keyword
case
followed by one or more enumeration cases,
separated by commas. The name of each case must be unique. Each case can also specify that it stores values
of a given type. These types are specified in the
associated value types
tuple, immediately following the name of
the case. For more information and to see examples of cases with associated value types, see
Associated
Values
.
​
Enumerations with Raw Cases Values
The following form declares an enumeration type that contains enumeration cases of the same basic type:
enum

enumeration name
:
raw value type
{

case

enumeration case 1
=
raw value 1

case

enumeration case 2
=
raw value 2
}
In this form, each case block consists of the keyword
case
, followed by one or more enumeration cases,
separated by commas. Unlike the cases in the first form, each case has an underlying value, called a
raw
value
, of the same basic type. The type of these values is specified in the
raw value type
and must represent a
literal integer, floating-point number, character, or string.
Each case must have a unique name and be assigned a unique raw value. If the raw value type is specified as
Int
and you don’t assign a value to the cases explicitly, they are implicitly assigned the values
0
,
1
,
2
, and so
on. Each unassigned case of type
Int
is implicitly assigned a raw value that is automatically incremented from
the raw value of the previous case.
1
enum

ExampleEnum
:
Int
{
2

case

A
,
B
,
C
=
5
,
D
3
}
In the above example, the value of
ExampleEnum.A
is
0
and the value of
ExampleEnum.B
is
1
. And
because the value of
ExampleEnum.C
is explicitly set to
5
, the value of
ExampleEnum.D
is automatically
incremented from
5
and is therefore
6
.
The raw value of an enumeration case can be accessed by calling its
toRaw
method, as in
ExampleEnum.B.toRaw()
. You can also use a raw value to find a corresponding case, if there is one, by
calling the
fromRaw
method, which returns an optional case. For more information and to see examples of
cases with raw value types, see
Raw Values
.
​
Accessing Enumeration Cases
To reference the case of an enumeration type, use dot (
.
) syntax, as in
EnumerationType.EnumerationCase
. When the enumeration type can be inferred from context, you
can omit it (the dot is still required), as described in
Enumeration Syntax
and
Implicit Member Expression
.

To check the values of enumeration cases, use a
switch
statement, as shown in
Matching Enumeration
Values with a Switch Statement
. The enumeration type is pattern-matched against the enumeration case
patterns in the case blocks of the
switch
statement, as described in
Enumeration Case Pattern
.
GRAMMAR OF AN ENUMERATION DECLARATION
​

enum-declaration
→

attributes
opt
union-style-enum

attributes
opt
raw-value-style-enum
​

union-style-enum
→

enum-name
generic-parameter-clause
opt
{
union-style-enum-
members
opt
}
​

union-style-enum-members
→

union-style-enum-member
union-style-enum-members
opt
​

union-style-enum-member
→

declaration

union-style-enum-case-clause
​

union-style-enum-case-clause
→

attributes
opt
case
union-style-enum-case-list
​

union-style-enum-case-list
→

union-style-enum-case

union-style-enum-case
,
union-style-
enum-case-list
​

union-style-enum-case
→

enum-case-name
tuple-type
opt
​

enum-name
→

identifier
​

enum-case-name
→

identifier
​

raw-value-style-enum
→

enum-name
generic-parameter-clause
opt
:
type-identifier
{
raw-
value-style-enum-members
opt
}
​

raw-value-style-enum-members
→

raw-value-style-enum-member
raw-value-style-enum-
members
opt
​

raw-value-style-enum-member
→

declaration

raw-value-style-enum-case-clause
​

raw-value-style-enum-case-clause
→

attributes
opt
case
raw-value-style-enum-case-list
​

raw-value-style-enum-case-list
→

raw-value-style-enum-case

raw-value-style-enum-
case
,
raw-value-style-enum-case-list
​

raw-value-style-enum-case
→

enum-case-name
raw-value-assignment
opt
​

raw-value-assignment
→

=
literal
​
Structure Declaration
A
structure declaration
introduces a named structure type into your program. Structure declarations are
declared using the keyword
struct
and have the following form:
struct

structure name
:
adopted protocols
{

declarations
}

The body of a structure contains zero or more
declarations
. These
declarations
can include both stored and
computed properties, static properties, instance methods, static methods, initializers, type aliases, and even
other structure, class, and enumeration declarations. Structure declarations can’t contain destructor or protocol
declarations. For a discussion and several examples of structures that include various kinds of declarations,
see
Classes and Structures
.
Structure types can adopt any number of protocols, but can’t inherit from classes, enumerations, or other
structures.
There are three ways create an instance of a previously declared structure:
The process of initializing a structure’s declared properties is described in
Initialization
.
Properties of a structure instance can be accessed using dot (
.
) syntax, as described in
Accessing Properties
.
Structures are value types; instances of a structure are copied when assigned to variables or constants, or
when passed as arguments to a function call. For information about value types, see
Structures and
Enumerations Are Value Types
.
You can extend the behavior of a structure type with an extension declaration, as discussed in
Extension
Declaration
.
GRAMMAR OF A STRUCTURE DECLARATION
​

