In ATL, you can't derive one coclass from another.

Click heading for discussion.

> I understand why you might want to have a deep hierarchy of
> implementation classes (this style was popular at one point - witness
> MFC - but has fallen out of fashion lately). But why would you want a
> deep hierarchy of _interfaces_?

Because the derived classes add additional functionality [and there must be a new interface to invoke the new functionality]. In my case--without going into irrelevant proprietary details--I have a 'painter' class that paints stuff, a 'control-base' class which implements a basic GUI control around the painted stuff, and a 'control' class which adds extra standard functionality.

Typically the user would either instantiate 'control-base' and customize it (picking and choosing the functionality desired and composing that functionality manually), or just use 'control' for a standard set of functionality. While I suppose this doesn't *have* to be designed as a hierarchy, the code was already written that way before I knew I would have to turn it into a COM component.

>You are busy confusing interface inheritance and implementation
>inheritance. These are two different things.

Actually I never confused the two, although I may have presented my thoughts in a highly confusing manner. Sorry about that! When I said IA and IB share a common interface, the common interface I meant was IA.

Anyway, I have now implemented my COM hierarchy and it's working great. Thanks for your help. I ended up doing pretty much what it says to do at http://vcfaq.mvps.org/com/8.htm -- to summarize what I did, I have

1. Implementation non-template classes A, B, and C (C derives from B derives from A) which don't necessarily use ATL or COM at all
2. COM interfaces IA, IB, IC (IC derives from IB derives from IA)
3. Wrapper classes CImpl<I>, BImpl<I> and AImpl<I> (CImpl<I> derives from BImpl<I> derives from AImpl<I> derives from I); AImpl<I> contains a pointer to an object of type A, B, or C depending on which ATL class is using it
4. ATL classes CA, CB, and CC which are totally separate and unrelated. CA, CB, and CC have base classes AImpl<IA>, BImpl<IB>, and CImpl<IC> respectively.