Why are methods that modify the state of an object declared as mutating in a class?

In Swift, methods that modify the properties of a structure or enumeration must be marked with the mutating keyword. This is because structures and enumerations are value types, and by default, their methods cannot modify the instance itself, as it is considered a constant within the method.

Declaring a method as mutating allows that method to modify the properties of the instance, as well as replace the instance with a new value. This is important for ensuring type safety and preventing unintended changes.

Example:

struct Counter {
    var count = 0
    mutating func increment() {
        count += 1
    }
}

var counter = Counter()
counter.increment()  // count is now 1

In classes, mutating is not required, as classes are reference types, and methods can modify the properties of an object without additional annotation.