Generic Enumeration

Optional

A type that represents either a wrapped value or nil, the absence of a value.

Overview

You use the Optional type whenever you use optional values, even if you never type the word Optional. Swift’s type system usually shows the wrapped type’s name with a trailing question mark (?) instead of showing the full type name. For example, if a variable has the type Int?, that’s just another way of writing Optional<Int>. The shortened form is preferred for ease of reading and writing code.

The types of shortForm and longForm in the following code sample are the same:

let shortForm: Int? = Int("42")
let longForm: Optional<Int> = Int("42")

The Optional type is an enumeration with two cases. Optional.none is equivalent to the nil literal. Optional.some(Wrapped) stores a wrapped value. For example:

let number: Int? = Optional.some(42)
let noNumber: Int? = Optional.none
print(noNumber == nil)
// Prints "true"

You must unwrap the value of an Optional instance before you can use it in many contexts. Because Swift provides several ways to safely unwrap optional values, you can choose the one that helps you write clear, concise code.

The following examples use this dictionary of image names and file paths:

let imagePaths = ["star": "/glyphs/star.png",
                  "portrait": "/images/content/portrait.jpg",
                  "spacer": "/images/shared/spacer.gif"]

Getting a dictionary’s value using a key returns an optional value, so imagePaths["star"] has type Optional<String> or, written in the preferred manner, String?.

Optional Binding

To conditionally bind the wrapped value of an Optional instance to a new variable, use one of the optional binding control structures, including if let, guard let, and switch.

if let starPath = imagePaths["star"] {
    print("The star image is at '\(starPath)'")
} else {
    print("Couldn't find the star image")
}
// Prints "The star image is at '/glyphs/star.png'"

Optional Chaining

To safely access the properties and methods of a wrapped instance, use the postfix optional chaining operator (postfix ?). The following example uses optional chaining to access the hasSuffix(_:) method on a String? instance.

if let isPNG = imagePaths["star"]?.hasSuffix(".png") {
    print("The star image is in PNG format")
}
// Prints "The star image is in PNG format"

Using the Nil-Coalescing Operator

Use the nil-coalescing operator (??) to supply a default value in case the Optional instance is nil. Here a default path is supplied for an image that is missing from imagePaths.

let defaultImagePath = "/images/default.png"
let heartPath = imagePaths["heart"] ?? defaultImagePath
print(heartPath)
// Prints "/images/default.png"

The ?? operator also works with another Optional instance on the right-hand side. As a result, you can chain multiple ?? operators together.

let shapePath = imagePaths["cir"] ?? imagePaths["squ"] ?? defaultImagePath
print(shapePath)
// Prints "/images/default.png"

Unconditional Unwrapping

When you’re certain that an instance of Optional contains a value, you can unconditionally unwrap the value by using the forced unwrap operator (postfix !). For example, the result of the failable Int initializer is unconditionally unwrapped in the example below.

let number = Int("42")!
print(number)
// Prints "42"

You can also perform unconditional optional chaining by using the postfix ! operator.

let isPNG = imagePaths["star"]!.hasSuffix(".png")
print(isPNG)
// Prints "true"

Unconditionally unwrapping a nil instance with ! triggers a runtime error.

Topics

Creating an Optional Value

case some(Wrapped)

The presence of a value, stored as Wrapped.

init(Wrapped)

Creates an instance that stores the given value.

Creating a Nil Value

case none

The absence of a value.

init(nilLiteral: ())

Creates an instance initialized with nil.

Transforming an Optional Value

func map<U>((Wrapped) -> U)

Evaluates the given closure when this Optional instance is not nil, passing the unwrapped value as a parameter.

func flatMap<U>((Wrapped) -> U?)

Evaluates the given closure when this Optional instance is not nil, passing the unwrapped value as a parameter.

Inspecting an Optional

var unsafelyUnwrapped: Wrapped

The wrapped value of this instance, unwrapped without checking whether the instance is nil.

var debugDescription: String

A textual representation of this instance, suitable for debugging.

Coalescing Nil Values

func ??<T>(T?, () -> T)

Performs a nil-coalescing operation, returning the wrapped value of an Optional instance or a default value.

func ??<T>(T?, () -> T?)

Performs a nil-coalescing operation, returning the wrapped value of an Optional instance or a default Optional value.

Comparing Optional Values

func ==<T>(T?, T?)

Returns a Boolean value indicating whether two optional instances are equal.

func !=<T>(T?, T?)

Returns a Boolean value indicating whether two optional instances are not equal.

Deprecated

enum ImplicitlyUnwrappedOptional

An optional type that allows implicit member access.