Extensions add new functionality to an existing class, structure, enumeration, or protocol 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 in Swift can:

  • Add computed instance properties and computed type properties (stored property는 추가 할수 없다.)
  • 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

In Swift, you can even extend a protocol to provide implementations of its requirements or add additional functionality that conforming types can take advantage of. For more details, see Protocol Extensions.


Extensions can add new functionality to a type, but they cannot override existing functionality.

Extension Syntax

Declare extensions with the extension keyword:

extension SomeType {
   // new functionality to add to SomeType goes here

An extension can extend an existing type to make it adopt one or more protocols. To add protocol conformance, you write the protocol names the same way as you write them for a class or structure:

extension SomeType: SomeProtocol, AnotherProtocol {
   // implementation of protocol requirements goes here

Adding protocol conformance in this way is described in Adding Protocol Conformance with an Extension.

An extension can be used to extend an existing generic type, as described in Extending a Generic Type. You can also extend a generic type to conditionally add functionality, as described in Extensions with a Generic Where Clause.


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

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:

extension Double {
   var km: Double { return self * 1_000.0 }
   var m: Double { return self }
   var cm: Double { return self / 100.0 }
   var mm: Double { return self / 1_000.0 }
   var ft: Double { return self / 3.28084 }
let oneInch = 25.4.mm
print(“One inch is (oneInch) meters”)
// Prints “One inch is 0.0254 meters”
let threeFeet = 3.ft
print(“Three feet is (threeFeet) meters”)
// Prints “Three feet is 0.914399970739201 meters”

let aMarathon = 42.km + 195.m
print(“A marathon is (aMarathon) meters long”)
// Prints “A marathon is 42195.0 meters long”


Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.


Extensions can add new initializers to existing types. 

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.


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.

struct Size {
   var width = 0.0, height = 0.0
struct Point {
   var x = 0.0, y = 0.0
struct Rect {
   var origin = Point()
   var size = Size()

let defaultRect = Rect()
let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0),
                         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:

extension Rect {
   init(center: Point, size: Size) {
       let originX = center.x – (size.width / 2)
       let originY = center.y – (size.height / 2)
       self.init(origin: Point(x: originX, y: originY), size: size)

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:

let centerRect = Rect(center: Point(x: 4.0, y: 4.0),
                     size: Size(width: 3.0, height: 3.0))
// centerRect’s origin is (2.5, 2.5) and its size is (3.0, 3.0)


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:

extension Int {
   func repetitions(task: () -> Void) {

      // 0과 self사이의 정수를 인덱스로 순환

      // 참고) https://stackoverflow.com/a/26083896

       for _ in 0..<self {

3.repetitions {
// Hello!
// Hello!
// Hello!

Mutating Instance Methods

(참고사항 tumblr #swift #mutating #enum: 

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its 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.

extension Int {
   mutating func square() {
       self = self * self
var someInt = 3
// someInt is now 9


Extensions can add new subscripts to an existing type. 

123456789[0] returns 9

123456789[1] returns 8

extension Int {
   subscript(digitIndex: Int) -> Int {
       var decimalBase = 1
       for _ in 0..<digitIndex {
           decimalBase *= 10
       return (self / decimalBase) % 10
// returns 5
// returns 9
// returns 2
// returns 7

// returns 0, as if you had requested:

Nested Types

Extensions can add new nested types to existing classes, structures, and enumerations:

extension Int {
   enum Kind {
       case negative, zero, positive
   var kind: Kind {
       switch self {
       case 0:
           return .zero
       case let x where x > 0:
           return .positive
           return .negative

func printIntegerKinds(_ numbers: [Int]) {
   for number in numbers {
       switch number.kind {
       case .negative:
           print(“- ”, terminator: “”)
       case .zero:
           print(“0 ”, terminator: “”)
       case .positive:
           print(“+ ”, terminator: “”)
printIntegerKinds([3, 19, -27, 0, -6, 0, 7])
// Prints “+ + – 0 – 0 + ”


number.kind is already known to be of type Int.Kind. Because of this, all of the Int.Kind case values can be written in shorthand form inside the switch statement, such as .negative rather than Int.Kind.negative

Nested Types

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

struct BlackjackCard {
   // nested Suit enumeration
   enum Suit: Character {
       case spades = “♠”, hearts = “♡”, diamonds = “♢”, clubs = “♣”
   // nested Rank enumeration
   enum Rank: Int {
       case two = 2, three, four, five, six, seven, eight, nine, ten
       case jack, queen, king, ace
       struct Values {
           let first: Int, second: Int?
       var values: Values {
           switch self {
           case .ace:
               return Values(first: 1, second: 11)
           case .jack, .queen, .king:
               return Values(first: 10, second: nil)
               return Values(first: self.rawValue, second: nil)
   // BlackjackCard properties and methods
   let rank: Rank, suit: Suit
   var description: String {
       var output = “suit is (suit.rawValue),”
       output += “ value is (rank.values.first)”
       if let second = rank.values.second {
           output += “ or (second)”
       return output

let theAceOfSpades = BlackjackCard(rank: .ace, suit: .spades)
print(“theAceOfSpades: (theAceOfSpades.description)”)
// Prints “theAceOfSpades: suit is ♠, value is 1 or 11”

let heartsSymbol = BlackjackCard.Suit.hearts.rawValue
// heartsSymbol is “♡”

What does “case” mean without switch statement in Swift?

