Swift Chapter 5 2018-12-19T13:08:16+00:00


Topics:(Enumerations, Raw Values, Classes and Structures, Choosing Between Classes and Structures,  Properties,Methods, Subscripts)


An enumeration defines a common type for a group of related values and enables you to work with those values in a type-safe way within your code. Enumerations in Swift are much more flexible, and do not have to provide a value for each case of the enumeration. If a value (known as a “raw” value) is provided for each enumeration case, the value can be a string, a character, or a value of any integer or floating-point type.
Alternatively, enumeration cases can specify associated values of any type to be stored along with each different case value, much as unions or variants do in other languages. You can define a common set of related cases as part of one enumeration, each of which has a different set of values of appropriate types associated with it.
Enumerations in Swift are first-class types in their own right. They adopt many features traditionally supported only by classes, such as computed properties to provide additional information about the enumeration’s current value, and instance methods to provide functionality related to the values the enumeration represents. Enumerations can also define initializers to provide an initial case value; can be extended to expand their functionality beyond their original implementation; and can conform to protocols to provide standard functionality.

‌Enumeration Syntax
You introduce enumerations with the enum keyword and place their entire definition within a pair of braces:
enum SomeEnumeration {
// enumeration definition goes here

Here’s an example for the four main points of a compass:
enum CompassPoint {
case north
case south
case east
case west

The values defined in an enumeration (such as north, south, east, and west) are its enumeration cases. You use the case keyword to introduce new enumeration cases.Swift enumeration cases are not assigned a default integer value when they are created. In the CompassPoint example above, north, south, east and west do not implicitly equal 0, 1, 2 and 3. Instead, the different enumeration cases are fully-fledged values in their own right, with an explicitly defined type of CompassPoint.

Multiple cases can appear on a single line, separated by commas:

enum Planet {
case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune
Each enumeration definition defines a brand new type. Like other types in Swift, their names (such as CompassPoint and Planet) should start with a capital letter. Give enumeration types singular rather than plural names, so that they read as self-evident:

var directionToHead = CompassPoint.west
The type of directionToHead is inferred when it is initialized with one of the possible values of CompassPoint. Once directionToHead is declared as a CompassPoint, you can set it to a different CompassPoint value using a shorter dot syntax:

directionToHead = .east
The type of directionToHead is already known, and so you can drop the type when setting its value. This makes for highly readable code when working with explicitly typed enumeration values.
‌Matching Enumeration Values with a Switch Statement
You can match individual enumeration values with a switch statement:

directionToHead = .south
switch directionToHead {
case .north:
print(“Lots of planets have a north”)
case .south:
print(“Watch out for penguins”)
case .east:
print(“Where the sun rises”)
case .west:
print(“Where the skies are blue”)
} // Prints “Watch out for penguins”

You can read this code as:
“Consider the value of directionToHead. In the case where it equals .north, print “Lots of planets have a north”. In the case where it equals .south, print “Watch out for penguins”.”  …and so on.Aswitch statement must be exhaustive when considering an enumeration’s cases. If the case for .west is omitted, this code does not compile, because it does not consider the complete list of CompassPoint cases. Requiring exhaustiveness ensures that enumeration cases are not accidentally omitted.
When it is not appropriate to provide a case for every enumeration case, you can provide a default case to cover any cases that are not addressed explicitly:

let somePlanet = Planet.earth
switch somePlanet {
case .earth:
print(“Mostly harmless”)
print(“Not a safe place for humans”)
// Prints “Mostly harmless”

Associated Values

The examples in the previous section show how the cases of an enumeration are a defined (and typed) value in their own right. You can set a constant or variable to Planet.earth, and check for this value later. However, it is sometimes useful to be able to store associated values of other types alongside these case values. This enables you to store additional custom information along with the case value, and permits this information to vary each time you use that case in your code.
You can define Swift enumerations to store associated values of any given type, and the value types can be different for each case of the enumeration if needed. Enumerations similar to these are known as discriminated unions, tagged unions, or variants in other programming languages.
For example, suppose an inventory tracking system needs to track products by two different types of barcode. Some products are labeled with 1D barcodes in UPC format, which uses the numbers 0 to 9. Each barcode has a “number system” digit, followed by five “manufacturer code” digits and five “product code” digits. These are followed by a “check” digit to verify that the code has been scanned correctly:

Other products are labeled with 2D barcodes in QR code format, which can use any ISO 8859-1 character and can encode a string up to 2,953 characters long:

It would be convenient for an inventory tracking system to be able to store UPC barcodes as a tuple of four integers, and QR code barcodes as a string of any length.
In Swift, an enumeration to define product barcodes of either type might look like this:

enum Barcode {
case upc(Int, Int, Int, Int)
case qrCode(String)

This can be read as:
“Define an enumeration type called Barcode, which can take either a value of upc with an associated value of type (Int, Int, Int, Int), or a value of qrCode with an associated value of type String.”
This definition does not provide any actual Int or String values—it just defines the type of associated values that Barcode constants and variables can store when they are equal to Barcode.upc or Barcode.qrCode.
New barcodes can then be created using either type:

var productBarcode = Barcode.upc(8, 85909, 51226, 3)

This example creates a new variable called productBarcode and assigns it a value of Barcode.upc with an associated tuple value of (8, 85909, 51226, 3).
The same product can be assigned a different type of barcode:

productBarcode = .qrCode(“ABCDEFGHIJKLMNOP”)

At this point, the original Barcode.upc and its integer values are replaced by the new Barcode.qrCode and its string value. Constants and variables of type Barcode can store either a .upc or a .qrCode (together with their associated values), but they can only store one of them at any given time.
The different barcode types can be checked using a switch statement, as before. This time, however, the associated values can be extracted as part of the switch statement. You extract each associated value as a constant (with the let prefix) or a variable (with the var prefix) for use within the switch case’s body:

switch productBarcode {
case .upc(let numberSystem, let manufacturer, let product, let check):
print(“UPC: \(numberSystem), \(manufacturer), \(product), \(check).”)
case .qrCode(let productCode):
print(“QR code: \(productCode).”)
}            // Prints “QR code: ABCDEFGHIJKLMNOP.”

