Open Radar

 
Please consider supporting the mutating construct that is already supported for structs onto classes. This will provide us with everything we need to create truly const classes as well as avoid the unfortunate pseudo-constant struct/class composition problems.

As an example, see this program:
class CPerson {
    var name : String
    init(name : String) { self.name = name }
}

struct SPerson {
    var age : Int
    var person : CPerson
    mutating func haveBirthday() {
        println("Happy birthday!")
        age++
    }
}

let david = SPerson(age: 32, person: CPerson(name: "David"))
var frank = SPerson(age: 35, person: CPerson(name: "Frank"))

//david.age = 23             // compiler error, as it should
david.person.name = "Sally"  // this SHOULD be a compiler error, but classes do not support 'mutating' or const in general
david.person.name            // prints "Sally"

frank.age = 23
frank.person.name = "David"
frank.person.name   // prints "David"

If classes supported mutating, the let keyword would provide const protection down the members of the struct avoiding being able to change the underlying values of the struct.