Guides and Sample Code


The Swift Programming Language (Swift 4.1)

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Classes and Structures

Classes and structures are general-purpose, flexible constructs that become the building blocks of your program’s code. You define properties and methods to add functionality to your classes and structures by using exactly the same syntax as for constants, variables, and functions.

Unlike other programming languages, Swift does not require you to create separate interface and implementation files for custom classes and structures. In Swift, you define a class or a structure in a single file, and the external interface to that class or structure is automatically made available for other code to use.

Comparing Classes and Structures

Classes and structures in Swift have many things in common. Both can:

  • Define properties to store values

  • Define methods to provide functionality

  • Define subscripts to provide access to their values using subscript syntax

  • Define initializers to set up their initial state

  • Be extended to expand their functionality beyond a default implementation

  • Conform to protocols to provide standard functionality of a certain kind

For more information, see Properties, Methods, Subscripts, Initialization, Extensions, and Protocols.

Classes have additional capabilities that structures do not:

  • Inheritance enables one class to inherit the characteristics of another.

  • Type casting enables you to check and interpret the type of a class instance at runtime.

  • Deinitializers enable an instance of a class to free up any resources it has assigned.

  • Reference counting allows more than one reference to a class instance.

For more information, see Inheritance, Type Casting, Deinitialization, and Automatic Reference Counting.

Definition Syntax

Classes and structures have a similar definition syntax. You introduce classes with the class keyword and structures with the struct keyword. Both place their entire definition within a pair of braces:

  1. class SomeClass {
  2. // class definition goes here
  3. }
  4. struct SomeStructure {
  5. // structure definition goes here
  6. }

Here’s an example of a structure definition and a class definition:

  1. struct Resolution {
  2. var width = 0
  3. var height = 0
  4. }
  5. class VideoMode {
  6. var resolution = Resolution()
  7. var interlaced = false
  8. var frameRate = 0.0
  9. var name: String?
  10. }

The example above defines a new structure called Resolution, to describe a pixel-based display resolution. This structure has two stored properties called width and height. Stored properties are constants or variables that are bundled up and stored as part of the class or structure. These two properties are inferred to be of type Int by setting them to an initial integer value of 0.

The example above also defines a new class called VideoMode, to describe a specific video mode for video display. This class has four variable stored properties. The first, resolution, is initialized with a new Resolution structure instance, which infers a property type of Resolution. For the other three properties, new VideoMode instances will be initialized with an interlaced setting of false (meaning “noninterlaced video”), a playback frame rate of 0.0, and an optional String value called name. The name property is automatically given a default value of nil, or “no name value”, because it is of an optional type.

Class and Structure Instances

The Resolution structure definition and the VideoMode class definition only describe what a Resolution or VideoMode will look like. They themselves do not describe a specific resolution or video mode. To do that, you need to create an instance of the structure or class.

The syntax for creating instances is very similar for both structures and classes:

  1. let someResolution = Resolution()
  2. let someVideoMode = VideoMode()

Structures and classes both use initializer syntax for new instances. The simplest form of initializer syntax uses the type name of the class or structure followed by empty parentheses, such as Resolution() or VideoMode(). This creates a new instance of the class or structure, with any properties initialized to their default values. Class and structure initialization is described in more detail in Initialization.

Accessing Properties

You can access the properties of an instance using dot syntax. In dot syntax, you write the property name immediately after the instance name, separated by a period (.), without any spaces:

  1. print("The width of someResolution is \(someResolution.width)")
  2. // Prints "The width of someResolution is 0"

In this example, someResolution.width refers to the width property of someResolution, and returns its default initial value of 0.

