I have an array of different structs, all implementing Equatable
protocol and am trying to pass it to a function that expects a collection where T.Iterator.Element: Equatable
. I know how to solve this problem by using classes and just creating a class Vehicle: Identifiable, Equatable
, and then make Car
and Tractor
implement Vehicle
. However I'd like to know if this is possible with using structs and protocols?
Here's a contrived example of what I'm trying to do
//: 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
The problem is, as the error says, you cannot use protocols with Self or associated type requirements as actual types – as you'd lose the type information for what those requirements were. In this case, you'd lose the type information for the parameters of the ==
implementation – as Equatable
says they must be the same type as the conforming type (ie Self
).
The solution is almost always to build a type eraser . In the case of expecting types to be equal if their id
properties are equal, this can be as simple as just storing the id
property and comparing it in the ==
implementation.
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
}
}
(Note that I renamed your ID
property to id
in order to conform with Swift naming convention)
However, a more general solution would be to store a function in the type eraser that can compare two arbitrary Vehicle
conforming instances based on their ==
implementation, after type-casting to ensure they are the same type as the concrete type that the type eraser was created with.
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)
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.