If all of the associated values for an enumeration case are extracted as constants, or if all are extracted as variables, you can place a single var or let annotation before the case name, for brevity:

switch productBarcode {
case let .upc(numberSystem, manufacturer, product, check):
print(“UPC : \(numberSystem), \(manufacturer), \(product), \(check).”)
case let .qrCode(productCode):
print(“QR code: \(productCode).”)
// Prints “QR code: ABCDEFGHIJKLMNOP.”

Raw Values

The barcode example in Associated Values shows how cases of an enumeration can declare that they store associated values of different types. As an alternative to associated values, enumeration cases can come prepopulated with default values (called raw values), which are all of the same type.
Here’s an example that stores raw ASCII values alongside named enumeration cases:

enum ASCIIControlCharacter: {
case tab = “\t”
case lineFeed = “\n”
case carriageReturn = “\r”

Here, the raw values for an enumeration called ASCIIControlCharacter are defined to be of type Character, and are set to some of the more common ASCII control characters. Character values are described in Strings and Characters.
Raw values can be strings, characters, or any of the integer or floating-point number types. Each raw value must be unique within its enumeration declaration.Raw values are not the same as associated values. Raw values are set to prepopulated values when you first define the enumeration in your code, like the three ASCII codes above. The raw value for a particular enumeration case is always the same. Associated values are set when you create a new constant or variable based on one of the enumeration’s cases, and can be different each time you do so.

‌Implicitly Assigned Raw Values

When you’re working with enumerations that store integer or string raw values, you don’t have to explicitly assign a raw value for each case. When you don’t, Swift will automatically assign the values for you.For instance, when integers are used for raw values, the implicit value for each case is one more than the previous case. If the first case doesn’t have a value set, its value is 0.
The enumeration below is a refinement of the earlier Planet enumeration, with integer raw values to represent each planet’s order from the sun:

enum Planet: {
case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune

In the example above, Planet.mercury has an explicit raw value of 1, Planet.venus has an implicit raw value of 2, and so on.When strings are used for raw values, the implicit value for each case is the text of that case’s name.
The enumeration below is a refinement of the earlier CompassPoint enumeration, with string raw values to represent each direction’s name:

enum CompassPoint: {
case north, south, east, west
In the example above, CompassPoint.south has an implicit raw value of “south”, and so on.
You access the raw value of an enumeration case with its rawValue property:

let earthsOrder = Planet.earth.rawValue
// earthsOrder is 3

let sunsetDirection = CompassPoint.west.rawValue
// sunsetDirection is “west”

Initializing from a Raw Value

If you define an enumeration with a raw-value type, the enumeration automatically receives an initializer that takes a value of the raw value’s type (as a parameter called rawValue) and returns either an enumeration case or nil. You can use this initializer to try to create a new instance of the enumeration.
This example identifies Uranus from its raw value of 7:

let possiblePlanet = Planet(rawValue: 7)
// possiblePlanet is of type Planet? and equals Planet.uranus

Not all possible Int values will find a matching planet, however. Because of this, the raw value initializer always returns an optional enumeration case. In the example above, possiblePlanet is of type Planet?, or “optional Planet.”The raw value initializer is a failable initializer, because not every raw value will return an enumeration case.

If you try to find a planet with a position of 11, the optional Planet value returned by the raw value initializer will be nil:

let positionToFind = 11
if let somePlanet = Planet(rawValue: positionToFind) {
switch somePlanet {
case .earth:
print(“Mostly harmless”)
print(“Not a safe place for humans”)
} else {
print(“There isn’t a planet at position \(positionToFind)”)
} // Prints “There isn’t a planet at position 11”

This example uses optional binding to try to access a planet with a raw value of 11. The statement if let somePlanet = Planet(rawValue: 11) creates an optional Planet, and sets somePlanet to the value of that optional Planet if it can be retrieved. In this case, it is not possible to retrieve a planet with a position of 11, and so the else branch is executed instead.

Recursive Enumerations

A recursive enumeration is an enumeration that has another instance of the enumeration as the associated value for one or more of the enumeration cases. You indicate that an enumeration case is recursive by writing indirect before it, which tells the compiler to insert the necessary layer of indirection.
For example, here is an enumeration that stores simple arithmetic expressions:

enum ArithmeticExpression {
case number(Int)
indirect case addition(ArithmeticExpression, ArithmeticExpression)
indirect case multiplication(ArithmeticExpression, ArithmeticExpression)

You can also write indirect before the beginning of the enumeration to enable indirection for all of the enumeration’s cases that have an associated value:

indirect enum ArithmeticExpression {
case number(Int)
case addition(ArithmeticExpression, ArithmeticExpression)
case multiplication(ArithmeticExpression, ArithmeticExpression)

This enumeration can store three kinds of arithmetic expressions: a plain number, the addition of two expressions, and the multiplication of two expressions. The addition and multiplication cases have associated values that are also arithmetic expressions—these associated values make it possible to nest expressions. For example, the expression (5 + 4) * 2 has a number on the right-hand side of the multiplication and another expression on the left-hand side of the multiplication. Because the data is nested, the enumeration used to store the data also needs to support nesting—this means the enumeration needs to be recursive. The code below shows the ArithmeticExpression recursive enumeration being created for (5 + 4) * 2:

switch expression {
case let .number(value):
return value
case let .addition(left, right):
return evaluate(left) + evaluate(right)
case let .multiplication(left, right):
return evaluate(left) * evaluate(right)

print(evaluate(product))           // Prints “18”

This function evaluates a plain number by simply returning the associated value. It evaluates an addition or multiplication by evaluating the expression on the left-hand side, evaluating the expression on the right-hand side, and then adding them or multiplying them.

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.

An instance of a class is traditionally known as an object. However, Swift classes and structures are much closer in functionality than in other languages, and much of this chapter describes functionality that can apply to instances of either a class or a structure type. Because of this, the more general term instance is used.

