Swift Object reference to array?

Question

I probably missed an important information about swift. I have a map contains a key / swift array pair. I changed the array and the array inside the map was not changed. Cou

Answer 1

Class

Let's suppose we have a type that is a class in Swift:

class MyClass {
    var a:Int
    init(i:Int) {
        a = i
    }
}

let aClass = MyClass(i: 10)
var bClass = aClass
bClass.a = 20
aClass.a // 20
aClass.a = 30
bClass.a // 30

Here we see pointer-style behaviour: aClass and bClass are pointing at the same object. If you change a property value in one of them then both change, because they are the same.

Struct

The behaviour changes when a struct is used, as can be seen here:

struct MyStruct {
    var a:Int
    init(i:Int) {
        a = i
    }
}
let aStruct = MyStruct(i: 10)
var bStruct = aStruct
bStruct.a = 20
aStruct.a // 10

With a struct (and an enum) instead of there being a single instance and multiple pointers to that instance, a new instance is created when we assign the value to a new instance. It is a copy of the original and they are independent of one another.

Array type

Since the Swift Array type is a struct it has the behaviour of copying rather than pointing. While NSArray and NSMutableArray remain classes (as in ObjC) and so have the pointing behaviour of a class.

Making a struct behave like a class

You've already seen others post about inout properties, and you can also use computed variables to partially replicate the behaviour of a pointer:

struct MyStruct {
    lazy var map = [String:[String]]()
    var list:[String] {
        mutating get {
            if map["list"] == nil {
                map["list"] = [String]()
            }
            return map["list"]!
        }
        set {
            map["list"] = newValue
        }
        
    }
}

var mS = MyStruct()
mS.list = ["test"]
mS.map // ["list": ["test"]]
mS.list // ["test"]

While outside a type instance you might do the following:

var map = [String:[String]]()
var list = [String]()
var appendToList = { (a:String) in
    list.append(a)
    map["list"] = list
}
appendToList("test") // list and map["list"] are both updated