[英]Operation on an array of structs implementing Equatable
我有一组不同的结构,都实现了Equatable
协议,我试图将它传递给一个需要where T.Iterator.Element: Equatable
集合的函数。 我知道如何通过使用类来解决这个问题,只需创建一个class Vehicle: Identifiable, Equatable
,然后使Car
和Tractor
实现Vehicle
。 但是,我想知道使用结构和协议是否可行?
这是我正在尝试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
问题是,正如错误所说,您不能将具有Self或相关类型要求的协议用作实际类型 - 因为您丢失了这些要求的类型信息。 在这种情况下,您将丢失==
实现的参数的类型信息 - 正如Equatable
所说,它们必须与符合类型(即Self
)的类型相同。
解决方案几乎总是构建一个类型橡皮擦 。 如果期望类型相等,如果它们的id
属性相等,这可以像存储id
属性并在==
实现中进行比较一样简单。
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
}
}
(注意我将您的ID
属性重命名为id
以符合Swift命名约定)
但是,更通用的解决方案是在类型擦除器中存储一个函数,它可以在类型转换后根据它们的 ==
实现比较两个任意的符合Vehicle
实例,以确保它们与类型橡皮擦的具体类型相同是用。创建的。
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)
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