struct-declaration
→

attributes
opt
struct
struct-name
generic-parameter-clause
opt
type-
inheritance-clause
opt
struct-body
​

struct-name
→

identifier
​

struct-body
→

{
declarations
opt
}
​
Call one of the initializers declared within the structure, as described in
Initializers
.
If no initializers are declared, call the structure’s memberwise initializer, as described in
Memberwise Initializers for Structure Types
.
If no initializers are declared, and all properties of the structure declaration were given initial
values, call the structure’s default initializer, as described in
Default Initializers
.

Class Declaration
A
class declaration
introduces a named class type into your program. Class declarations are declared using the
keyword
class
and have the following form:
class

class name
:
superclass
,
adopted protocols
{

declarations
}
The body of a class contains zero or more
declarations
. These
declarations
can include both stored and
computed properties, instance methods, class methods, initializers, a single destructor method, type aliases,
and even other class, structure, and enumeration declarations. Class declarations can’t contain protocol
declarations. For a discussion and several examples of classes that include various kinds of declarations, see
Classes and Structures
.
A class type can inherit from only one parent class, its
superclass
, but can adopt any number of protocols. The
superclass
appears first in the
type-inheritance-clause
, followed by any
adopted protocols
.
As discussed in
Initializer Declaration
, classes can have designated and convenience initializers. When you
declare either kind of initializer, you can require any subclass to override it by marking the initializer with the
required
attribute. The designated initializer of a class must initialize all of the class’s declared properties
and it must do so before calling any of its superclass’s designated initializers.
A class can override properties, methods, and initializers of its superclass. Overridden methods and properties
must be marked with the
override
keyword.
Although properties and methods declared in the
superclass
are inherited by the current class, designated
initializers declared in the
superclass
are not. That said, if the current class overrides all of the superclass’s
designated initializers, it inherits the superclass’s convenience initializers. Swift classes do not inherit from a
universal base class.
There are two ways create an instance of a previously declared class:
Call one of the initializers declared within the class, as described in
Initializers
.
If no initializers are declared, and all properties of the class declaration were given initial values,

Access properties of a class instance with dot (
.
) syntax, as described in
Accessing Properties
.
Classes are reference types; instances of a class are referred to, rather than copied, when assigned to
variables or constants, or when passed as arguments to a function call. For information about reference types,
see
Structures and Enumerations Are Value Types
.
You can extend the behavior of a class type with an extension declaration, as discussed in
Extension
Declaration
.
GRAMMAR OF A CLASS DECLARATION
​

class-declaration
→

attributes
opt
class
class-name
generic-parameter-clause
opt
type-
inheritance-clause
opt
class-body
​

class-name
→

identifier
​

class-body
→

{
declarations
opt
}
​
Protocol Declaration
A
protocol declaration
introduces a named protocol type into your program. Protocol declarations are declared
using the keyword
protocol
and have the following form:
protocol

protocol name
:
inherited protocols
{

protocol member declarations
}
The body of a protocol contains zero or more
protocol member declarations
, which describe the conformance
requirements that any type adopting the protocol must fulfill. In particular, a protocol can declare that conforming
types must implement certain properties, methods, initializers, and subscripts. Protocols can also declare
special kinds of type aliases, called
associated types
, that can specify relationships among the various
declarations of the protocol. The
protocol member declarations
are discussed in detail below.
Protocol types can inherit from any number of other protocols. When a protocol type inherits from other
call the class’s default initializer, as described in
Default Initializers
.

protocols, the set of requirements from those other protocols are aggregated, and any type that inherits from the
current protocol must conform to all those requirements. For an example of how to use protocol inheritance,
see
Protocol Inheritance
.
NOTE
You can also aggregate the conformance requirements of multiple protocols using protocol
composition types, as described in
Protocol Composition Type
and
Protocol Composition
.
You can add protocol conformance to a previously declared type by adopting the protocol in an extension
declaration of that type. In the extension, you must implement all of the adopted protocol’s requirements. If the
type already implements all of the requirements, you can leave the body of the extension declaration empty.
By default, types that conform to a protocol must implement all properties, methods, and subscripts declared in
the protocol. That said, you can mark these protocol member declarations with the
optional
attribute to
specify that their implementation by a conforming type is optional. The
optional
attribute can be applied only
to protocols that are marked with the
objc
attribute. As a result, only class types can adopt and conform to a
protocol that contains optional member requirements. For more information about how to use the
optional
attribute and for guidance about how to access optional protocol members—for example, when you’re not sure
whether a conforming type implements them—see
Optional Protocol Requirements
.
To restrict the adoption of a protocol to class types only, mark the entire protocol declaration with the
class_protocol
attribute. Any protocol that inherits from a protocol marked with the
class_protocol
attribute can likewise be adopted only by a class type.
NOTE
If a protocol is already marked with the
objc
attribute, the
class_protocol
attribute is
implicitly applied to that protocol; there’s no need to mark the protocol with the
class_protocol
attribute explicitly.

