[Python-Dev] type categories

[David Abrahams]

From: "Guido van Rossum" <guido@python.org>

> > > We'd need a trick to deny an interface that would be inherited by
> > > default.  Something like private inheritance.
> >
> > I think it's more than that. You might need to "uninherit": Say
> > Interface A begets class B which begets class C. What if C doesn't
> > fulfill A?
>
> Sorry, I meant to include that case.  How do you do that in C++?

We don't use inheritance for this kind of interface. When we're making a
Java-style interface, sure, inheritance works fine in C++. However, because
of Python's dynamic, generic nature what we've been calling an interface
for Python is much more like a "concept", which has no direct expression in
code:

http://www.boost.org/more/generic_programming.html#concept

Actually if you read a little further on down the page (the Traits and Tag
Dispatching sections), you'll see that it's possible to create an
expression in code of a concept in C++. Usually you want to do that when
concepts form a refinement hierarchy (e.g. bidirectional_iterator refines
forward_iterator) which may or may not correspond to inheritance
relationships.

> Inherit privately from B and publicly from A, and making A virtual
> base everywhere?


I guess you /could/ do that. I don't think anyone does, though ;-)

I was going to say that is seems to me if you can dynamically inject base
classes in Python there's no problem using inheritance to do this sort of
labelling. However, on third though, maybe there is a problem. Suppose you
have an inheritance chain A->B->C...->Z and I come a long later to say that
A fulfills interface II and add II to A's bases. Which of A's subclasses
also fulfill II. I might not know. I might not even know about them. For
this, maybe you'd need a way to express inheritance that goes just "one
level deep" (i.e. A inherits II publicly, but nothing else does). And that
might just screw with the notion of inheritance enough that you want a
separate parallel mechanism.

So I guess I'm back to where I was before. Inheritance probably doesn't
work out too well for expressing "satisfies interface".


-----------------------------------------------------------
           David Abrahams * Boost Consulting
dave@boost-consulting.com * http://www.boost-consulting.com