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iOS Generics in Swift Generics and Protocols Associated Types

Peter Correa
Peter Correa
17,155 Points
Posted by Peter Correa
Peter Correa
17,155 Points

Why do we have the need to use *mutating func* and not just *func*?

Why do we have the need to use mutating func and not just func?

1 Answer

Tassia Castro
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Tassia Castro
iOS Development Techdegree Student 9,170 Points
Tassia Castro
.a{fill-rule:evenodd;}techdegree
Tassia Castro
iOS Development Techdegree Student 9,170 Points

Hey Peter,

You use mutating func when the func can change (or mutate) one or more properties of a Struct.

Check out this example.

struct Rectangle {
    var width = 1
    var height = 1

    func area() -> Int {
        return width * height
    }
}

As you can see the func here is not mutating and the reason is because the method only returns the width times the height without changing it.

struct Rectangle {
    var width = 1
    var height = 1

    mutating func scaleBy(value: Int) {
        width *= value
        height *= value
    }
}

However if I need to create a method that actually change those variables I need to define it as mutating. scaleBy does exactly that, it receives a value and set width and height with them.

Peter Correa
Peter Correa
17,155 Points
Peter Correa
Peter Correa
17,155 Points

Great explanation! Thank you!