Operator overload "<" declared in Swift class is sometimes not called when used in Objective-C

Question

I've come across a strange issue related to Swift/Objective-C interoperability. The case is this:

I have an @objc public class GKDistance:NSObject,NSCoding,Comparable written in Swift. In order to compare distances, I have added an operator overload for < as follows:

public func <(a:GKDistance, b:GKDistance) -> Bool {
    return a.value < b.value
}

public func ==(a:GKDistance, b:GKDistance) -> Bool {
    return a.value == b.value
}

It's then used in an Objective-C method like so:

if (distance < averageDistance){
    // code
}

When the Objective-C method is called I can add a print() statement and breakpoint in the < method to confirm when the operator overload is being used. In one case, it mysteriously skips the operator overload defined in Swift, and uses the regular Objective-C comparator between two GKDistance objects.

When run with distance.value == 2375.1842554877021 and averageDistance.value == 75.671794891357421, distance < averageDistance returns true, the Swift operator overload is never hit, and Objective-C executes the code inside the conditional.

If I convert the Objective-C method in question to Swift, it behaves as expected, but I'm concerned that there are other GKDistance comparisons in various Objective-C methods throughout our app that could be failing to see the Swift operator overload.

Has anyone run into similar issues with Swift/Objective-C interoperability as it pertains to operator overloading?

Answer 1

Your

public func <(a:GKDistance, b:GKDistance) -> Bool { }

is – like all Swift operators – a top-level function, and top-level functions are not exported to Objective-C, see "Swift Type Compatibility" in "Interacting with Objective-C APIs".

Also note that you cannot override operators (like <) in (Objective-)C, see e.g.

• Operator overloading in C
• Is it possible to overload operators in C?

Therefore in

if (distance < averageDistance) {
    // code
}

just the two pointers are compared, and the condition is true if the object pointed to by distance resides in a lower memory address than the object pointer to by averageDistance (which is obviously not what you intended).

What you could do is to implement a compare: method in your class, similar to the one of NSString or NSNumber.

Note also that for NSObject subclasses, you should override isEqual: instead of implementing ==, compare

• NSObject subclass in Swift: hash vs hashValue, isEqual vs ==