swift - 我可以将 `inout` 与协议(protocol)扩展一起使用吗？

Question

我有一个协议(protocol)及其扩展，以及一个符合该协议(protocol)的类。

protocol WarAbilities {
    var strength: Int { get set }
    func attack(inout opponent: WarAbilities)
}

extension WarAbilities {
    func attack(inout opponent: WarAbilities) {
        opponent.strength -= 1
    }
}

class Warrior: WarAbilities {
    var strength: Int

    init(strength: Int) {
        self.strength = strength
    }
}

现在如果我想让两个战士打架:

let thug1 = Warrior(strength: 10)
let thug2 = Warrior(strength: 30)

thug1.attack(&thug2)

我收到此错误消息:

error: cannot pass immutable value of type 'WarAbilities' as inout argument

添加 mutating 看起来很有希望:

protocol WarAbilities {
    var strength: Int { get set }
    mutating func attack(inout opponent: WarAbilities)
}

extension WarAbilities {
    mutating func attack(inout opponent: WarAbilities) {
        opponent.strength -= 1
    }
}

但编译器也不高兴，我无法理解新错误消息的含义:

error: cannot pass immutable value as inout argument: implicit conversion from 'Warrior' to 'WarAbilities' requires a temporary

因为 Warrior 符合 WarAbilities，我认为其中一个会起作用——但看起来 Swift 没有这种……协方差？我什至不确定我在说什么。

我的错误是什么？

Answer 1

让它成为一个 class 协议(protocol)并摆脱(当时)不必要的 inout 东西:

protocol WarAbilities : class {
    var strength: Int { get set }
    func attack(opponent: WarAbilities)
}

extension WarAbilities {
    func attack(opponent: WarAbilities) {
        opponent.strength -= 1
    }
}

class Warrior: WarAbilities {
    var strength: Int

    init(strength: Int) {
        self.strength = strength
    }
}

let thug1 = Warrior(strength: 10)
let thug2 = Warrior(strength: 30)

thug1.attack(thug2)

thug2.strength // 29

(事实上，我不太清楚为什么你在这里需要一个协议(protocol)；因为 Warrior 是一个类，你可以将 WarAbilities 作为它的父类(super class)。)