You can drill down into sub-properties, such as the width property in the resolution property of a VideoMode:

  1. print("The width of someVideoMode is \(someVideoMode.resolution.width)")
  2. // Prints "The width of someVideoMode is 0"

You can also use dot syntax to assign a new value to a variable property:

  1. someVideoMode.resolution.width = 1280
  2. print("The width of someVideoMode is now \(someVideoMode.resolution.width)")
  3. // Prints "The width of someVideoMode is now 1280"

Memberwise Initializers for Structure Types

All structures have an automatically-generated memberwise initializer, which you can use to initialize the member properties of new structure instances. Initial values for the properties of the new instance can be passed to the memberwise initializer by name:

  1. let vga = Resolution(width: 640, height: 480)

Unlike structures, class instances do not receive a default memberwise initializer. Initializers are described in more detail in Initialization.

Structures and Enumerations Are Value Types

A value type is a type whose value is copied when it is assigned to a variable or constant, or when it is passed to a function.

You’ve actually been using value types extensively throughout the previous chapters. In fact, all of the basic types in Swift—integers, floating-point numbers, Booleans, strings, arrays and dictionaries—are value types, and are implemented as structures behind the scenes.

All structures and enumerations are value types in Swift. This means that any structure and enumeration instances you create—and any value types they have as properties—are always copied when they are passed around in your code.

Consider this example, which uses the Resolution structure from the previous example:

  1. let hd = Resolution(width: 1920, height: 1080)
  2. var cinema = hd

This example declares a constant called hd and sets it to a Resolution instance initialized with the width and height of full HD video (1920 pixels wide by 1080 pixels high).

It then declares a variable called cinema and sets it to the current value of hd. Because Resolution is a structure, a copy of the existing instance is made, and this new copy is assigned to cinema. Even though hd and cinema now have the same width and height, they are two completely different instances behind the scenes.

Next, the width property of cinema is amended to be the width of the slightly-wider 2K standard used for digital cinema projection (2048 pixels wide and 1080 pixels high):

  1. cinema.width = 2048

Checking the width property of cinema shows that it has indeed changed to be 2048:

  1. print("cinema is now \(cinema.width) pixels wide")
  2. // Prints "cinema is now 2048 pixels wide"

However, the width property of the original hd instance still has the old value of 1920:

  1. print("hd is still \(hd.width) pixels wide")
  2. // Prints "hd is still 1920 pixels wide"

When cinema was given the current value of hd, the values stored in hd were copied into the new cinema instance. The end result is two completely separate instances, which just happened to contain the same numeric values. Because they are separate instances, setting the width of cinema to 2048 doesn’t affect the width stored in hd.

The same behavior applies to enumerations:

  1. enum CompassPoint {
  2. case north, south, east, west
  3. }
  4. var currentDirection = CompassPoint.west
  5. let rememberedDirection = currentDirection
  6. currentDirection = .east
  7. if rememberedDirection == .west {
  8. print("The remembered direction is still .west")
  9. }
  10. // Prints "The remembered direction is still .west"

When rememberedDirection is assigned the value of currentDirection, it is actually set to a copy of that value. Changing the value of currentDirection thereafter does not affect the copy of the original value that was stored in rememberedDirection.

Classes Are Reference Types

Unlike value types, reference types are not copied when they are assigned to a variable or constant, or when they are passed to a function. Rather than a copy, a reference to the same existing instance is used instead.

Here’s an example, using the VideoMode class defined above:

  1. let tenEighty = VideoMode()
  2. tenEighty.resolution = hd
  3. tenEighty.interlaced = true
  4. = "1080i"
  5. tenEighty.frameRate = 25.0

This example declares a new constant called tenEighty and sets it to refer to a new instance of the VideoMode class. The video mode is assigned a copy of the HD resolution of 1920 by 1080 from before. It is set to be interlaced, and is given a name of "1080i". Finally, it is set to a frame rate of 25.0 frames per second.

Next, tenEighty is assigned to a new constant, called alsoTenEighty, and the frame rate of alsoTenEighty is modified:

  1. let alsoTenEighty = tenEighty
  2. alsoTenEighty.frameRate = 30.0

Because classes are reference types, tenEighty and alsoTenEighty actually both refer to the same VideoMode instance. Effectively, they are just two different names for the same single instance.