‌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 initialisers 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, Initialisation, 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, Deinitialisation, and Automatic Reference Counting.

Structures are always copied when they are passed around in your code, and do not use 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:

class SomeClass {
// class definition goes here
struct SomeStructure {
// structure definition goes here

Whenever you define a new class or structure, you effectively define a brand new Swift type. Give types UpperCamelCase names (such as SomeClass and SomeStructure here) to match the capitalization of standard Swift types (such as String, Int, and Bool). Conversely, always give properties and methods lowerCamelCase names (such as frameRate and incrementCount) to differentiate them from type names.

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

struct Resolution {
var width = 0
var height = 0
class VideoMode {
var resolution = Resolution()
var interlaced = false
var frameRate = 0.0
var name: ?

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:

let someResolution = Resolution()
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:

print(“The width of someResolution is \(someResolution.width)”)
// 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:

print(“The width of someVideoMode is \(someVideoMode.resolution.width)”)
// Prints “The width of someVideoMode is 0”

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

someVideoMode.resolution.width = 1280
print(“The width of someVideoMode is now \(someVideoMode.resolution.width)”)
// Prints “The width of someVideoMode is now 1280”

Swift enables you to set sub-properties of a structure property directly. In the last example above, the width property of the resolution property of someVideoMode is set directly, without your needing to set the entire resolution property to a new value.

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:

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. 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:

let hd = Resolution(width: 1920, height: 1080)
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):

cinema.width = 2048

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

print(“cinema is now \(cinema.width) pixels wide”)
// Prints “cinema is now 2048 pixels wide”

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

print(“hd is still \(hd.width) pixels wide”)
// 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:

enum CompassPoint {
case north, south, east, west
var currentDirection = CompassPoint.west
let rememberedDirection = currentDirection
currentDirection = .east
if rememberedDirection == .west {
print(“The remembered direction is still .west”)
// 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 Type

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:

let tenEighty = VideoMode()
tenEighty.resolution = hd
tenEighty.interlaced = true
tenEighty.name = “1080i”
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:

let alsoTenEighty = tenEighty
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:

print(“The frameRate property of tenEighty is now \(tenEighty.frameRate)”)
// 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:

if tenEighty === alsoTenEighty {
print(“tenEighty and alsoTenEighty refer to the same VideoMode instance.”)
// 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”.


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.

The description above refers to the “copying” of strings, arrays, and dictionaries. The behavior you see in your code will always be as if a copy took place. However, Swift only performs an actual copy behind the scenes when it is absolutely necessary to do so. Swift manages all value copying to ensure optimal performance, and you should not avoid assignment to try to preempt this optimization.


Properties associate values with a particular class, structure, or enumeration. Stored properties store constant and variable values as part of an instance, whereas computed properties calculate (rather than store) a value. Computed properties are provided by classes, structures, and enumerations. Stored properties are provided only by classes and structures.Stored and computed properties are usually associated with instances of a particular type. However, properties can also be associated with the type itself. Such properties are known as type properties.
In addition, you can define property observers to monitor changes in a property’s value, which you can respond to with custom actions. Property observers can be added to stored properties you define yourself, and also to properties that a subclass inherits from its superclass.

Stored Properties

In its simplest form, a stored property is a constant or variable that is stored as part of an instance of a particular class or structure. Stored properties can be either variable stored properties (introduced by the var keyword) or constant stored properties (introduced by the let keyword).
You can provide a default value for a stored property as part of its definition, as described in Default Property Values. You can also set and modify the initial value for a stored property during initialization. This is true even for constant stored properties, as described in Assigning Constant Properties During Initialization.
The example below defines a structure called FixedLengthRange, which describes a range of integers whose range length cannot be changed after it is created:

struct FixedLengthRange {
var firstValue:
let length:
var rangeOfThreeItems = FixedLengthRange(firstValue: 0, length: 3)
// the range represents integer values 0, 1, and 2
rangeOfThreeItems.firstValue = 6
// the range now represents integer values 6, 7, and 8

Instances of FixedLengthRange have a variable stored property called firstValue and a constant stored property called length. In the example above, length is initialized when the new range is created and cannot be changed thereafter, because it is a constant property.

‌Stored Properties of Constant Structure Instances

If you create an instance of a structure and assign that instance to a constant, you cannot modify the instance’s properties, even if they were declared as variable properties:

let rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4)
// this range represents integer values 0, 1, 2, and 3
rangeOfFourItems.firstValue = 6
// this will report an error, even though firstValue is a variable property

Because rangeOfFourItems is declared as a constant (with the let keyword), it is not possible to change its firstValue property, even though firstValue is a variable property.This behavior is due to structures being value types. When an instance of a value type is marked as a constant, so are all of its properties.
The same is not true for classes, which are reference types. If you assign an instance of a reference type to a constant, you can still change that instance’s variable properties.

 Lazy Stored Properties

A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the lazy modifier before its declaration.You must always declare a lazy property as a variable (with the var keyword), because its initial value might not be retrieved until after instance initialization completes. Constant properties must always have a value before initialization completes, and therefore cannot be declared as lazy.

