enum Optional<Wrapped>

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

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 imagePaths["star"]?.hasSuffix(".png") == true {
    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
// 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
// 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")!
// Prints "42"

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

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

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

Inheritance CustomDebugStringConvertible, CustomReflectable, Decodable, Encodable, Equatable, ExpressibleByNilLiteral, Hashable View Protocol Hierarchy →
Import import Swift


case none

The absence of a value.

In code, the absence of a value is typically written using the nil literal rather than the explicit .none enumeration case.


case some(Wrapped)

The presence of a value, stored as Wrapped.




Creates an instance that stores the given value.


init(_ some: Wrapped)

Creates a new instance by decoding from the given decoder.

This initializer throws an error if reading from the decoder fails, or if the data read is corrupted or otherwise invalid.

decoder: The decoder to read data from.


init(from decoder: Decoder)

Creates an instance initialized with nil.

Do not call this initializer directly. It is used by the compiler when you initialize an Optional instance with a nil literal. For example:

var i: Index? = nil

In this example, the assignment to the i variable calls this initializer behind the scenes.


init(nilLiteral: ())

Instance Variables

var customMirror: Mirror

The custom mirror for this instance.

If this type has value semantics, the mirror should be unaffected by subsequent mutations of the instance.


var customMirror: Mirror { get }
var debugDescription: String

A textual representation of this instance, suitable for debugging.


var debugDescription: String { get }
var hashValue: Int

The hash value for the optional instance.

Two optionals that are equal will always have equal hash values.

Hash values are not guaranteed to be equal across different executions of your program. Do not save hash values to use during a future execution.


var hashValue: Int { get }
var unsafelyUnwrapped: Wrapped

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

The unsafelyUnwrapped property provides the same value as the forced unwrap operator (postfix !). However, in optimized builds (-O), no check is performed to ensure that the current instance actually has a value. Accessing this property in the case of a nil value is a serious programming error and could lead to undefined behavior or a runtime error.

In debug builds (-Onone), the unsafelyUnwrapped property has the same behavior as using the postfix ! operator and triggers a runtime error if the instance is nil.

The unsafelyUnwrapped property is recommended over calling the unsafeBitCast(_:) function because the property is more restrictive and because accessing the property still performs checking in debug builds.

Warning: This property trades safety for performance. Use unsafelyUnwrapped only when you are confident that this instance will never be equal to nil and only after you've tried using the postfix ! operator.


var unsafelyUnwrapped: Wrapped { get }

Instance Methods

func encode(to:)

Encodes this optional value into the given encoder.

This function throws an error if any values are invalid for the given encoder's format.

encoder: The encoder to write data to.


func encode(to encoder: Encoder) throws
func flatMap(_:)

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

Use the flatMap method with a closure that returns an optional value. This example performs an arithmetic operation with an optional result on an optional integer.

let possibleNumber: Int? = Int("42")
let nonOverflowingSquare = possibleNumber.flatMap { x -> Int? in
    let (result, overflowed) = x.multipliedReportingOverflow(by: x)
    return overflowed ? nil : result
// Prints "Optional(1764)"

transform: A closure that takes the unwrapped value of the instance.
Returns: The result of the given closure. If this instance is nil, returns nil.


func flatMap<U>(_ transform: (Wrapped) throws -> U?) rethrows -> U?
func hash(into:)

Hashes the essential components of this value by feeding them into the given hasher.

hasher: The hasher to use when combining the components of this instance.


func hash(into hasher: inout Hasher)
func map(_:)

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

Use the map method with a closure that returns a nonoptional value. This example performs an arithmetic operation on an optional integer.

let possibleNumber: Int? = Int("42")
let possibleSquare = possibleNumber.map { $0 * $0 }
// Prints "Optional(1764)"

let noNumber: Int? = nil
let noSquare = noNumber.map { $0 * $0 }
// Prints "nil"

transform: A closure that takes the unwrapped value of the instance. Returns: The result of the given closure. If this instance is nil, returns nil.


func map<U>(_ transform: (Wrapped) throws -> U) rethrows -> U?