Re: "The last feature": overridable methods in interfaces

[Andrei Alexandrescu]

Lars T. Kyllingstad wrote:

Andrei Alexandrescu wrote:

Walter has now implemented final methods in interfaces and also contracts in interfaces, both of which I think are just awesome.

We figured that essentially he artificially disallows interfaces from providing bodies for methods. I think that's a gratuitous limitation; the only distinguishing quality of an interface is that it has no state. Other than that, interfaces can always offer overridable functions that by default offer functionality in terms of existing interface functions. For example:

interface Stack(T)
{
    void push(T);
    void pop();
    @property ref T top();
    @property bool empty();
    T belowTop()
    {
        auto t = top;
        pop();
        auto result = top;
        push(t);
    }
}

The default implementation of belowTop does a fair amount of work. A particular implementation might just use that or override it with a more efficient implementation.

Many more examples can be imagined, but I'm looking for a killer one, or perhaps a killer counterexample (e.g. when would an interface-defined method be really bad?)

Your thoughts welcome.

What happens when two interfaces implement the same method?

Ambiguity error.

I thought multiple subtyping without multiple inheritance was the raison d'être for interfaces.

Interfaces do foster multiple inheritance!

Andrei