Lazy properties are useful when the initial value for a property is dependent on outside factors whose values are not known until after an instance’s initialization is complete. Lazy properties are also useful when the initial value for a property requires complex or computationally expensive setup that should not be performed unless or until it is needed.The example below uses a lazy stored property to avoid unnecessary initialization of a complex class. This example defines two classes called DataImporter and DataManager, neither of which is shown in full:

class DataImporter {
DataImporter is a class to import data from an external file.
The class is assumed to take a nontrivial amount of time to initialize.
var filename = “data.txt”
// the DataImporter class would provide data importing functionality here

class DataManager {
lazy var importer = DataImporter()
var data = [String]()
// the DataManager class would provide data management functionality here

let manager = DataManager()
manager.data.append(“Some data”)
manager.data.append(“Some more data”)
// the DataImporter instance for the importer property has not yet been created

The DataManager class has a stored property called data, which is initialized with a new, empty array of String values. Although the rest of its functionality is not shown, the purpose of this DataManager class is to manage and provide access to this array of String data.Part of the functionality of the DataManager class is the ability to import data from a file. This functionality is provided by the DataImporter class, which is assumed to take a nontrivial amount of time to initialize. This might be because a DataImporter instance needs to open a file and read its contents into memory when the DataImporter instance is initialized.
It is possible for a DataManager instance to manage its data without ever importing data from a file, so there is no need to create a new DataImporter instance when the DataManager itself is created. Instead, it makes more sense to create the DataImporter instance if and when it is first used.
Because it is marked with the lazy modifier, the DataImporter instance for the importer property is only created when the importer property is first accessed, such as when its filename property is queried:

// the DataImporter instance for the importer property has now been created
// Prints “data.txt”

If a property marked with the lazy modifier is accessed by multiple threads simultaneously and the property has not yet been initialized, there is no guarantee that the property will be initialized only once.

Stored Properties and Instance Variables

A Swift property does not have a corresponding instance variable, and the backing store for a property is not accessed directly. This approach avoids confusion about how the value is accessed in different contexts and simplifies the property’s declaration into a single, definitive statement. All information about the property—including its name, type, and memory management characteristics—is defined in a single location as part of the type’s definition.

Computed Properties

In addition to stored properties, classes, structures, and enumerations can define computed properties, which do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly.

struct Point {
var x = 0.0, y = 0.0
struct Size {
var width = 0.0, height = 0.0
struct Rect {
var origin = Point()
var size = Size()
var center: {
get {
let centerX = origin.x + (size.width / 2)
let centerY = origin.y + (size.height / 2)
return Point(x: centerX, y: centerY)
set(newCenter) {
origin.x = newCenter.x – (size.width / 2)
origin.y = newCenter.y – (size.height / 2)
var square = Rect(origin: Point(x: 0.0, y: 0.0),
size: Size(width: 10.0, height: 10.0))
let initialSquareCenter = square.center
square.center = Point(x: 15.0, y: 15.0)
print(“square.origin is now at (\(square.origin.x), \(square.origin.y))”)
// Prints “square.origin is now at (10.0, 10.0)”

This example defines three structures for working with geometric shapes:
Point encapsulates the x- and y-coordinate of a point.
Size encapsulates a width and a height.
Rect defines a rectangle by an origin point and a size.
The Rect structure also provides a computed property called center. The current center position of a Rect can always be determined from its origin and size, and so you don’t need to store the center point as an explicit Point value. Instead, Rect defines a custom getter and setter for a computed variable called center, to enable you to work with the rectangle’s center as if it were a real stored property.The preceding example creates a new Rect variable called square. The square variable is initialized with an origin point of (0, 0), and a width and height of 10. This square is represented by the blue square in the diagram below.
The square variable’s center property is then accessed through dot syntax (square.center), which causes the getter for center to be called, to retrieve the current property value. Rather than returning an existing value, the getter actually calculates and returns a new Point to represent the center of the square. As can be seen above, the getter correctly returns a center point of (5, 5).
The center property is then set to a new value of (15, 15), which moves the square up and to the right, to the new position shown by the orange square in the diagram below. Setting the center property calls the setter for center, which modifies the x and y values of the stored origin property, and moves the square to its new position.

Shorthand Setter Declaration

If a computed property’s setter does not define a name for the new value to be set, a default name of newValue is used. Here’s an alternative version of the Rect structure, which takes advantage of this shorthand notation:

struct AlternativeRect {
var origin = Point()
var size = Size()
var center: {
get {
let centerX = origin.x + (size.width / 2)
let centerY = origin.y + (size.height / 2)
return Point(x: centerX, y: centerY)
set {
origin.x = newValue.x – (size.width / 2)
origin.y = newValue.y – (size.height / 2)

Read-Only Computed Properties

A computed property with a getter but no setter is known as a read-only computed property. A read-only computed property always returns a value, and can be accessed through dot syntax, but cannot be set to a different value.You must declare computed properties—including read-only computed properties—as variable properties with the var keyword, because their value is not fixed. The let keyword is only used for constant properties, to indicate that their values cannot be changed once they are set as part of instance initialization.

You can simplify the declaration of a read-only computed property by removing the get keyword and its braces:

struct Cuboid {
var width = 0.0, height = 0.0, depth = 0.0
var volume: {
return width * height * depth
let fourByFiveByTwo = Cuboid(width: 4.0, height: 5.0, depth: 2.0)
print(“the volume of fourByFiveByTwo is \(fourByFiveByTwo.volume)”)
// Prints “the volume of fourByFiveByTwo is 40.0”

This example defines a new structure called Cuboid, which represents a 3D rectangular box with width, height, and depth properties. This structure also has a read-only computed property called volume, which calculates and returns the current volume of the cuboid. It doesn’t make sense for volume to be settable, because it would be ambiguous as to which values of width, height, and depth should be used for a particular volume value. Nonetheless, it is useful for a Cuboid to provide a read-only computed property to enable external users to discover its current calculated volume.

Property Observers

Property observers observe and respond to changes in a property’s value. Property observers are called every time a property’s value is set, even if the new value is the same as the property’s current value.You can add property observers to any stored properties you define, except for lazy stored properties. You can also add property observers to any inherited property (whether stored or computed) by overriding the property within a subclass. You don’t need to define property observers for non-overridden computed properties, because you can observe and respond to changes to their value in the computed property’s setter.

You have the option to define either or both of these observers on a property:
willSet is called just before the value is stored.
didSet is called immediately after the new value is stored.
If you implement a willSet observer, it’s passed the new property value as a constant parameter. You can specify a name for this parameter as part of your willSet implementation. If you don’t write the parameter name and parentheses within your implementation, the parameter is made available with a default parameter name of newValue.
Similarly, if you implement a didSet observer, it’s passed a constant parameter containing the old property value. You can name the parameter or use the default parameter name of oldValue. If you assign a value to a property within its own didSet observer, the new value that you assign replaces the one that was just set.The willSet and didSet observers of superclass properties are called when a property is set in a subclass initializer, after the superclass initializer has been called. They are not called while a class is setting its own properties, before the superclass initializer has been called.

