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5/12/2020 Initialization — The Swift Programming Language (Swift 5.2)

https://docs.swift.org/swift-book/LanguageGuide/Initialization.html 2/35

Initialization

Initialization is the process of preparing an instance of a class, structure, or

enumeration for use. This process involves setting an initial value for each stored

property on that instance and performing any other setup or initialization that is required

before the new instance is ready for use.

You implement this initialization process by defining initializers, which are like special

methods that can be called to create a new instance of a particular type. Unlike

Objective-C initializers, Swift initializers do not return a value. Their primary role is to

ensure that new instances of a type are correctly initialized before they are used for the

first time.

Instances of class types can also implement a deinitializer, which performs any custom

cleanup just before an instance of that class is deallocated. For more information about

deinitializers, see Deinitialization.

Setting Initial Values for Stored Properties

Classes and structures must set all of their stored properties to an appropriate initial

value by the time an instance of that class or structure is created. Stored properties

cannot be left in an indeterminate state.

You can set an initial value for a stored property within an initializer, or by assigning a

default property value as part of the property’s definition. These actions are described

in the following sections.

N O T E

When you assign a default value to a stored property, or set its initial value within an

initializer, the value of that property is set directly, without calling any property observers.

Initializers

Initializers are called to create a new instance of a particular type. In its simplest form,

an initializer is like an instance method with no parameters, written using the init

keyword:

1 init() {

2 // perform some initialization here

3 }

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5/12/2020 Initialization — The Swift Programming Language (Swift 5.2)

https://docs.swift.org/swift-book/LanguageGuide/Initialization.html 4/35

You can customize the initialization process with input parameters and optional property

types, or by assigning constant properties during initialization, as described in the

following sections.

Initialization Parameters

You can provide initialization parameters as part of an initializer’s definition, to define

the types and names of values that customize the initialization process. Initialization

parameters have the same capabilities and syntax as function and method parameters.

The following example defines a structure called Celsius, which stores temperatures

expressed in degrees Celsius. The Celsius structure implements two custom initializers

called init(fromFahrenheit:) and init(fromKelvin:), which initialize a new instance

of the structure with a value from a different temperature scale:

1 struct Celsius {

2 var temperatureInCelsius: Double

3 init(fromFahrenheit fahrenheit: Double) {

4 temperatureInCelsius = (fahrenheit - 32.0) / 1.8

5 }

6 init(fromKelvin kelvin: Double) {

7 temperatureInCelsius = kelvin - 273.15

8 }

9 }

10 let boilingPointOfWater = Celsius(fromFahrenheit: 212.0)

11 // boilingPointOfWater.temperatureInCelsius is 100.0

12 let freezingPointOfWater = Celsius(fromKelvin: 273.15)

13 // freezingPointOfWater.temperatureInCelsius is 0.0

The first initializer has a single initialization parameter with an argument label of

fromFahrenheit and a parameter name of fahrenheit. The second initializer has a

single initialization parameter with an argument label of fromKelvin and a parameter

name of kelvin. Both initializers convert their single argument into the corresponding

Celsius value and store this value in a property called temperatureInCelsius.

Parameter Names and Argument Labels

As with function and method parameters, initialization parameters can have both a

parameter name for use within the initializer’s body and an argument label for use when

calling the initializer.

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