What is meant by Mutating in the context of programming?

In programming, the term "mutating" refers to operations or methods that modify the state of an object after they are called. In the context of iOS and Swift, mutating methods are methods that can change the properties of a structure or enumeration.

In Swift, structures and enumerations are value types and are immutable by default, so to allow a method to modify the properties of an instance, it must be marked with the mutating keyword.

Example:

struct Point {
    var x: Int
    var y: Int

    mutating func moveBy(dx: Int, dy: Int) {
        x += dx
        y += dy
    }
}

var p = Point(x: 0, y: 0)
p.moveBy(dx: 5, dy: 3)  // now p.x == 5, p.y == 3

Thus, mutating methods allow changing the properties of value types, which would be impossible without this keyword.