Protocols are named types, and thus they can appear in all the same places in your code as other named types,
as discussed in
Protocols as Types
. However, you can’t construct an instance of a protocol, because protocols
do not actually provide the implementations for the requirements they specify.
You can use protocols to declare which methods a delegate of a class or structure should implement, as
described in
Delegation
.
GRAMMAR OF A PROTOCOL DECLARATION
​

protocol-declaration
→

attributes
opt
protocol
protocol-name
type-inheritance-
clause
opt
protocol-body
​

protocol-name
→

identifier
​

protocol-body
→

{
protocol-member-declarations
opt
}
​

protocol-member-declaration
→

protocol-property-declaration
​

protocol-member-declaration
→

protocol-method-declaration
​

protocol-member-declaration
→

protocol-initializer-declaration
​

protocol-member-declaration
→

protocol-subscript-declaration
​

protocol-member-declaration
→

protocol-associated-type-declaration
​

protocol-member-declarations
→

protocol-member-declaration
protocol-member-declarations
opt
​
Protocol Property Declaration
Protocols declare that conforming types must implement a property by including a
protocol property declaration
in the body of the protocol declaration. Protocol property declarations have a special form of a variable
declaration:
var

property name
:
type
{
get

set
}
As with other protocol member declarations, these property declarations declare only the getter and setter
requirements for types that conform to the protocol. As a result, you don’t implement the getter or setter directly
in the protocol in which it is declared.
The getter and setter requirements can be satisfied by a conforming type in a variety of ways. If a property
declaration includes both the
get
and
set
keywords, a conforming type can implement it with a stored
variable property or a computed property that is both readable and writeable (that is, one that implements both a
getter and a setter). However, that property declaration can’t be implemented as a constant property or a read-

only computed property. If a property declaration includes only the
get
keyword, it can be implemented as any
kind of property. For examples of conforming types that implement the property requirements of a protocol, see
Property Requirements
.
See also
Variable Declaration
.
GRAMMAR OF A PROTOCOL PROPERTY DECLARATION
​

protocol-property-declaration
→

variable-declaration-head
variable-name
type-
annotation
getter-setter-keyword-block
​
Protocol Method Declaration
Protocols declare that conforming types must implement a method by including a protocol method declaration
in the body of the protocol declaration. Protocol method declarations have the same form as function
declarations, with two exceptions: They don’t include a function body, and you can’t provide any default
parameter values as part of the function declaration. For examples of conforming types that implement the
method requirements of a protocol, see
Method Requirements
.
To declare a class or static method requirement in a protocol declaration, mark the method declaration with the
class
keyword. Classes that implement this method also declare the method with the
class
keyword.
Structures that implement it must declare the method with the
static
keyword instead. If you’re
implementing the method in an extension, use the
class
keyword if you’re extending a class and the
static
keyword if you’re extending a structure.
See also
Function Declaration
.
GRAMMAR OF A PROTOCOL METHOD DECLARATION
​

protocol-method-declaration
→

function-head
function-name
generic-parameter-
clause
opt
function-signature
​
Protocol Initializer Declaration

Protocols declare that conforming types must implement an initializer by including a protocol initializer
declaration in the body of the protocol declaration. Protocol initializer declarations have the same form as
initializer declarations, except they don’t include the initializer’s body.
See also
Initializer Declaration
.
GRAMMAR OF A PROTOCOL INITIALIZER DECLARATION
​

protocol-initializer-declaration
→

initializer-head
generic-parameter-clause
opt
parameter-
clause
​
Protocol Subscript Declaration
Protocols declare that conforming types must implement a subscript by including a protocol subscript
declaration in the body of the protocol declaration. Protocol property declarations have a special form of a
subscript declaration:
subscript
(
parameters
) ->
return type
{
get

set
}
Subscript declarations only declare the minimum getter and setter implementation requirements for types that
conform to the protocol. If the subscript declaration includes both the
get
and
set
keywords, a conforming
type must implement both a getter and a setter clause. If the subscript declaration includes only the
get
keyword, a conforming type must implement
at least
a getter clause and optionally can implement a setter
clause.
See also
Subscript Declaration
.
GRAMMAR OF A PROTOCOL SUBSCRIPT DECLARATION
​

protocol-subscript-declaration
→

subscript-head
subscript-result
getter-setter-keyword-block
​
Protocol Associated Type Declaration

