Swift Initializers

Swift Initializers

Initialization is the process of preparing an instance of a class, structure, or enumeration for use.

Unlike Objective-C, Swift initializers do not return a value.

Their primary role is to ensure that every property on a new instance is correctly set to an initial value before the object is used for the first time.

The init Method

You define an initializer by writing the init keyword. It behaves exactly like an instance method, but without the func keyword.

class Fahrenheit {
    var temperature: Double
    // This runs immediately when the object is created
    init() {
        temperature = 32.0
    }
}

let currentTemp = Fahrenheit()
print("The default temperature is \(currentTemp.temperature)")

Customizing Initialization

Initializers can take parameters, just like normal functions.

This allows you to pass in custom data the exact moment an object is created.

struct Person {
    var name: String
    var age: Int
    init(name: String, age: Int) {
        // 'self' distinguishes the property from the parameter
        self.name = name
        self.age = age
    }
}

// Passing data directly upon creation
let user = Person(name: "Akash", age: 25)
print(user.name)

Memberwise Initializers for Structs

If you declare a struct, Swift is incredibly helpful: it automatically generates a "memberwise initializer" for you behind the scenes!

This means you don't even have to write an init method for structs if you don't want to.

struct Size {
    var width: Double
    var height: Double
}

// Swift automatically generated this initializer for us!
let mySize = Size(width: 10.0, height: 20.0)
print(mySize.width)

Note: Classes do NOT receive an automatic memberwise initializer.

Failable Initializers

Sometimes initialization can fail (e.g., if invalid data is passed in). You can define a failable initializer by writing init?. It returns an Optional that will be nil if initialization fails.