LESSON 288 OF 678 · FUNCTIONS & CLASSES

Constructors in Swift

Lesson

TL ; DR:

A constructor is a special method that runs automatically when you create an object. It's used to set up the object's initial state.

In Swift, a constructor is called an initializer and is defined using init:

class Dog {
    var name: String
    var age: Int

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

Then create objects with arguments: Dog(name: "Rex", age: 4) — the initializer fills in the fields for you.

Full lesson:

In the previous lesson you set fields after creating the object:

var dog = Dog()
dog.name = "Rex"
dog.age = 4

That works, but it's noisy and easy to forget. Initializers let you initialize the fields the moment Dog(...) is called.

Defining an initializer

A Swift initializer has these features:

It is always named init (not the class name).
It has no return type.

init(name: String, age: Int) {
    self.name = name
    self.age = age
}

The self keyword

The parameter name and the field name have the same identifier. Without self, both sides of name = name would refer to the parameter (the local one wins). self is a built-in reference to "the current object", so self.name is the field, and bare name is the parameter.

Calling the initializer

let rex  = Dog(name: "Rex", age: 4)
let luna = Dog(name: "Luna", age: 7)

By the time these lines finish, rex.name is already "Rex", rex.age is 4, and the same for Luna.

The default initializer

If every stored property has a default value (as in the previous lesson), Swift gives the class a default initializer, Dog(). If a property has no default value, you must write an init that sets it. The moment you define your own initializer, Dog() is no longer available: you must call Dog with the arguments you specified.

Your task

The Dog class already has the name and age fields. Add an initializer that takes name: String and age: Int and assigns them using self. Then in the main code, create two dogs:

let rex  = Dog(name: "Rex", age: 4)
let luna = Dog(name: "Luna", age: 7)

Expected output:

Rex is 4
Luna is 7

STEP 1 OF 2

Guided

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