Checking the frameRate property of tenEighty shows that it correctly reports the new frame rate of 30.0 from the underlying VideoMode instance:

  1. print("The frameRate property of tenEighty is now \(tenEighty.frameRate)")
  2. // Prints "The frameRate property of tenEighty is now 30.0"

Note that tenEighty and alsoTenEighty are declared as constants, rather than variables. However, you can still change tenEighty.frameRate and alsoTenEighty.frameRate because the values of the tenEighty and alsoTenEighty constants themselves do not actually change. tenEighty and alsoTenEighty themselves do not “store” the VideoMode instance—instead, they both refer to a VideoMode instance behind the scenes. It is the frameRate property of the underlying VideoMode that is changed, not the values of the constant references to that VideoMode.

Identity Operators

Because classes are reference types, it is possible for multiple constants and variables to refer to the same single instance of a class behind the scenes. (The same is not true for structures and enumerations, because they are always copied when they are assigned to a constant or variable, or passed to a function.)

It can sometimes be useful to find out if two constants or variables refer to exactly the same instance of a class. To enable this, Swift provides two identity operators:

  • Identical to (===)

  • Not identical to (!==)

Use these operators to check whether two constants or variables refer to the same single instance:

  1. if tenEighty === alsoTenEighty {
  2. print("tenEighty and alsoTenEighty refer to the same VideoMode instance.")
  3. }
  4. // Prints "tenEighty and alsoTenEighty refer to the same VideoMode instance."

Note that “identical to” (represented by three equals signs, or ===) does not mean the same thing as “equal to” (represented by two equals signs, or ==):

  • “Identical to” means that two constants or variables of class type refer to exactly the same class instance.

  • “Equal to” means that two instances are considered “equal” or “equivalent” in value, for some appropriate meaning of “equal”, as defined by the type’s designer.

When you define your own custom classes and structures, it is your responsibility to decide what qualifies as two instances being “equal”. The process of defining your own implementations of the “equal to” and “not equal to” operators is described in Equivalence Operators.


If you have experience with C, C++, or Objective-C, you may know that these languages use pointers to refer to addresses in memory. A Swift constant or variable that refers to an instance of some reference type is similar to a pointer in C, but is not a direct pointer to an address in memory, and does not require you to write an asterisk (*) to indicate that you are creating a reference. Instead, these references are defined like any other constant or variable in Swift.

Choosing Between Classes and Structures

You can use both classes and structures to define custom data types to use as the building blocks of your program’s code.

However, structure instances are always passed by value, and class instances are always passed by reference. This means that they are suited to different kinds of tasks. As you consider the data constructs and functionality that you need for a project, decide whether each data construct should be defined as a class or as a structure.

As a general guideline, consider creating a structure when one or more of these conditions apply:

  • The structure’s primary purpose is to encapsulate a few relatively simple data values.

  • It is reasonable to expect that the encapsulated values will be copied rather than referenced when you assign or pass around an instance of that structure.

  • Any properties stored by the structure are themselves value types, which would also be expected to be copied rather than referenced.

  • The structure does not need to inherit properties or behavior from another existing type.

Examples of good candidates for structures include:

  • The size of a geometric shape, perhaps encapsulating a width property and a height property, both of type Double.

  • A way to refer to ranges within a series, perhaps encapsulating a start property and a length property, both of type Int.

  • A point in a 3D coordinate system, perhaps encapsulating x, y and z properties, each of type Double.

In all other cases, define a class, and create instances of that class to be managed and passed by reference. In practice, this means that most custom data constructs should be classes, not structures.

Assignment and Copy Behavior for Strings, Arrays, and Dictionaries

In Swift, many basic data types such as String, Array, and Dictionary are implemented as structures. This means that data such as strings, arrays, and dictionaries are copied when they are assigned to a new constant or variable, or when they are passed to a function or method.

This behavior is different from Foundation: NSString, NSArray, and NSDictionary are implemented as classes, not structures. Strings, arrays, and dictionaries in Foundation are always assigned and passed around as a reference to an existing instance, rather than as a copy.