Here’s an example of willSet and didSet in action. The example below defines a new class called StepCounter, which tracks the total number of steps that a person takes while walking. This class might be used with input data from a pedometer or other step counter to keep track of a person’s exercise during their daily routine.

class StepCounter {
var totalSteps: = 0 {
willSet(newTotalSteps) {
print(“About to set totalSteps to \(newTotalSteps)”)
didSet {
if totalSteps > oldValue {
print(“Added \(totalSteps – oldValue) steps”)
let stepCounter = StepCounter()
stepCounter.totalSteps = 200
// About to set totalSteps to 200
// Added 200 steps
stepCounter.totalSteps = 360
// About to set totalSteps to 360
// Added 160 steps
stepCounter.totalSteps = 896
// About to set totalSteps to 896
// Added 536 steps

The StepCounter class declares a totalSteps property of type Int. This is a stored property with willSet and didSet observers.The willSet and didSet observers for totalSteps are called whenever the property is assigned a new value. This is true even if the new value is the same as the current value.
This example’s willSet observer uses a custom parameter name of newTotalSteps for the upcoming new value. In this example, it simply prints out the value that is about to be set.The didSet observer is called after the value of totalSteps is updated. It compares the new value of totalSteps against the old value. If the total number of steps has increased, a message is printed to indicate how many new steps have been taken. The didSet observer does not provide a custom parameter name for the old value, and the default name of oldValue is used instead.If you pass a property that has observers to a function as an in-out parameter, the willSet and didSet observers are always called. This is because of the copy-in copy-out memory model for in-out parameters: The value is always written back to the property at the end of the function.

Global and Local Variables

The capabilities described above for computing and observing properties are also available to global variables and local variables. Global variables are variables that are defined outside of any function, method, closure, or type context. Local variables are variables that are defined within a function, method, or closure context.The global and local variables you have encountered in previous chapters have all been stored variables. Stored variables, like stored properties, provide storage for a value of a certain type and allow that value to be set and retrieved.However, you can also define computed variables and define observers for stored variables, in either a global or local scope. Computed variables calculate their value, rather than storing it, and they are written in the same way as computed properties.

Global constants and variables are always computed lazily, in a similar manner to Lazy Stored Properties. Unlike lazy stored properties, global constants and variables do not need to be marked with the lazy modifier.Local constants and variables are never computed lazily.

Type Properties

Instance properties are properties that belong to an instance of a particular type. Every time you create a new instance of that type, it has its own set of property values, separate from any other instance.You can also define properties that belong to the type itself, not to any one instance of that type. There will only ever be one copy of these properties, no matter how many instances of that type you create. These kinds of properties are called type properties.
Type properties are useful for defining values that are universal to all instances of a particular type, such as a constant property that all instances can use , or a variable property that stores a value that is global to all instances of that type.Stored type properties can be variables or constants. Computed type properties are always declared as variable properties, in the same way as computed instance properties.

Unlike stored instance properties, you must always give stored type properties a default value. This is because the type itself does not have an initializer that can assign a value to a stored type property at initialization time.Stored type properties are lazily initialized on their first access. They are guaranteed to be initialized only once, even when accessed by multiple threads simultaneously, and they do not need to be marked with the lazy modifier.

Type Property Syntax

In Swift, type properties are written as part of the type’s definition, within the type’s outer curly braces, and each type property is explicitly scoped to the type it supports.You define type properties with the static keyword. For computed type properties for class types, you can use the class keyword instead to allow subclasses to override the superclass’s implementation. The example below shows the syntax for stored and computed type properties:

struct SomeStructure {
static var storedTypeProperty = “Some value.”
static var computedTypeProperty: {
return 1
enum SomeEnumeration {
static var storedTypeProperty = “Some value.”
static var computedTypeProperty: {
return 6
class SomeClass {
static var storedTypeProperty = “Some value.”
static var computedTypeProperty: {
return 27
class var overrideableComputedTypeProperty: {
return 107

The computed type property examples above are for read-only computed type properties, but you can also define read-write computed type properties with the same syntax as for computed instance properties.

Querying and Setting Type Properties

Type properties are queried and set with dot syntax, just like instance properties. However, type properties are queried and set on the type, not on an instance of that type. For example:

print(SomeStructure.storedTypeProperty)     // Prints “Some value.”
SomeStructure.storedTypeProperty = “Another value.”
print(SomeStructure.storedTypeProperty)      // Prints “Another value.”
print(SomeEnumeration.computedTypeProperty)     // Prints “6”
print(SomeClass.computedTypeProperty)      // Prints “27”

The examples that follow use two stored type properties as part of a structure that models an audio level meter for a number of audio channels. Each channel has an integer audio level between 0 and 10 inclusive.
The figure below illustrates how two of these audio channels can be combined to model a stereo audio level meter. When a channel’s audio level is 0, none of the lights for that channel are lit. When the audio level is 10, all of the lights for that channel are lit. In this figure, the left channel has a current level of 9, and the right channel has a current level of 7:

The audio channels described above are represented by instances of the AudioChannel structure:

struct AudioChannel {
static let thresholdLevel = 10
static var maxInputLevelForAllChannels = 0
var currentLevel: = 0 {
didSet {
if currentLevel > AudioChannel.thresholdLevel {
// cap the new audio level to the threshold level
currentLevel = AudioChannel.thresholdLevel
if currentLevel > AudioChannel.maxInputLevelForAllChannels {
// store this as the new overall maximum input level
AudioChannel.maxInputLevelForAllChannels = currentLevel

The AudioChannel structure defines two stored type properties to support its functionality. The first, thresholdLevel, defines the maximum threshold value an audio level can take. This is a constant value of 10 for all AudioChannel instances. If an audio signal comes in with a higher value than 10, it will be capped to this threshold value (as described below).
The second type property is a variable stored property called maxInputLevelForAllChannels. This keeps track of the maximum input value that has been received by any AudioChannel instance. It starts with an initial value of 0.The AudioChannel structure also defines a stored instance property called currentLevel, which represents the channel’s current audio level on a scale of 0 to 10.
The currentLevel property has a didSet property observer to check the value of currentLevel whenever it is set. This observer performs two checks:
If the new value of currentLevel is greater than the allowed thresholdLevel, the property observer caps currentLevel to thresholdLevel.
If the new value of currentLevel (after any capping) is higher than any value previously received by any AudioChannel instance, the property observer stores the new currentLevel value in the maxInputLevelForAllChannels type property.In the first of these two checks, the didSet observer sets currentLevel to a different value. This does not, however, cause the observer to be called again.

