COM inheritence

[Ann and Jason Edmeades]

Hiya,

If you havent noticed, I'm trying to port the d3d8 code into wined3d and
make it common for use from d3d9, and this is all COM objects. I have come
across something which in theory would enable me to tidy my code simply, but
I cant see how to make it work in C.

Now I have a class, for example FRED who implements some common functions,
and all the code for it would be in fred.c. I also have a subclass, defined
called JIM, which needs to have entrypoints as per fred, plus some more, and
its code resides in jim.c. Still with me...?

Now previously, inside jim, I have duplicated all the common functions, but
I want to avoid this and just call the one definition. My plan was to change
the Vtbl to look like:
IJimVtbl Jim_Vtbl =
{
    IJimImpl_QueryInterface,
    IJimImpl_AddRef,
    IJimImpl_Release,
    IFredImpl_CommonFunc1,
    IFredImpl_CommonFunc2,
      :
    IJimImpl_UniqueFunc1 etc
}

I was also ensuring the variable definitions in the IJimImpl structure map
precisely over the IFredImpl structure, before any unique variables are
defined (I think this is a necessity for emulating subclassing in C?).

Now, obviously if the common funcs need changing I can copy in the code, and
change the vtbl to have a local override but at the moment I dont need to
(they are mostly stubs even in the d3d8 code today).

However, doing this fails to compile - It wont allow me to have a vtbl in
jim.c which includes pointers to functions which are in fred.c, even if I
have extern prototypes defined. Basically this is because array
initialization requires constant values.

However, this would seem something which is so common - ie inheritence is
used all over the place. How do people handle this? I've looked through a
few places in the code and cant see how this can be done. I could 'pretend'
by initializing the Vtbl as I create the com object and do some LoadLibrary
type dynamic addressing but I really want link time resolution not load time
resolution.

One solution would be to put the common routines in a header file, and
#include it, but code in headers or #including modules is uaually frowned
upon! I cant see any sensible way of doing this and I'd rather not do lots
of code duplication if avoidable as this common class is used in about 7
other classes!

Jason