# Type Casting

> Version: *Swift 5.6*\
> Source: [*swift-book: Type Casting*](https://docs.swift.org/swift-book/LanguageGuide/TypeCasting.html)\
> Digest Date: *February 28, 2022*

* [Type Casting](#type-casting)
  * [Overview](#overview)
  * [Defining a Class Hierarchy for Type Casting](#defining-a-class-hierarchy-for-type-casting)
  * [Checking Type](#checking-type)
  * [Downcasting](#downcasting)
  * [Type Casting for Any and AnyObject](#type-casting-for-any-and-anyobject)

## Overview

*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.

You can also use type casting to check whether a type conforms to a protocol, as described in [Checking for Protocol Conformance](https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID283).

## 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’s assumed that all media items, including all movies and songs, will have a name.)

```swift
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:

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:

```swift
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’s not.

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

```swift
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"
```

## 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 aren’t 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.

```swift
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.

Use `Any` and `AnyObject` only when you explicitly need the behavior and capabilities they provide. It’s always better to be specific about the types you expect to work with in your code.

Here’s an example of using `Any` to work with a mix of different types, including function types and nonclass types.

The example creates an array called `things`, which can store values of type `Any`:

```swift
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.

To discover the specific type of a constant or variable that’s known only to be of type `Any` or `AnyObject`, you can use an `is` or `as` pattern in a `switch` statement’s cases.

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:

```swift
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(stringConverter("Michael"))
    default:
        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
```

**NOTE**:

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.

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