Protocols declare associated types using the keyword
typealias
. An associated type provides an alias for a
type that is used as part of a protocol’s declaration. Accosiated types are similiar to type paramters in generic
parameter clauses, but they’re associated with
Self
in the protocol in which they’re declared. In that context,
Self
refers to the eventual type that conforms to the protocol. For more information and examples, see
Associated Types
.
See also
Type Alias Declaration
.
GRAMMAR OF A PROTOCOL ASSOCIATED TYPE DECLARATION
​

protocol-associated-type-declaration
→

typealias-head
type-inheritance-clause
opt
typealias-
assignment
opt
​
Initializer Declaration
An
initializer declaration
introduces an initializer for a class, structure, or enumeration into your program.
Initializer declarations are declared using the keyword
init
and have two basic forms.
Structure, enumeration, and class types can have any number of initializers, but the rules and associated
behavior for class initializers are different. Unlike structures and enumerations, classes have two kinds of
initializers: designated initializers and convenience initializers, as described in
Initialization
.
The following form declares initializers for structures, enumerations, and designated initializers of classes:
init
(
parameters
) {

statements
}
A designated initializer of a class initializes all of the class’s properties directly. It can’t call any other initializers
of the same class, and if the class has a superclass, it must call one of the superclass’s designated initializers.
If the class inherits any properties from its superclass, one of the superclass’s designated initializers must be
called before any of these properties can be set or modified in the current class.
Designated initializers can be declared in the context of a class declaration only and therefore can’t be added to

a class using an extension declaration.
Initializers in structures and enumerations can call other declared initializers to delegate part or all of the
initialization process.
To declare convenience initializers for a class, prefix the initializer declaration with the context-sensitive
keyword
convenience
.
convenience

init
(
parameters
) {

statements
}
Convenience initializers can delegate the initialization process to another convenience initializer or to one of the
class’s designated initializers. That said, the initialization processes must end with a call to a designated
initializer that ultimately initializes the class’s properties. Convenience initializers can’t call a superclass’s
initializers.
You can mark designated and convenience initializers with the
required
attribute to require that every
subclass implement the initializer. Because designated initializers are not inherited by subclasses, they must
be implemented directly. Required convenience initializers can be either implemented explicitly or inherited
when the subclass directly implements all of the superclass’s designated initializers (or overrides the
designated initializers with convenience initializers). Unlike methods, properties, and subscripts, you don’t need
to mark overridden initializers with the
override
keyword.
To see examples of initializers in various type declarations, see
Initialization
.
GRAMMAR OF AN INITIALIZER DECLARATION
​

initializer-declaration
→

initializer-head
generic-parameter-clause
opt
parameter-
clause
initializer-body
​

initializer-head
→

attributes
opt
convenience
opt
init
​

initializer-body
→

code-block
​
Deinitializer Declaration

A
deinitializer declaration
declares a deinitializer for a class type. Deinitializers take no parameters and have
the following form:
deinit
{

statements
}
A deinitializer is called automatically when there are no longer any references to a class object, just before the
class object is deallocated. A deinitializer can be declared only in the body of a class declaration—but not in an
extension of a class—and each class can have at most one.
A subclass inherits its superclass’s deinitializer, which is implicitly called just before the subclass object is
deallocated. The subclass object is not deallocated until all deinitializers in its inheritance chain have finished
executing.
Deinitializers are not called directly.
For an example of how to use a deinitializer in a class declaration, see
Deinitialization
.
GRAMMAR OF A DEINITIALIZER DECLARATION
​

deinitializer-declaration
→

attributes
opt
deinit
code-block
​
Extension Declaration
An
extension declaration
allows you to extend the behavior of existing class, structure, and enumeration types.
Extension declarations begin with the keyword
extension
and have the following form:
extension

type
:
adopted protocols
{

declarations
}

The body of an extension declaration contains zero or more
declarations
. These
declarations
can include
computed properties, computed static properties, instance methods, static and class methods, initializers,
subscript declarations, and even class, structure, and enumeration declarations. Extension declarations can’t
contain destructor or protocol declarations, store properties, property observers, or other extension
declarations. For a discussion and several examples of extensions that include various kinds of declarations,
see
Extensions
.
Extension declarations can add protocol conformance to an existing class, structure, and enumeration type in
the
adopted protocols
. Extension declarations can’t add class inheritance to an existing class, and therefore the
type-inheritance-clause
in an extension declaration contains only a list of protocols.
Properties, methods, and initializers of an existing type can’t be overridden in an extension of that type.
Extension declarations can contain initializer declarations. That said, if the type you’re extending is defined in
another module, an initializer declaration must delegate to an initializer already defined in that module to ensure
members of that type are properly initialized.
GRAMMAR OF AN EXTENSION DECLARATION
​

extension-declaration
→

extension
type-identifier
type-inheritance-clause
opt
extension-
body
​

extension-body
→

{
declarations
opt
}
​
Subscript Declaration
A
subscript
declaration allows you to add subscripting support for objects of a particular type and are typically
used to provide a convenient syntax for accessing the elements in a collection, list, or sequence. Subscript
declarations are declared using the keyword
subscript
and have the following form:
subscript
(
parameters
) ->
return type
{