You can use the AudioChannel structure to create two new audio channels called leftChannel and rightChannel, to represent the audio levels of a stereo sound system:

var leftChannel = AudioChannel()
var rightChannel = AudioChannel()
If you set the currentLevel of the left channel to 7, you can see that the maxInputLevelForAllChannels type property is updated to equal 7:

leftChannel.currentLevel = 7
print(leftChannel.currentLevel)      // Prints “7”
print(AudioChannel.maxInputLevelForAllChannels)     // Prints “7”

If you try to set the currentLevel of the right channel to 11, you can see that the right channel’s currentLevel property is capped to the maximum value of 10, and the maxInputLevelForAllChannels type property is updated to equal 10:

rightChannel.currentLevel = 11
print(rightChannel.currentLevel)       // Prints “10”
print(AudioChannel.maxInputLevelForAllChannels)     // Prints “10”


Methods are functions that are associated with a particular type. Classes, structures, and enumerations can all define instance methods, which encapsulate specific tasks and functionality for working with an instance of a given type. Classes, structures, and enumerations can also define type methods, which are associated with the type itself.  In Swift, you can choose whether to define a class, structure, or enumeration, and still have the flexibility to define methods on the type you create.

Instance Methods

Instance methods are functions that belong to instances of a particular class, structure, or enumeration. They support the functionality of those instances, either by providing ways to access and modify instance properties, or by providing functionality related to the instance’s purpose. Instance methods have exactly the same syntax as functions, as described in Functions.You write an instance method within the opening and closing braces of the type it belongs to. An instance method has implicit access to all other instance methods and properties of that type. An instance method can be called only on a specific instance of the type it belongs to. It cannot be called in isolation without an existing instance.
Here’s an example that defines a simple Counter class, which can be used to count the number of times an action occurs:

class Counter {
var count = 0
func increment() {
count += 1
func increment(by amount: ) {
count += amount
func reset() {
count = 0

The Counter class defines three instance methods:
increment() increments the counter by 1.increment(by: Int) increments the counter by a specified integer amount.reset() resets the counter to zero.
The Counter class also declares a variable property, count, to keep track of the current counter value.
You call instance methods with the same dot syntax as properties:

let counter = Counter()       // the initial counter value is 0
counter.increment()        // the counter’s value is now 1
counter.increment(by: 5)       // the counter’s value is now 6
counter.reset()          // the counter’s value is now 0

Function parameters can have both a name (for use within the function’s body) and an argument label (for use when calling the function), as described in Function Argument Labels and Parameter Names. The same is true for method parameters, because methods are just functions that are associated with a type.

The self Property

Every instance of a type has an implicit property called self, which is exactly equivalent to the instance itself. You use the self property to refer to the current instance within its own instance methods.The increment() method in the example above could have been written like this:

func increment() {
self.count += 1

In practice, you don’t need to write self in your code very often. If you don’t explicitly write self, Swift assumes that you are referring to a property or method of the current instance whenever you use a known property or method name within a method. This assumption is demonstrated by the use of count (rather than self.count) inside the three instance methods for Counter.
The main exception to this rule occurs when a parameter name for an instance method has the same name as a property of that instance. In this situation, the parameter name takes precedence, and it becomes necessary to refer to the property in a more qualified way. You use the self property to distinguish between the parameter name and the property name.
Here, self disambiguates between a method parameter called x and an instance property that is also called x:

struct Point {
var x = 0.0, y = 0.0
func isToTheRightOf(x: ) -> {
return self.x > x
let somePoint = Point(x: 4.0, y: 5.0)
if somePoint.isToTheRightOf(x: 1.0) {
print(“This point is to the right of the line where x == 1.0”)
// Prints “This point is to the right of the line where x == 1.0”

Without the self prefix, Swift would assume that both uses of x referred to the method parameter called x.

Modifying Value Types from Within Instance Methods

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.However, if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method. The method can then mutate (that is, change) its properties from within the method, and any changes that it makes are written back to the original structure when the method ends. The method can also assign a completely new instance to its implicit self property, and this new instance will replace the existing one when the method ends.You can opt in to this behavior by placing the mutating keyword before the func keyword for that method:

struct Point {
var x = 0.0, y = 0.0
mutating func moveBy(x deltaX: , y deltaY: ) {
x += deltaX
y += deltaY
var somePoint = Point(x: 1.0, y: 1.0)
somePoint.moveBy(x: 2.0, y: 3.0)
print(“The point is now at (\(somePoint.x), \(somePoint.y))”)
// Prints “The point is now at (3.0, 4.0)”

The Point structure above defines a mutating moveBy(x:y:) method, which moves a Point instance by a certain amount. Instead of returning a new point, this method actually modifies the point on which it is called. The mutating keyword is added to its definition to enable it to modify its properties.
Note that you cannot call a mutating method on a constant of structure type, because its properties cannot be changed, even if they are variable properties, as described in Stored Properties of Constant Structure Instances:

let fixedPoint = Point(x: 3.0, y: 3.0)
fixedPoint.moveBy(x: 2.0, y: 3.0)
// this will report an error

Assigning to self Within a Mutating Method

Mutating methods can assign an entirely new instance to the implicit self property. The Point example shown above could have been written in the following way instead:

struct Point {
var x = 0.0, y = 0.0
mutating func moveBy(x deltaX: , y deltaY: ) {
self = Point(x: x + deltaX, y: y + deltaY)

This version of the mutating moveBy(x:y:) method creates a brand new structure whose x and y values are set to the target location. The end result of calling this alternative version of the method will be exactly the same as for calling the earlier version.Mutating methods for enumerations can set the implicit self parameter to be a different case from the same enumeration:

enum TriStateSwitch {
case off, low, high
mutating func next() {
switch self {
case .off:
self = .low
case .low:
self = .high
case .high:
self = .off
var ovenLight = TriStateSwitch.low
// ovenLight is now equal to .high
// ovenLight is now equal to .off

This example defines an enumeration for a three-state switch. The switch cycles between three different power states (off, low and high) every time its next() method is called.