How case works in if-case

Type Casting

Type casting is a way to check the type of an instance, or to treat that instance as 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.

Defining a Class Hierarchy for Type Casting

class MediaItem {
   var name: String
   init(name: String) {
       self.name = name

class Movie: MediaItem {
   var director: String
   init(name: String, director: String) {
       self.director = director
       super.init(name: name)
class Song: MediaItem {
   var artist: String
   init(name: String, artist: String) {
       self.artist = artist
       super.init(name: name)

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:

let library = [
   Movie(name: “Casablanca”, director: “Michael Curtiz”),
   Song(name: “Blue Suede Shoes”, artist: “Elvis Presley”),
   Movie(name: “Citizen Kane”, director: “Orson Welles”),
   Song(name: “The One And Only”, artist: “Chesney Hawkes”),
   Song(name: “Never Gonna Give You Up”, artist: “Rick Astley”)
// 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.

var movieCount = 0
var songCount = 0
for item in library {
   if item is Movie {
       movieCount += 1
   } else if item is Song {
       songCount += 1
print(“Media library contains (movieCount) movies and (songCount) songs”)
// Prints “Media library contains 2 movies and 3 songs”


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 a type cast operator (as? or as!).

Because downcasting can fail, the type cast operator comes in two different forms. The conditional 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 conditional 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.

for item in library {
   if let movie = item as? Movie {
       print(“Movie: (movie.name), dir. (movie.director)”)
   } else if let song = item as? Song {
       print(“Song: (song.name), by (song.artist)”)
// Movie: Casablanca, dir. Michael Curtiz
// Song: Blue Suede Shoes, by Elvis Presley
// Movie: Citizen Kane, dir. Orson Welles
// Song: The One And Only, by Chesney Hawkes
// Song: Never Gonna Give You Up, by Rick Astley

Type Casting for Any and AnyObject

Swift provides two special types for working with nonspecific types:

  • Any can represent an instance of any type at all, including function types.
  • AnyObject can represent an instance of any class type.

var things = [Any]()
things.append((3.0, 5.0))
things.append(Movie(name: “Ghostbusters”, director: “Ivan Reitman”))
things.append({ (name: String) -> String in “Hello, (name)” })

(참고사항 if case https://stackoverflow.com/a/37888514)


Optional Pattern


for thing in things {
   switch thing {
   case 0 as Int:
       print(“zero as an Int”)
   case 0 as Double:
       print(“zero as a Double”)
   case let someInt as Int:
       print(“an integer value of (someInt)”)
   case let someDouble as Double where someDouble > 0:
       print(“a positive double value of (someDouble)”)
   case is Double:
       print(“some other double value that I don’t want to print”)
   case let someString as String:
       print(“a string value of “(someString)””)
   case let (x, y) as (Double, Double):
       print(“an (x, y) point at (x), (y)”)
   case let movie as Movie:
       print(“a movie called (movie.name), dir. (movie.director)”)
   case let stringConverter as (String) -> String:
       print(“something else”)
// zero as an Int
// zero as a Double
// an integer value of 42
// a positive double value of 3.14159
// a string value of “hello”
// an (x, y) point at 3.0, 5.0
// a movie called Ghostbusters, dir. Ivan Reitman
// Hello, Michael


The Any type represents values of any type, including optional types. Swift gives you a warning if you use an optional value where a value of type Any is expected. If you really do need to use an optional value as an Any value, you can use the as operator to explicitly cast the optional to Any, as shown below.

let optionalNumber: Int? = 3
things.append(optionalNumber)        // Warning
things.append(optionalNumber as Any) // No warning

Where “where” may be used?

Error Handling

Representing and Throwing Errors

In Swift, errors are represented by values of types that conform to the Error protocol. This empty protocol indicates that a type can be used for error handling.

Swift enumerations are particularly well suited to modeling a group of related error conditions, with associated values allowing for additional information about the nature of an error to be communicated.

enum VendingMachineError: Error {
   case invalidSelection
   case insufficientFunds(coinsNeeded: Int)
   case outOfStock

 You use a throw statement to throw an error. 

throw VendingMachineError.insufficientFunds(coinsNeeded: 5)