Swift: Mutating Methods

Mutating Methods in Swift:

A mutating method may sound like something out of a mad scientist’s lab, but it’s actually how we change the properties of an instance using an instance method in Swift. So, we’ll need the mutating keyword to do so:

struct Car {
  var name : String ; 
  var speed : Int ;
  init (name : String, speed : Int) {
    self.name = name ;
    self.speed = speed ;

  // modify() is a mutating method:
  mutating func modify() -> Int {
    print("Car Modified") ;
    self.speed += 40 ;
    return self.speed ;

We added the mutating method modify() to our Car structure using the mutating keyword. This method prints "Car Modified" and can also affect self (note how self.speed can be increased). The method returns self.speed at the end.

Take a look at what happens when we call the racingCar: method on our Car instance.

var racingCar = Car(name : "BMW i8", speed : 340) ;
var newSpeed = racingCar.modify() ;
// Prints: Car Modified

print(newSpeed) ;   // Prints: 380

Our method worked as expected in the examples above:

  • Car Modified was printed.
  • The return value of racingCar.modify() was successfully saved to newSpeed.
  • Printing out the speed of the Car increased by 40, according to newSpeed!

If we tried to change a property using a regular method without the mutating keyword, it would look like this:

func modify() -> Int {
    print("Car Modified") ;
    self.speed += 40 ;
    return self.speed ;

Swift’s compiler would throw an error if you called racingCar.modify().

error: left side of mutating operator isn't mutable: 'self' is immutable
note: mark method 'mutating' to make 'self' mutable

This error indicates that we are unable to reassign a value to self. So, the note goes on to say that we should use the mutating keyword to let the method reassign self.

Helpful Tools:

Here are some useful tools to help you along your journey!

Setting up an IDE (Integrated Development Environment) can be difficult for beginners. The Online Compiler will enable you to run your code inside your browser without the need to install an IDE. If you need a more detailed explanation of a specific topic, the best place to find answers is in the Official Documentation.

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