Immutable value of type Array<String> only has mutating member named 'append'

Question

I have simple case where I call some list and try to append new value.

class User{

    var list:Array<String> = []

    func getList()->Array<String>{
      return list
    }
}

var user = User()
user.getList().append("aaa") // <-- ERROR
user.list.append("aaa") // OK

Immutable value of type Array<String> only has mutating member named 'append'

Why It doesn't work if user.getList() returns list.

I know there is no encapsulation like in Java but it seems strange.

[EDIT]

Regards to @MichaelDautermann answer:

var user = User()                        // {["c"]}
var temp:Array<String> = user.getList()  // ["c"]
temp += ["aaa"]                          // ["c", "aaa"]

var z:Array<String> = user.getList()     // ["c"]

Michael Dautermann · Accepted Answer · 2014-09-07 06:13:14Z

3

Your "getList" function is returning an immutable copy of the list array, which is why you can't modify via an array retrieved via the getList() function.

But you can modify the original array when you access the member variable, as you've discovered in your "OK" case.

Basically you'll probably either need to append on the original "list" array, or write an "append" func for your "User" class.

answered Sep 7, 2014 at 6:13

Michael Dautermann

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2 Comments

snaggs Over a year ago

So if it returns copy even if ill write: var temp =user.getList(); temp.append("bb") it doesn't modify original list, right?

David Berry Over a year ago

@fessy, since Array is a struct and returned by value and not reference, it would be meaningless to allow you to modify it in the single statement, as the modified copy would be immediately discarded at the end of the statement (it would then have no remaining references) To make this point clearer, the compiler treats it as a "let" temporary. As Martin and Michael have both pointed out, the only way to modify the copy in the class is to access it directly, or via an append method on the class itself.

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