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The Swift Programming Language

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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.)

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

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:

  • 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.

The example below defines two variables, movieCount and songCount, which count the number of Movie and Song instances in the library array:

  • var movieCount = 0
  • var songCount = 0
  • for item in library {
  • if item is Movie {
  • ++movieCount
  • } else if item is Song {
  • ++songCount
  • }
  • }
  • println("Media library contains \(movieCount) movies and \(songCount) songs")
  • // 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 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.

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 conditional form of the type cast operator (as?) to check the downcast each time through the loop:

  • for item in library {
  • if let movie = item as? Movie {
  • println("Movie: '\(movie.name)', dir. \(movie.director)")
  • } else if let song = item as? Song {
  • println("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

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 be 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 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.

Type Casting for Any and AnyObject

Swift provides two special type aliases for working with non-specific types:

  • AnyObject can represent an instance of any class type.

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

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:

  • let someObjects: [AnyObject] = [
  • Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"),
  • Movie(name: "Moon", director: "Duncan Jones"),
  • Movie(name: "Alien", director: "Ridley Scott")
  • ]

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!):

  • for object in someObjects {
  • let movie = object as! Movie
  • println("Movie: '\(movie.name)', dir. \(movie.director)")
  • }
  • // Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
  • // Movie: 'Moon', dir. Duncan Jones
  • // 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:

  • for movie in someObjects as! [Movie] {
  • println("Movie: '\(movie.name)', dir. \(movie.director)")
  • }
  • // Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
  • // Movie: 'Moon', dir. Duncan Jones
  • // Movie: 'Alien', dir. Ridley Scott

Any

Here’s an example of using Any to work with a mix of different types, including function types and non-class types. The example creates an array called things, which can store values of type Any:

  • var things = [Any]()
  • things.append(0)
  • things.append(0.0)
  • things.append(42)
  • things.append(3.14159)
  • things.append("hello")
  • things.append((3.0, 5.0))
  • things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))
  • things.append({ (name: String) -> String in "Hello, \(name)" })

The things array contains two Int values, two Double values, a String value, a tuple of type (Double, Double), the movie “Ghostbusters”, and a closure expression that takes a String value and returns another String value.

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:

  • for thing in things {
  • switch thing {
  • case 0 as Int:
  • println("zero as an Int")
  • case 0 as Double:
  • println("zero as a Double")
  • case let someInt as Int:
  • println("an integer value of \(someInt)")
  • case let someDouble as Double where someDouble > 0:
  • println("a positive double value of \(someDouble)")
  • case is Double:
  • println("some other double value that I don't want to print")
  • case let someString as String:
  • println("a string value of \"\(someString)\"")
  • case let (x, y) as (Double, Double):
  • println("an (x, y) point at \(x), \(y)")
  • case let movie as Movie:
  • println("a movie called '\(movie.name)', dir. \(movie.director)")
  • case let stringConverter as String -> String:
  • println(stringConverter("Michael"))
  • default:
  • println("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