== overload for custom class is not always called

Question

I have a custom operator defined globally like so:

func ==(lhs: Item!, rhs: Item!)->Bool {
    return lhs?.dateCreated == rhs?.dateCreated
}

Answer 1

There are two main problems with what you're trying to do here.

1. Overload resolution favours supertypes over optional promotion

You've declared your == overload for Item! parameters rather than Item parameters. By doing so, the type checker is weighing more in favour of statically dispatching to NSObject's overload for ==, as it appears that the type checker favours subclass to superclass conversions over optional promotion (I haven't been able to find a source to confirm this though).

Usually, you shouldn't need to define your own overload to handle optionals. By conforming a given type to Equatable, you'll automatically get an == overload which handles equality checking between optional instances of that type.

A simpler example that demonstrates the favouring of a superclass overload over an optional subclass overload would be:

// custom operator just for testing.
infix operator <===>

class Foo {}
class Bar : Foo {}

func <===>(lhs: Foo, rhs: Foo) {
    print("Foo's overload")
}

func <===>(lhs: Bar?, rhs: Bar?) {
    print("Bar's overload")
}

let b = Bar()

b <===> b // Foo's overload

If the Bar? overload is changed to Bar – that overload will be called instead.

Therefore you should change your overload to take Item parameters instead. You'll now be able to use that overload to compare two Item instances for equality. However, this won't fully solve your problem due to the next issue.

2. Subclasses can't directly re-implement protocol requirements

Item doesn't directly conform to Equatable. Instead, it inherits from NSObject, which already conforms to Equatable. Its implementation of == just forwards onto isEqual(_:) – which by default compares memory addresses (i.e checks to see if the two instances are the exact same instance).

What this means is that if you overload == for Item, that overload is not able to be dynamically dispatched to. This is because Item doesn't get its own protocol witness table for conformance to Equatable – it instead relies on NSObject's PWT, which will dispatch to its == overload, simply invoking isEqual(_:).

^{(Protocol witness tables are the mechanism used in order to achieve dynamic dispatch with protocols – see this WWDC talk on them for more info.)}

This will therefore prevent your overload from being called in generic contexts, including the aforementioned free == overload for optionals – explaining why it doesn't work when you attempt to compare Item? instances.

This behaviour can be seen in the following example:

class Foo : Equatable {}
class Bar : Foo {}

func ==(lhs: Foo, rhs: Foo) -> Bool { // gets added to Foo's protocol witness table.
    print("Foo's overload")           // for conformance to Equatable.
    return true
}

func ==(lhs: Bar, rhs: Bar) -> Bool { // Bar doesn't have a PWT for conformance to
    print("Bar's overload")           // Equatable (as Foo already has), so cannot 
    return true                       // dynamically dispatch to this overload.
}

func areEqual<T : Equatable>(lhs: T, rhs: T) -> Bool {
    return lhs == rhs // dynamically dispatched via the protocol witness table.
}

let b = Bar()

areEqual(lhs: b, rhs: b) // Foo's overload

So, even if you were to change your overload such that it takes an Item input, if == was ever called from a generic context on an Item instance, your overload won't get called. NSObject's overload will.

This behaviour is somewhat non-obvious, and has been filed as a bug – SR-1729. The reasoning behind it, as explained by Jordan Rose is:

[...] The subclass does not get to provide new members to satisfy the conformance. This is important because a protocol can be added to a base class in one module and a subclass created in another module.

Which makes sense, as the module in which the subclass resides would have to be recompiled in order to allow it to satisfy the conformance – which would likely result in problematic behaviour.

It's worth noting however that this limitation is only really problematic with operator requirements, as other protocol requirements can usually be overridden by subclasses. In such cases, the overriding implementations are added to the subclass' vtable, allowing for dynamic dispatch to take place as expected. However, it's currently not possible to achieve this with operators without the use of a helper method (such as isEqual(_:)).

The Solution

The solution therefore is to override NSObject's isEqual(_:) method and hash property rather than overloading == (see this Q&A for how to go about this). This will ensure that your equality implementation will always be called, regardless of the context – as your override will be added to the class' vtable, allowing for dynamic dispatch.

The reasoning behind overriding hash as well as isEqual(_:) is that you need to maintain the promise that if two objects compare equal, their hashes must be the same. All sorts of weirdness can occur otherwise, if an Item is ever hashed.

Obviously, the solution for non-NSObject derived classes would be to define your own isEqual(_:) method, and have subclasses override it (and then just have the == overload chain to it).