get
{

statements
}

set
(
setter name
) {

statements
}
}
Subscript declarations can appear only in the context of a class, structure, enumeration, extension, or protocol
declaration.
The
parameters
specify one or more indexes used to access elements of the corresponding type in a subscript
expression (for example, the
i
in the expression
object[i]
). Although the indexes used to access the
elements can be of any type, each parameter must include a type annotation to specify the type of each index.
The
return type
specifies the type of the element being accessed.
As with computed properties, subscript declarations support reading and writing the value of the accessed
elements. The getter is used to read the value, and the setter is used to write the value. The setter clause is
optional, and when only a getter is needed, you can omit both clauses and simply return the requested value
directly. That said, if you provide a setter clause, you must also provide a getter clause.
The
setter name
and enclosing parentheses are optional. If you provide a setter name, it is used as the name of
the parameter to the setter. If you do not provide a setter name, the default parameter name to the setter is
value
. That type of the
setter name
must be the same as the
return type
.
You can overload a subscript declaration in the type in which it is declared, as long as the
parameters
or the
return type
differ from the one you’re overloading. You can also override a subscript declaration inherited from a
superclass. When you do so, you must mark the overridden subscript declaration with the
override
keyword.
You can also declare subscripts in the context of a protocol declaration, as described in
Protocol Subscript
Declaration
.
For more information about subscripting and to see examples of subscript declarations, see
Subscripts
.
GRAMMAR OF A SUBSCRIPT DECLARATION
​

subscript-declaration
→

subscript-head
subscript-result
code-block
​

subscript-declaration
→

subscript-head
subscript-result
getter-setter-block
​

subscript-declaration
→

subscript-head
subscript-result
getter-setter-keyword-block

​

subscript-head
→

attributes
opt
subscript
parameter-clause
​

subscript-result
→

->
attributes
opt
type
​
Operator Declaration
An
operator declaration
introduces a new infix, prefix, or postfix operator into your program and is declared
using the contextual keyword
operator
.
You can declare operators of three different fixities: infix, prefix, and postfix. The
fixity
of an operator specifies
the relative position of an operator to its operands.
There are three basic forms of an operator declaration, one for each fixity. The fixity of the operator is specified
by including the contextual keyword
infix
,
prefix
, or
postfix
between
operator
and the name of the
operator. In each form, the name of the operator can contain only the operator characters defined in
Operators
.
The following form declares a new infix operator:
operator

infix

operator name
{

precedence

precedence level

associativity

associativity
}
An
infix operator
is a binary operator that is written between its two operands, such as the familiar addition
operator (
+
) in the expression
1 + 2
.
Infix operators can optionally specify a precedence, associativity, or both.
The
precedence
of an operator specifies how tightly an operator binds to its operands in the absence of grouping
parentheses. You specify the precedence of an operator by writing the contextual keyword
precedence
followed by the
precedence level
. The
precedence level
can be any whole number (decimal integer) from 0 to
255; unlike decimal integer literals, it can’t contain any underscore characters. Although the precedence level is
a specific number, it is significant only relative to another operator. That is, when two operators compete with
each other for their operands, such as in the expression
2 + 3 * 5
, the operator with the higher precedence

level binds more tightly to its operands.
The
associativity
of an operator specifies how a sequence of operators with the same precedence level are
grouped together in the absence of grouping parentheses. You specify the associativity of an operator by writing
the contextual keyword
associativity
followed by the
associativity
, which is one of the contextual
keywords
left
,
right
, or
none
. Operators that are left-associative group left-to-right. For example, the
subtraction operator (
–
) is left-associative, and therefore the expression
4 – 5 – 6
is grouped as
(4 – 5)
– 6
and evaluates to
-7
. Operators that are right-associative group right-to-left, and operators that are
specified with an associativity of
none
don’t associate at all. Nonassociative operators of the same precedence
level can’t appear adjacent to each to other. For example,
1 < 2 < 3
is not a valid expression.
Infix operators that are declared without specifying a precedence or associativity are initialized with a
precedence level of 100 and an associativity of
none
.
The following form declares a new prefix operator:
operator

prefix

operator name
{}
A
prefix operator
is a unary operator that is written immediately before its operand, such as the prefix increment
operator (
++
) is in the expression
++i
.
Prefix operators declarations don’t specify a precedence level. Prefix operators are nonassociative.
The following form declares a new postfix operator:
operator

postfix

operator name
{}
A
postfix operator
is a unary operator that is written immediately after its operand, such as the postfix increment
operator (
++
) is in the expression
i++
.
As with prefix operators, postfix operator declarations don’t specify a precedence level. Postfix operators are
nonassociative.
After declaring a new operator, you implement it by declaring a function that has the same name as the

operator. To see an example of how to create and implement a new operator, see
Custom Operators
.
GRAMMAR OF AN OPERATOR DECLARATION
​

