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

After lots of research and some trial and error I finally came up with a working solution. The first step was moving the == operator from inside the class to the global scope. This fixed the errors about static and final.

For the base class this became:

func == (lhs: Base, rhs: Base) -> Bool {    return lhs.x == rhs.x}class Base : Equatable {    var x : Int}

And for the subclass:

func == (lhs: Subclass, rhs: Subclass) -> Bool {    return true}class Subclass : Base {    var y : String}

Now the only part left is figuring out how to call the == operator of the base class from the == operator of the subclass. This led me to the final solution:

func == (lhs: Subclass, rhs: Subclass) -> Bool {    if lhs.y == rhs.y {        if lhs as Base == rhs as Base {            return true        }    }    return false}

That first if statement results in a call to the == operator in the base class.

The final solution:

Base.swift:

func == (lhs: Base, rhs: Base) -> Bool {    return lhs.x == rhs.x}class Base : Equatable {    var x : Int}

Subclass.swift:

func == (lhs: Subclass, rhs: Subclass) -> Bool {    if lhs.y == rhs.y {        if lhs as Base == rhs as Base {            return true        }    }    return false}class Subclass : Base {    var y : String}

Following the other answers I came up with this:

class Base : Equatable {    var x : Int    static func == (lhs: Base, rhs: Base) -> Bool {        return lhs.x == rhs.x    }}class Subclass : Base {    var y : String    static func == (lhs: Subclass, rhs: Subclass) -> Bool {        return lhs.y == rhs.y && (lhs as Base) == (rhs as Base)    }}

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? = nillet grandpa4: Grandpa? = nillet father1 = Father()let father2 = Father()let father3 = Father()father3.y = 1let son1 = Son()let son2 = Son()let son3 = Son()son3.z = 1print("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: truegrandpa1 == grandpa3: falsegrandpa3 == grandpa4: truegrandpa1 == father1: falsefather1 == father2: truefather1 == father3: falseson1 == son2: trueson1 == son3: false