While a mutating method may sound like a term out of a mad scientist’s lab, it’s actually how we change an instance’s properties using an instance method. To do so, we need the mutating keyword:

struct Dog {
  var age : Int
  var isGood : Bool

  init(age: Int, isGood: Bool) {
    self.age = age
    self.isGood = isGood
  }

  // birthday() is a mutating method:
  mutating func birthday() -> Int {
    print("Best doggy")
    self.age += 1
    return self.age
  }
}

In our Dog structure, we used the mutating keyword to add the mutating method birthday(). This method prints out "Best doggy", is also able to affect self — note how we can increment self.age. In the end, the method returns self.age.

Let’s take a look at when we call the method on our Dog instance, bucket:

var bucket = Dog(age: 7, isGood: true)
var newAge = bucket.birthday() 
// Prints: Best doggy

print(newAge) // Prints: 8

Above, our method worked as expected:

• Best doggy printed out.
• We were able to save the return value of bucket.birthday() to newAge.
• Printing out newAge showed that bucket‘s age increased by 1!

If we tried to use a regular method to change a property, without the mutating keyword, like so:

func birthday() -> Int {
  print("Best doggy")
  self.age += 1
  return self.age
}

Calling bucket.birthday() Swift’s compiler would raise an error.

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

This error lets us know that we can’t reassign a value for self. The note then tells us to use the mutating keyword to allow the method to reassign self.