Swift Tutorial · Collections and Types · Lesson 13 of 30

Swift Structs

Define value types with stored and computed properties, methods, mutating methods and memberwise initializers.

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Structs are value types

Structs are the default way to model data in Swift. They get a free memberwise initializer, and copies are independent. A method that changes a property must be marked mutating.

A struct with a mutating method

struct Point {
    var x: Int
    var y: Int

    mutating func move(dx: Int, dy: Int) {
        x += dx
        y += dy
    }
}

var a = Point(x: 1, y: 2)       // memberwise initializer
var b = a                       // a copy
b.move(dx: 10, dy: 0)
print(a, b)
print(a.x == 1)
a.move(dx: 0, dy: 1)
print(a)

Output

Point(x: 1, y: 2) Point(x: 11, y: 2)
true
Point(x: 1, y: 3)

Run this on your computer — see “Get Started” for setup.

Computed properties and observers

A computed property calculates its value each time; didSet runs after a stored property changes.

Computed properties and didSet

struct Cart {
    var items: [Double] = [] {
        didSet { print("cart changed: \(items.count) items") }
    }
    var total: Double { items.reduce(0, +) }       // computed
    var isEmpty: Bool { items.isEmpty }
}

var cart = Cart()
cart.items.append(12.5)
cart.items.append(30)
print(cart.total, cart.isEmpty)

Output

cart changed: 1 items
cart changed: 2 items
42.5 false

Run this on your computer — see “Get Started” for setup.

Test yourself

Why must some struct methods be marked mutating?

To make them public
Structs are value types, so changing self needs to be explicit
To make them async
Only static methods need it

Show answer

Answer: Structs are value types, so changing self needs to be explicit

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