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

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 Swift so I do not want to involve NSObject or the NSCopying protocol.
I started with a base class and a subclass:
class Base {
var x : Int
}
class Subclass : Base {
var y : String
}
Now I wanted to add Equatable and the == operator to Base. Seems simple enough. Copy the == operator signature from the documentation:
class Base : Equatable {
var x : Int
static func == (lhs: Base, rhs: Base) -> Bool {
return lhs.x == rhs.x
}
}
So far so good. Now for the subclass:
class Subclass : Base {
static override func == (lhs: Base, rhs: Base) -> Bool {
return true
}
}
But this results in an error:
Operator function overrides a 'final' operator function
OK. After some research (still learning Swift 3) I learn that static can be replaced with class to indicate the type method can be overridden.
So I attempt to change static to class in Base:
class Base : Equatable {
var x : Int
class func == (lhs: Base, rhs: Base) -> Bool {
return lhs.x == rhs.x
}
}
But that results in a new error:
Operator '==' declared in non-final class 'Base' must be 'final'
Ugh. This is far more complicated than it should be.
How do I implement the Equatable protocol and the == operator properly in a base class and a subclass?

Answer 1

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
}

Answer 2

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)
}
}

Answer 3

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

Answer 4

Following rmaddy's answer, I came up with a guard approach for testing equality:
Base.swift
static func ==(lhs: Base, rhs: Base) -> Bool {
// ensure class properties match
guard lhs.x == rhs.x else {
return false
}
return true
}
Subclass.swift
static func ==(lhs: Subclass, rhs: Subclass) -> Bool {
// ensure base class properties match
guard lhs as Base == rhs as Base else {
return false
}
// ensure class properties match
guard lhs.y == rhs.y else {
return false
}
return true
}
```