在实现Equatable的结构体上进行 *** 作

在实现Equatable的结构体上进行 *** 作

正如错误所指出的那样，问题是，您不能将具有“自我”或相关类型要求的协议用作实际类型，因为您会丢失有关这些要求的类型信息。在这种情况下，您将丢失

==

实现参数的类型信息，因为

Equatable

它们必须与一致类型（即

Self

）具有相同的类型。

解决方案几乎总是建立一个类型的橡皮擦。在期望类型相等的情况下，如果它们的

id

属性相等，则可以简单地

id

将其存储起来并在

==

实现中进行比较。

struct AnyVehicle : Equatable {    static func ==(lhs: AnyVehicle, rhs: AnyVehicle) -> Bool {        return lhs.id == rhs.id    }    let id : String    init<T : Vehicle>(_ base: T) {        id = base.id    }}

（请注意，为了符合Swift命名约定，我将您的

ID

属性重

id

命名为）

但是，更通用的解决方案是在类型橡皮擦中存储一个函数，该函数可以在类型转换之后

Vehicle

根据 其

==

实现比较两个任意符合的实例，以确保它们与创建类型橡皮擦的具体类型相同。 。

struct AnyVehicle : Equatable {    static func ==(lhs: AnyVehicle, rhs: AnyVehicle) -> Bool {        // forward to both lhs's and rhs's _isEqual in order to determine equality.        // the reason that both must be called is to preserve symmetry for when a        // superclass is being compared with a subclass.        // if you know you're always working with value types, you can omit one of them.        return lhs._isEqual(rhs) || rhs._isEqual(lhs)    }    let base: Identifiable    private let _isEqual: (_ to: AnyVehicle) -> Bool    init<T : Vehicle>(_ base: T) {        self.base = base        _isEqual = { // attempt to cast the passed instance to the concrete type that // AnyVehicle was initialised with, returning the result of that // type's == implementation, or false otherwise. if let other = 
print(AnyVehicle(Car(id: "foo")) == AnyVehicle(Tractor(id: "foo"))) // falseprint(AnyVehicle(Car(id: "foo")) == AnyVehicle(Car(id: "bar"))) // falseprint(AnyVehicle(Car(id: "foo")) == AnyVehicle(Car(id: "foo"))) // truevar array = [AnyVehicle]()array.append(AnyVehicle(Car(id: "VW")))array.append(AnyVehicle(Car(id: "Porsche")))array.append(AnyVehicle(Tractor(id: "John Deere")))array.append(AnyVehicle(Tractor(id: "Steyr")))var op = Operator()// compiles fine as AnyVehicle conforms to Equatable.op.operationOnCollectionOfEquatables(array: array)
.base as? T {     return base == other } else {     return false }        }    }}

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