为什么我不能在 Swift 中使用泛型类型的子类？

Question

为什么 Swift 不允许我将值 Foo<U> 分配给 Foo<T> 类型的变量，其中 U 是 T 的子类？

例如：

class Cheese {
    let smell: Int
    let hardness: Int
    let name: String

    init(smell: Int, hardness: Int, name: String) {
        self.smell = smell
        self.hardness = hardness
        self.name = name
    }

    func cut() {
        print("Peeyoo!")
    }
}

class Gouda: Cheese {
    let aged: Bool

    init(smell: Int, hardness: Int, name: String, aged: Bool) {
        self.aged = aged
        super.init(smell: smell, hardness: hardness, name: name)
    }

    override func cut() {
        print("Smells delicious")
    }
}

class Platter<Food> {
    var food: Food

    init(food: Food) {
        self.food = food
    }
}

let goudaCheese = Gouda(smell: 6, hardness: 5, name: "Gouda", aged: false)
let goudaPlatter = Platter(food: goudaCheese)  //Platter<Gouda>

//error: cannot assign value of type 'Platter<Gouda>' to type 'Platter<Cheese>'
let platter: Platter<Cheese> = goudaPlatter

但为什么它不起作用？您可以为变量分配一个对象，该对象是其类型的子类，例如

let gouda = Gouda(smell: 6, hardness: 5, name: "Gouda", aged: false)
let cheese: Cheese = gouda

您可以将子类添加到集合中：

let plainCheese = Cheese(smell: 2, hardness: 5, name: "American")
let gouda = Gouda(smell: 6, hardness: 5, name: "Gouda", aged: false)
var cheeses: [Cheese] = [plainCheese]
cheeses.append(gouda)

那么let platter: Platter<Cheese> = goudaPlatter 有什么不同呢？是否有任何情况下如果它起作用会不安全？仅仅是当前版本 Swift 的限制吗？

Answer 1

您可以使用称为type erasure 的技术解决此问题。基本上，您创建了一个“包装器”结构，它从泛型中隐藏了底层类的详细信息。这并不理想，但它可以让您完成与您正在尝试做的事情类似的事情。

class Cheese {
    func doSomethingCheesy() {
        print("I'm cheese")
    }
}

class Gouda: Cheese {
    override func doSomethingCheesy() {
        print("I'm gouda")
    }
}

struct AnyCheese {
    let cheese: Cheese
}

class Container<T> {
    init(object: T) {
        self.object = object
    }
    let object: T
}

let cheese = Cheese()
let cheeseContainer: Container<AnyCheese> = Container(object: AnyCheese(cheese: cheese))

let gouda = Gouda()
let goudaContainer: Container<AnyCheese> = Container(object: AnyCheese(cheese: gouda))

cheeseContainer.object.cheese.doSomethingCheesy() // prints "I'm cheese"
goudaContainer.object.cheese.doSomethingCheesy()  // prints "I'm gouda"