‌Type Methods

Instance methods, as described above, are methods that are called on an instance of a particular type. You can also define methods that are called on the type itself. These kinds of methods are called type methods. You indicate type methods by writing the static keyword before the method’s func keyword. Classes may also use the class keyword to allow subclasses to override the superclass’s implementation of that method. In Swift, you can define type-level methods for all classes, structures, and enumerations. Each type method is explicitly scoped to the type it supports.

Type methods are called with dot syntax, like instance methods. However, you call type methods on the type, not on an instance of that type. Here’s how you call a type method on a class called SomeClass:

class SomeClass {
class func someTypeMethod() {
// type method implementation goes here

Within the body of a type method, the implicit self property refers to the type itself, rather than an instance of that type. This means that you can use self to disambiguate between type properties and type method parameters, just as you do for instance properties and instance method parameters.
More generally, any unqualified method and property names that you use within the body of a type method will refer to other type-level methods and properties. A type method can call another type method with the other method’s name, without needing to prefix it with the type name. Similarly, type methods on structures and enumerations can access type properties by using the type property’s name without a type name prefix.
The example below defines a structure called LevelTracker, which tracks a player’s progress through the different levels or stages of a game. It is a single-player game, but can store information for multiple players on a single device.
All of the game’s levels (apart from level one) are locked when the game is first played. Every time a player finishes a level, that level is unlocked for all players on the device. The LevelTracker structure uses type properties and methods to keep track of which levels of the game have been unlocked. It also tracks the current level for an individual player.

struct LevelTracker {
static var highestUnlockedLevel = 1
var currentLevel = 1

static func unlock(_ level: ) {
if level > highestUnlockedLevel { highestUnlockedLevel = level }

static func isUnlocked(_ level: ) -> {
return level <= highestUnlockedLevel } @discardableResult mutating func advance(to level: ) -> {
if LevelTracker.isUnlocked(level) {
currentLevel = level
return true
} else {
return false

The LevelTracker structure keeps track of the highest level that any player has unlocked. This value is stored in a type property called highestUnlockedLevel.
LevelTracker also defines two type functions to work with the highestUnlockedLevel property. The first is a type function called unlock(_:), which updates the value of highestUnlockedLevel whenever a new level is unlocked. The second is a convenience type function called isUnlocked(_:), which returns true if a particular level number is already unlocked. (Note that these type methods can access the highestUnlockedLevel type property without your needing to write it as LevelTracker.highestUnlockedLevel.)
In addition to its type property and type methods, LevelTracker tracks an individual player’s progress through the game. It uses an instance property called currentLevel to track the level that a player is currently playing.
To help manage the currentLevel property, LevelTracker defines an instance method called advance(to:). Before updating currentLevel, this method checks whether the requested new level is already unlocked. The advance(to:) method returns a Boolean value to indicate whether or not it was actually able to set currentLevel. Because it’s not necessarily a mistake for code that calls the advance(to:) method to ignore the return value, this function is marked with the @discardableResult attribute. For more information about this attribute, see Attributes.
The LevelTracker structure is used with the Player class, shown below, to track and update the progress of an individual player:

class Player {
var tracker = LevelTracker()
let playerName:
func complete(level: ) {
LevelTracker.unlock(level + 1)
tracker.advance(to: level + 1)
init(name: ) {
playerName = name

The Player class creates a new instance of LevelTracker to track that player’s progress. It also provides a method called complete(level:), which is called whenever a player completes a particular level. This method unlocks the next level for all players and updates the player’s progress to move them to the next level. (The Boolean return value of advance(to:) is ignored, because the level is known to have been unlocked by the call to LevelTracker.unlock(_:) on the previous line.)
You can create an instance of the Player class for a new player, and see what happens when the player completes level one:

var player = Player(name: “Argyrios”)
player.complete(level: 1)
print(“highest unlocked level is now \(LevelTracker.highestUnlockedLevel)”)
// Prints “highest unlocked level is now 2”

If you create a second player, whom you try to move to a level that is not yet unlocked by any player in the game, the attempt to set the player’s current level fails:

player = Player(name: “Beto”)
if player.tracker.advance(to: 6) {
print(“player is now on level 6”)
} else {
print(“level 6 has not yet been unlocked”)
// Prints “level 6 has not yet been unlocked”


Classes, structures, and enumerations can define subscripts, which are shortcuts for accessing the member elements of a collection, list, or sequence. You use subscripts to set and retrieve values by index without needing separate methods for setting and retrieval. For example, you access elements in an Array instance as someArray[index] and elements in a Dictionary instance as someDictionary[key].
You can define multiple subscripts for a single type, and the appropriate subscript overload to use is selected based on the type of index value you pass to the subscript. Subscripts are not limited to a single dimension, and you can define subscripts with multiple input parameters to suit your custom type’s needs.