operator-declaration
→

prefix-operator-declaration

postfix-operator-declaration

infix-operator-
declaration
​

prefix-operator-declaration
→

operator
prefix
operator
{
}
​

postfix-operator-declaration
→

operator
postfix
operator
{
}
​

infix-operator-declaration
→

operator
infix
operator
{
infix-operator-attributes
opt
}
​

infix-operator-attributes
→

precedence-clause
opt
associativity-clause
opt
​

precedence-clause
→

precedence
precedence-level
​

precedence-level
→

Digit 0 through 255
​

associativity-clause
→

associativity
associativity
​

associativity
→

left

right

none

​
​
Attributes
Attributes
provide more information about a declaration or type. There are two kinds of attributes in Swift, those
that apply to declarations and those that apply to types. For instance, the
required
attribute—when applied to
a designated or convenience initializer declaration of a class—indicates that every subclass must implement
that initializer. And the
noreturn
attribute—when applied to a function or method type—indicates that the
function or method doesn’t return to its caller.
You specify an attribute by writing the
@
symbol followed by the attribute’s name and any arguments that the
attribute accepts:
@
attribute name
@
attribute name
(
attribute arguments
)
Some declaration attributes accept arguments that specify more information about the attribute and how it
applies to a particular declaration. These
attribute arguments
are enclosed in parentheses, and their format is
defined by the attribute they belong to.
​
Declaration Attributes
You can apply a declaration attribute to declarations only. However, you can also apply the
noreturn
attribute
to a function or method
type
.
Apply this attribute to functions that overload a compound assignment operator. Functions that
overload a compound assignment operator must mark their initial input parameter as
inout
. For
an example of how to use the
assignment
attribute, see
Compound Assignment Operators
.
Apply this attribute to a protocol to indicate that the protocol can be adopted by class types only.
assignment
class_protocol

If you apply the
objc
attribute to a protocol, the
class_protocol
attribute is implicitly applied
to that protocol; there’s no need to mark the protocol with the
class_protocol
attribute
explicitly.
Apply this attribute to an import declaration to export the imported module, submodule, or
declaration from the current module. If another module imports the current module, that other
module can access the items exported by the current module.
Apply this attribute to a class or to a property, method, or subscript member of a class. It’s applied
to a class to indicate that the class can’t be subclassed. It’s applied to a property, method, or
subscript of a class to indicate that that class member can’t be overridden in any subclass.
Apply this attribute to a stored variable property of a class or structure to indicate that the property’s
initial value is calculated and stored at most once, when the property is first accessed. For an
example of how to use the
lazy
attribute, see
Lazy Stored Properties
.
Apply this attribute to a function or method declaration to indicate that the corresponding type of that
function or method,
T
, is
@noreturn T
. You can mark a function or method type with this
attribute to indicate that the function or method doesn’t return to its caller.
You can override a function or method that is not marked with the
noreturn
attribute with a
function or method that is. That said, you can’t override a function or method that is marked with the
noreturn
attribute with a function or method that is not. Similar rules apply when you implement
a protocol method in a conforming type.
Apply this attribute to a stored variable property of a class. This attribute causes the property’s
setter to be synthesized with a
copy
of the property’s value—returned by the
copyWithZone
method—instead of the value of the property itself. The type of the property must conform to the
NSCopying
protocol.
The
NSCopying
attribute behaves in a way similar to the Objective-C
copy
property attribute.
exported
final
lazy
noreturn
NSCopying
NSManaged

Apply this attribute to a stored variable property of a class that inherits from
NSManagedObject
to indicate that the storage and implementation of the property are provided dynamically by Core
Data at runtime based on the associated entity description.
Apply this attribute to any declaration that can be represented in Objective-C—for example, non-
nested classes, protocols, properties and methods (including getters and setters) of classes and
protocols, initializers, deinitializers, and subscripts. The
objc
attribute tells the compiler that a
declaration is available to use in Objective-C code.
If you apply the
objc
attribute to a class or protocol, it’s implicitly applied to the members of that
class or protocol. The compiler also implicitly adds the
objc
attribute to a class that inherits from
another class marked with the
objc
attribute. Protocols marked with the
objc
attribute can’t
inherit from protocols that aren’t.
The
objc
attribute optionally accepts a single attribute argument, which consists of an identifier.
Use this attribute when you want to expose a different name to Objective-C for the entity the
objc
attribute applies to. You can use this argument to name classes, protocols, methods, getters,
setters, and initializers. The example below exposes the getter for the
enabled
property of the
ExampleClass
to Objective-C code as
isEnabled
rather than just as the name of the property
itself.
1
@objc
2
class

ExampleClass
{
3

var

enabled
:
Bool
{
4

@objc
(
isEnabled
)
get
{
5

// Return the appropriate value
6
}
7
}
8
}
Apply this attribute to a protocol’s property, method, or subscript members to indicate that a
conforming type isn’t required to implement those members.
You can apply the
optional
attribute only to protocols that are marked with the
objc
attribute.
NSManaged
objc
optional

