在实现Equatable的结构体上进行操作

Question

我有一系列不同的结构，都实现了Equatable协议，并试图将其传递给需要collection的函数where T.Iterator.Element: Equatable。我知道如何使用类来解决此问题，只需创建一个class Vehicle: Identifiable, Equatable，然后制作Car并Tractor实现Vehicle。但是我想知道使用结构和协议是否可行？

这是我想要做的一个人为的例子

//: Playground - noun: a place where people can play

protocol Identifiable {
    var ID: String { get set }
    init(ID: String)
    init()
}

extension Identifiable {
    init(ID: String) {
        self.init()
        self.ID = ID
    }
}

typealias Vehicle = Identifiable & Equatable

struct Car: Vehicle {
    var ID: String

    init() {
        ID = ""
    }

    public static func ==(lhs: Car, rhs: Car) -> Bool {
        return lhs.ID == rhs.ID
    }
}

struct Tractor: Vehicle {
    var ID: String

    init() {
        ID = ""
    }

    public static func ==(lhs: Tractor, rhs: Tractor) -> Bool {
        return lhs.ID == rhs.ID
    }
}

class Operator {
    func operationOnCollectionOfEquatables<T: Collection>(array: T) where T.Iterator.Element: Equatable {
    }
}

var array = [Vehicle]() //Protocol 'Equatable' can only be used as a generic constraint because Self or associated type requirements

array.append(Car(ID:"VW"))
array.append(Car(ID:"Porsche"))
array.append(Tractor(ID:"John Deere"))
array.append(Tractor(ID:"Steyr"))

var op = Operator()
op.operationOnCollectionOfEquatables(array: array) //Generic parameter 'T' could not be inferred

Answer 1

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 = $0.base as? T {
                return base == other
            } else {
                return false
            }
        }
    }
}

print(AnyVehicle(Car(id: "foo")) == AnyVehicle(Tractor(id: "foo"))) // false
print(AnyVehicle(Car(id: "foo")) == AnyVehicle(Car(id: "bar"))) // false
print(AnyVehicle(Car(id: "foo")) == AnyVehicle(Car(id: "foo"))) // true

var 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)