Mutating keyword in Swift

Mutating keyword in Swift:

The mutating keyword in Swift is used to indicate that a method or function can potentially modify the instance it is called on. This is particularly relevant for methods defined within value types, such as structs and enums. Value types have value semantics, which means that by default, methods cannot modify properties of the instance they are called on.

Here’s a brief explanation of why the mutating keyword is necessary:

1. Structs and Enums are Value Types:
   • In Swift, structs and enums are value types. When you pass a value type instance to a function or method, you’re working with a copy of that instance.
   • By default, methods on value types are not allowed to modify properties of the instance.
2. Mutating Methods:
   • If you want a method of a value type to be able to modify the properties of the instance it’s called on, you need to mark that method with the mutating keyword.
   • The mutating keyword signals to the compiler that the method might modify the instance, and therefore, the method can only be called on variables declared with var (mutable variables), not on constants declared with let.

Here’s an example using a mutating method in a struct:

struct Point {
    var x, y: Double

    // A mutating method that modifies the properties of the struct
    mutating func moveBy(x deltaX: Double, y deltaY: Double) {
       x += deltaX
       y += deltaY
    }
}

// Creating a mutable instance of Point
var myPoint = Point(x: 10.0, y: 20.0)

// Calling the mutating method on the mutable instance
myPoint.moveBy(x: 5.0, y: 5.0)

// The properties of the struct have been modified
print("New Point: (\(myPoint.x), \(myPoint.y))") // Output: New Point: (15.0, 25.0)

Without the mutating keyword, the moveBy method would not be allowed to modify the properties of Point. The use of mutating ensures that the method can be called on mutable instances, and it communicates to the developer that the method has the potential to change the state of the instance.