As a result, only class types can adopt and conform to a protocol that contains optional member
requirements. For more information about how to use the
optional
attribute and for guidance
about how to access optional protocol members—for example, when you’re not sure whether a
conforming type implements them—see
Optional Protocol Requirements
.
Apply this attribute to a designated or convenience initializer of a class to indicate that every
subclass must implement that initializer.
Required designated initializers must be implemented explicitly. Required convenience initializers
can be either implemented explicitly or inherited when the subclass directly implements all of the
superclass’s designated initializers (or when the subclass overrides the designated initializers with
convenience initializers).
​
Declaration Attributes Used by Interface Builder
Interface Builder attributes are declaration attributes used by Interface Builder to synchronize with Xcode. Swift
provides the following Interface Builder attributes:
IBAction
,
IBDesignable
,
IBInspectable
, and
IBOutlet
. These attributes are conceptually the same as their Objective-C counterparts.
You apply the
IBOutlet
and
IBInspectable
attributes to property declarations of a class. You apply the
IBAction
attribute to method declarations of a class and the
IBDesignable
attribute to class declarations.
​
Type Attributes
You can apply type attributes to types only. However, you can also apply the
noreturn
attribute to a function
or method
declaration
.
This attribute is used to delay the evaluation of an expression by automatically wrapping that
expression in a closure with no arguments. Apply this attribute to a function or method type that
takes no arguments and that returns the type of the expression. For an example of how to use the
auto_closure
attribute, see
Function Type
.
required
auto_closure

Apply this attribute to the type of a function or method to indicate that the function or method doesn’t
return to its caller. You can also mark a function or method declaration with this attribute to indicate
that the corresponding type of that function or method,
T
, is
@noreturn T
.
GRAMMAR OF AN ATTRIBUTE
​

attribute
→

@
attribute-name
attribute-argument-clause
opt
​

attribute-name
→

identifier
​

attribute-argument-clause
→

(
balanced-tokens
opt
)
​

attributes
→

attribute
attributes
opt
​

balanced-tokens
→

balanced-token
balanced-tokens
opt
​

balanced-token
→

(
balanced-tokens
opt
)
​

balanced-token
→

[
balanced-tokens
opt
]
​

balanced-token
→

{
balanced-tokens
opt
}
​

balanced-token
→

Any identifier, keyword, literal, or operator
​

balanced-token
→

Any punctuation except
(
,
)
,
[
,
]
,
{
, or
}
noreturn

​
​
Patterns
A
pattern
represents the structure of a single value or a composite value. For example, the structure of a tuple
(1, 2)
is a comma-separated list of two elements. Because patterns represent the structure of a value rather
than any one particular value, you can match them with a variety of values. For instance, the pattern
(x, y)
matches the tuple
(1, 2)
and any other two-element tuple. In addition matching a pattern with a value, you
can extract part or all of a composite value and bind each part to a constant or variable name.
In Swift, patterns occur in variable and constant declarations (on their left-hand side), in
for
–
in
statements,
and in
switch
statements (in their case labels). Although any pattern can occur in the case labels of a
switch
statement, in the other contexts, only wildcard patterns, identifier patterns, and patterns containing
those two patterns can occur.
You can specify a type annotation for a wildcard pattern, an identifier pattern, and a tuple pattern to constraint the
pattern to match only values of a certain type.
GRAMMAR OF A PATTERN
​

pattern
→

wildcard-pattern
type-annotation
opt
​

pattern
→

identifier-pattern
type-annotation
opt
​

pattern
→

value-binding-pattern
​

pattern
→

tuple-pattern
type-annotation
opt
​

pattern
→

enum-case-pattern
​

pattern
→

type-casting-pattern
​

pattern
→

expression-pattern
​
Wildcard Pattern
A
wildcard pattern
matches and ignores any value and consists of an underscore (
_
). Use a wildcard pattern
when you don’t care about the values being matched against. For example, the following code iterates through
the closed range
1..3
, ignoring the current value of the range on each iteration of the loop:
1
for

_

in

1
…
3
{
2

// Do something three times.

3
}
GRAMMAR OF A WILDCARD PATTERN
​

wildcard-pattern
→

_
​
Identifier Pattern
An
identifier pattern
matches any value and binds the matched value to a variable or constant name. For
example, in the following constant declaration,
someValue
is an identifier pattern that matches the value
42
of
type
Int
:
1
let

someValue
=
42
When the match succeeds, the value
42
is bound (assigned) to the constant name
someValue
.
When the pattern on the left-hand side of a variable or constant declaration is an identifier pattern, the identifier
pattern is implicitly a subpattern of a value-binding pattern.
GRAMMAR OF AN IDENTIFIER PATTERN
​

identifier-pattern
→

identifier
​
Value-Binding Pattern
A
value-binding pattern
binds matched values to variable or constant names. Value-binding patterns that bind a
matched value to the name of a constant begin with the keyword
let
; those that bind to the name of variable
begin with the keyword
var
.
Identifiers patterns within a value-binding pattern bind new named variables or constants to their matching
values. For example, you can decompose the elements of a tuple and bind the value of each element to a
corresponding identifier pattern.