Subscript Syntax

Subscripts enable you to query instances of a type by writing one or more values in square brackets after the instance name. Their syntax is similar to both instance method syntax and computed property syntax. You write subscript definitions with the subscript keyword, and specify one or more input parameters and a return type, in the same way as instance methods. Unlike instance methods, subscripts can be read-write or read-only. This behavior is communicated by a getter and setter in the same way as for computed properties:

subscript(index: ) -> {
get {
// return an appropriate subscript value here
set(newValue) {
// perform a suitable setting action here

The type of newValue is the same as the return value of the subscript. As with computed properties, you can choose not to specify the setter’s (newValue) parameter. A default parameter called newValue is provided to your setter if you do not provide one yourself.
As with read-only computed properties, you can simplify the declaration of a read-only subscript by removing the get keyword and its braces:

subscript(index: ) -> {
// return an appropriate subscript value here

Here’s an example of a read-only subscript implementation, which defines a TimesTable structure to represent an n-times-table of integers:

struct TimesTable {
let multiplier:
subscript(index: ) -> {
return multiplier * index
let threeTimesTable = TimesTable(multiplier: 3)
print(“six times three is \(threeTimesTable[6])”)
// Prints “six times three is 18”

In this example, a new instance of TimesTable is created to represent the three-times-table. This is indicated by passing a value of 3 to the structure’s initializer as the value to use for the instance’s multiplier parameter.You can query the threeTimesTable instance by calling its subscript, as shown in the call to threeTimesTable[6]. This requests the sixth entry in the three-times-table, which returns a value of 18, or 3 times 6.An n-times-table is based on a fixed mathematical rule. It is not appropriate to set threeTimesTable[someIndex] to a new value, and so the subscript for TimesTable is defined as a read-only subscript.

‌Subscript Usage

The exact meaning of “subscript” depends on the context in which it is used. Subscripts are typically used as a shortcut for accessing the member elements in a collection, list, or sequence. You are free to implement subscripts in the most appropriate way for your particular class or structure’s functionality.
For example, Swift’s Dictionary type implements a subscript to set and retrieve the values stored in a Dictionary instance. You can set a value in a dictionary by providing a key of the dictionary’s key type within subscript brackets, and assigning a value of the dictionary’s value type to the subscript:

var numberOfLegs = [“spider”: 8, “ant”: 6, “cat”: 4]
numberOfLegs[“bird”] = 2

The example above defines a variable called numberOfLegs and initializes it with a dictionary literal containing three key-value pairs. The type of the numberOfLegs dictionary is inferred to be [String: Int]. After creating the dictionary, this example uses subscript assignment to add a String key of “bird” and an Int value of 2 to the dictionary.
Swift’s Dictionary type implements its key-value subscripting as a subscript that takes and returns an optional type. For the numberOfLegs dictionary above, the key-value subscript takes and returns a value of type Int?, or “optional int”. The Dictionary type uses an optional subscript type to model the fact that not every key will have a value, and to give a way to delete a value for a key by assigning a nil value for that key.

‌Subscript Options

Subscripts can take any number of input parameters, and these input parameters can be of any type. Subscripts can also return any type. Subscripts can use variadic parameters, but they can’t use in-out parameters or provide default parameter values.
A class or structure can provide as many subscript implementations as it needs, and the appropriate subscript to be used will be inferred based on the types of the value or values that are contained within the subscript brackets at the point that the subscript is used. This definition of multiple subscripts is known as subscript overloading.While it is most common for a subscript to take a single parameter, you can also define a subscript with multiple parameters if it is appropriate for your type. The following example defines a Matrix structure, which represents a two-dimensional matrix of Double values. The Matrix structure’s subscript takes two integer parameters:

struct Matrix {
let rows: , columns:
var grid: [] init(rows: , columns: ) {
self.rows = rows
self.columns = columns
grid = Array(repeating: 0.0, count: rows * columns)
func indexIsValid(row: , column: ) -> {
return row >= 0 && row < rows && column >= 0 && column < columns } subscript(row: , column: ) -> {
get {
assert(indexIsValid(row: row, column: column), “Index out of range”)
return grid[(row * columns) + column] }
set {
assert(indexIsValid(row: row, column: column), “Index out of range”)
grid[(row * columns) + column] = newValue

Matrix provides an initializer that takes two parameters called rows and columns, and creates an array that is large enough to store rows * columns values of type Double. Each position in the matrix is given an initial value of 0.0. To achieve this, the array’s size, and an initial cell value of 0.0, are passed to an array initializer that creates and initializes a new array of the correct size. You can construct a new Matrix instance by passing an appropriate row and column count to its initializer:

var matrix = Matrix(rows: 2, columns: 2)

The preceding example creates a new Matrix instance with two rows and two columns. The grid array for this Matrix instance is effectively a flattened version of the matrix, as read from top left to bottom right:

Values in the matrix can be set by passing row and column values into the subscript, separated by a comma:

matrix[0, 1] = 1.5
matrix[1, 0] = 3.2
These two statements call the subscript’s setter to set a value of 1.5 in the top right position of the matrix (where row is 0 and column is 1), and 3.2 in the bottom left position (where row is 1 and column is 0):

The Matrix subscript’s getter and setter both contain an assertion to check that the subscript’s row and column values are valid. To assist with these assertions, Matrix includes a convenience method called indexIsValid(row:column:), which checks whether the requested row and column are inside the bounds of the matrix:

func indexIsValid(row: , column: ) -> {
return row >= 0 && row < rows && column >= 0 && column < columns
An assertion is triggered if you try to access a subscript that is outside of the matrix bounds:

let someValue = matrix[2, 2]
// this triggers an assert, because [2, 2] is outside of the matrix bounds

This Is A Custom Widget

This Sliding Bar can be switched on or off in theme options, and can take any widget you throw at it or even fill it with your custom HTML Code. Its perfect for grabbing the attention of your viewers. Choose between 1, 2, 3 or 4 columns, set the background color, widget divider color, activate transparency, a top border or fully disable it on desktop and mobile.

This Is A Custom Widget

This Sliding Bar can be switched on or off in theme options, and can take any widget you throw at it or even fill it with your custom HTML Code. Its perfect for grabbing the attention of your viewers. Choose between 1, 2, 3 or 4 columns, set the background color, widget divider color, activate transparency, a top border or fully disable it on desktop and mobile.