Why does my Xcode compiler tell me I use a value type even though I use classes?

Question

I have two classes: Player and Enemy that both conform to the protocol CharacterCharacteristicsProtocol:

class Enemy: CharacterCharacteristicsProtocol {...
class Player: CharacterCharacteristicsProtocol {...

They are both references and not values like structs are; still when I send in the objects as arguments like this:

viewModel.combatButtonIsClicked(attacker: self.player, defender: self.enemy) { result in...

I get the error

Passing value of type 'CharacterCharacteristicsProtocol' to an inout parameter requires explicit '&'

Why is this showing up? Isn't this only supposed to happen with structs?

If I do as the compiler wish and inset inout and & at the appropriate places things work except for in closures where the error now is

Escaping closure captures 'inout' parameter 'characterVM'

Here's where it happens (just for completion):

func enemyTurn(enemyVM: CharacterCharacteristicsProtocol, characterVM: inout CharacterCharacteristicsProtocol, completion: @escaping(_ enemyReponse: String) -> Void){
    let xEnemy = enemyVM.getX()
    let yEnemy = enemyVM.getY()
        
    viewModel.enemyShouldMove = true

    viewModel.proximityCheck(checkProxyForWho: .enemy, i: xEnemy, j: yEnemy, completion: {
        let combat = Combat()
        combat.combat(attacker: enemyVM, defender: &characterVM, completion: { result in...

I have searched on how to solve this error and the get the following suggestion: "change the struct to a class"...

Check this: https://stackoverflow.com/q/31960117/1187415 – you probably want to declare CharacterCharacteristicsProtocol as a “class protocol” — Martin R, Sep 09 '20 at 19:06

score 1 · Accepted Answer · answered Sep 09 '20 at 19:07

1

Is your protocol class bound? If not, the compiler needs to assume that a struct may also implement it and needs to apply value semantics.
To make a protocol class bound you simply need to do like this:
protocol CharacterCharacteristicsProtocol: class

Then you will only be able to implement it with classess, and not structs, and the compiler will be able to assume that only reference semantics apply.

answered Sep 09 '20 at 19:07

Losiowaty

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Just a minor side note: `: class` is considered less favorable; `: AnyObject` is a preferred way to limit a protocol adoption to class types. – Vadim Belyaev Sep 09 '20 at 21:08
@Losiowaty Ah, ok, I didn't know that. So even though you only use classes you specifically need to resdtrict the protocol. Thanks a bunch for clearing that up! – Joakim Sjöstedt Sep 10 '20 at 06:50
@VadimBelyaev Ok, good to know! Could you just elaborate a bit on why it is more favorable to use AnyObject. And doesn't AnyObject incluce structs as well? – Joakim Sjöstedt Sep 10 '20 at 06:51
@JoakimSjöstedt, the `: class` is no longer mentioned in the official Swift documentation therefore I'd speculate it may be deprecated eventually. There's a subtle difference between `: class` and `: AnyObject` as discussed here: https://stackoverflow.com/questions/30176814/whats-the-difference-between-a-protocol-extended-from-anyobject-and-a-class-onl/32895975#32895975 In practice the two will have identical behavior in most cases. A struct can't conform to `AnyObject` (compiler will raise an error), therefore a struct can't conform to a class-only protocol declared with `: AnyObject`. – Vadim Belyaev Sep 10 '20 at 20:29

Why does my Xcode compiler tell me I use a value type even though I use classes?

1 Answers1