How to properly implement the Equatable protocol in a class hierarchy?

Question

I\'m trying to implement the == operator (from Equatable) in a base class and its subclasses in Swift 3. All of the classes will only be used in Sw

Answer 1

I know it's been a while since the question is posted, but I hope my answer helps.

TLDR -- Instead of trying to override ==, you provide a custom comparing method, make == call it, and override the custom comparing method if needed.

---

So you said

All of the classes will only be used in Swift so I do not want to involve NSObject or the NSCopying protocol.

But if you were to subclass NSObject, how will you write your custom comparison method? You will override isEqual(Any?), right? And if you try to conform to Equatable protocol in your subclass, compiler will complain about "Redundant conformance to protocol Equatable" because NSObject already conformed to Equatable.

Now that gives us some hints about how NSObject handles this problem -- it provides a custom comparing method isEqual(Any?), call it inside ==, and its subclasses can override it if needed. You can do the same in your own base class.

Without further ado, let's do some experiments(in Swift 4). Define some classes

class Grandpa: Equatable {
    var x = 0

    static func ==(lhs: Grandpa, rhs: Grandpa) -> Bool {
        return lhs.isEqual(to: rhs)
    }

    func isEqual(to object: Any?) -> Bool {
        guard object != nil && type(of: object!) == Grandpa.self else {
            return false
        }
        let value = object as! Grandpa
        return x == value.x
    }
}

class Father: Grandpa {
    var y = 0

    override func isEqual(to object: Any?) -> Bool {
        guard object != nil && type(of: object!) == Father.self else {
            return false
        }
        let value = object as! Father
        return x == value.x && y == value.y
    }
}

class Son: Father {
    var z = 0

    override func isEqual(to object: Any?) -> Bool {
        guard object != nil && type(of: object!) == Son.self else {
            return false
        }
        let value = object as! Son
        return x == value.x && y == value.y && z == value.z
    }
}

And write some test code

let grandpa1 = Grandpa()
let grandpa2 = Grandpa()
let grandpa3: Grandpa? = nil
let grandpa4: Grandpa? = nil
let father1 = Father()
let father2 = Father()
let father3 = Father()
father3.y = 1
let son1 = Son()
let son2 = Son()
let son3 = Son()
son3.z = 1

print("grandpa1 == grandpa2: \(grandpa1 == grandpa2)")
print("grandpa1 == grandpa3: \(grandpa1 == grandpa3)")
print("grandpa3 == grandpa4: \(grandpa3 == grandpa4)")
print("grandpa1 == father1: \(grandpa1 == father1)")
print("father1 == father2: \(father1 == father2)")
print("father1 == father3: \(father1 == father3)")
print("son1 == son2: \(son1 == son2)")
print("son1 == son3: \(son1 == son3)")

Run it and you should get

grandpa1 == grandpa2: true
grandpa1 == grandpa3: false
grandpa3 == grandpa4: true
grandpa1 == father1: false
father1 == father2: true
father1 == father3: false
son1 == son2: true
son1 == son3: false