Mutating methods

If a struct has a variable property but the instance of the struct was created as a constant, that property can’t be changed – the struct is constant, so all its properties are also constant regardless of how they were created.

The problem is that when you create the struct Swift has no idea whether you will use it with constants or variables, so by default it takes the safe approach: Swift won’t let you write methods that change properties unless you specifically request it.

When you want to change a property inside a method, you need to mark it using the mutating keyword, like this:

struct Person {
    var name: String

    mutating func makeAnonymous() {
        name = "Anonymous"
    }
}

Because it changes the property, Swift will only allow that method to be called on Person instances that are variables:

var person = Person(name: "Ed")
person.makeAnonymous()

SAVE 50% To celebrate WWDC23, all our books and bundles are half price, so you can take your Swift knowledge further without spending big! Get the Swift Power Pack to build your iOS career faster, get the Swift Platform Pack to builds apps for macOS, watchOS, and beyond, or get the Swift Plus Pack to learn advanced design patterns, testing skills, and more.

Save 50% on all our books and bundles!