NEW: Start my new Ultimate Portfolio App course with a free Hacking with Swift+ trial! >>

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()
Hacking with Swift is sponsored by ViRE

SPONSORED ViRE offers discoverable way of working with regex. It provides really readable regex experience, code complete & cheat sheet, unit tests, powerful replace system, step-by-step search & replace, regex visual scheme, regex history & playground. ViRE is available on Mac & iPad.

Download on the App Store

Sponsor Hacking with Swift and reach the world's largest Swift community!

BUY OUR BOOKS
Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.7/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.