1
let

point
= (
3
,
2
)
2
switch

point
{
3

// Bind x and y to the elements of point.
4
case

let
(
x
,
y
):
5

println
(
"The point is at (
\(
x
)
,
\(
y
)
)."
)
6
}
7
// prints "The point is at (3, 2)."
In the example above,
let
distributes to each identifier pattern in the tuple pattern
(x, y)
. Because of this
behavior, the
switch
cases
case let (x, y):
and
case (let x, let y):
match the same
values.
GRAMMAR OF A VALUE-BINDING PATTERN
​

value-binding-pattern
→

var
pattern

let
pattern
​
Tuple Pattern
A
tuple pattern
is a comma-separated list of zero or more patterns, enclosed in parentheses. Tuple patterns
match values of corresponding tuple types.
You can constrain a tuple pattern to match certain kinds of tuple types by using type annotations. For example,
the tuple pattern
(x, y): (Int, Int)
in the constant declaration
let (x, y): (Int, Int) =
(1, 2)
matches only tuple types in which both elements are of type
Int
. To constrain only some elements of
a tuple pattern, provide type annotations directly to those individual elements. For example, the tuple pattern in
let (x: String, y)
matches any two-element tuple type, as long as the first element is of type
String
.
When a tuple pattern is used as the pattern in a
for
–
in
statement or in a variable or constant declaration, it can
contain only wildcard patterns, identifier patterns, or other tuple patterns that contain those. For example, the
following code isn’t valid because the element
0
in the tuple pattern
(x, 0)
is an expression pattern:
1
let

points
= [(
0
,
0
), (
1
,
0
), (
1
,
1
), (
2
,
0
), (
2
,
1
)]
2
// This code isn't valid.
3
for
(
x
,
0
)
in

points
{

4

/* … */
5
}
The parentheses around a tuple pattern that contains a single element have no effect. The pattern matches
values of that single element’s type. For example, the following are equivalent:
1
let

a
=
2

// a: Int = 2
2
let
(
a
) =
2

// a: Int = 2
3
let
(
a
):
Int
=
2

// a: Int = 2
GRAMMAR OF A TUPLE PATTERN
​

tuple-pattern
→

(
tuple-pattern-element-list
opt
)
​

tuple-pattern-element-list
→

tuple-pattern-element

tuple-pattern-element
,
tuple-pattern-
element-list
​

tuple-pattern-element
→

pattern
​
Enumeration Case Pattern
An
enumeration case pattern
matches a case of an existing enumeration type. Enumeration case patterns
appear only in
switch
statement case labels.
If the enumeration case you’re trying to match has any associated values, the corresponding enumeration case
pattern must specify a tuple pattern that contains one element for each associated value. For an example that
uses a
switch
statement to match enumeration cases containing associated values, see
Associated Values
.
GRAMMAR OF AN ENUMERATION CASE PATTERN
​

enum-case-pattern
→

type-identifier
opt
.
enum-case-name
tuple-pattern
opt
​
Type-Casting Patterns
There are two type-casting patterns, the
is
pattern and the
as
pattern. Both type-casting patterns appear only in
switch
statement case labels. The
is
and
as
patterns have the following form:

is

type
pattern

as

type
The
is
pattern matches a value if the type of that value at runtime is the same as the type specified in the right-
hand side of the
is
pattern—or a subclass of that type. The
is
pattern behaves like the
is
operator in that they
both perform a type cast but discard the returned type.
The
as
pattern matches a value if the type of that value at runtime is the same as the type specified in the right-
hand side of the
as
pattern—or a subclass of that type. If the match succeeds, the type of the matched value is
cast to the
pattern
specified in the left-hand side of the
as
pattern.
For an example that uses a
switch
statement to match values with
is
and
as
patterns, see
Type Casting for
Any and AnyObject
.
GRAMMAR OF A TYPE CASTING PATTERN
​

type-casting-pattern
→

is-pattern

as-pattern
​

is-pattern
→

is
type
​

as-pattern
→

pattern
as
type
​
Expression Pattern
An
expression pattern
represents the value of an expression. Expression patterns appear only in
switch
statement case labels.
The expression represented by the expression pattern is compared with the value of an input expression using
the Swift standard library
~=
operator. The matches succeeds if the
~=
operator returns
true
. By default, the
~=
operator compares two values of the same type using the
==
operator. It can also match an integer value
with a range of integers in an
Range
object, as the following example shows:
1
let

point
= (
1
,
2
)
2
switch

point
{