NEW! Master Swift design patterns with my latest book! >>

< Previous: Methods   Next: Properties and methods of strings >

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()

Go from iOS to macOS the easy way!

If you like Hacking with Swift, you'll love Hacking with macOS – learn to build macOS apps today, using 18 real-world projects!

< Previous: Methods   Next: Properties and methods of strings >
MASTER SWIFT NOW
Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Practical iOS 11 Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Click here to visit the Hacking with Swift store >>