Swift override function in extension

Question

If I have a class:

class Spaceship<FuelType> {
    function prepareForLiftoff() throws {
        //Start the countdown!
    }
}

I originally assumed that I would be able to override prepareForLiftoff without subclassing by adding an extension:

extension Spaceship where FuelType: CollectionType {
    func prepareForLiftoff() throws {}
} 

This code doesn't compile though, the error says invalid redeclaration of the function, which makes sense.

My question is: Is there anyway to override a function of a particular class? In other words can I replace the functionality under certain conditions like the example above where FuelType: CollectionType. If not, is there any other workaround or way to achieve that behavior (maybe declaring another protocol, idk)

Now that I think about it more, I would have to say that's not possible because what's to stop someone from overriding any of the standard library functions?

Answer 1

From the documentation:

Extensions can add new functionality to a type, but they cannot override existing functionality.

The documentation lists carefully and precisely what an extension is allowed to do.

As to your question:

Is there anyway to override a function of a particular class

Yes, it's called subclassing.