Mutating Methods - Swift Programming

A struct method marked mutating can change the instance's stored properties.

Change a value type

mutating_methods.swift

struct Counter {
    var value: Int

    mutating func advance(by amount: Int) {
        value = value + amount
    }
}

let step = 
var counter = Counter(value: 10)
counter.advance(by: step)
counter.advance(by: step)

print("step=\(step)")
print("value=\(counter.value)")

struct Counter {
    var value: Int

    mutating func advance(by amount: Int) {
        value = value + amount
    }
}

let step = 
var counter = Counter(value: 10)
counter.advance(by: step)
counter.advance(by: step)

print("step=\(step)")
print("value=\(counter.value)")

struct Counter {
    var value: Int

    mutating func advance(by amount: Int) {
        value = value + amount
    }
}

let step = 
var counter = Counter(value: 10)
counter.advance(by: step)
counter.advance(by: step)

print("step=\(step)")
print("value=\(counter.value)")

mutating Struct instances are values. A `mutating` method is explicit about changing the stored properties of that value.