COM inheritance Q

[Ann and Jason Edmeades]

>>I have code which looks like:
>> 
>>HRESULT WINAPI IWineD3DSurfaceImpl_GetParent(IWineD3DSurface *iface,
>>IUnknown **pParent) {
>>    return IWineD3DResource_GetParent((IWineD3DResource *)iface, pParent);
>>}
>>
>>IWineD3DResource_GetParent is a macro:
>>#define IWineD3DSurface_GetParent(p,a)
>>(p)->lpVtbl->GetParent(p,a)
>>
>>So we then go iface->lpVtbl->GetTable. Since iface is a surface, once cast
>>to a resource its GetParent is in the same place in the table... Hence the
>>call ends up being effectively
>>
>>HRESULT WINAPI IWineD3DSurfaceImpl_GetParent(IWineD3DSurface *iface,
>>IUnknown **pParent) {
>>    return IWineD3DSurfaceImpl_GetParent (iface, pParent);
>>}
>>
>>Whats the solution?
>>
>>In d3d8 I would have coded IWineD3DSurfaceImpl_GetParent as
>>IWineD3DResourceImpl_GetParent(iface, pParent) because the Impl versions
>>were prototyped - Is this the only way to solve this?
>>  
>>

>I don't think I've got enough information to answer you correctly. Is 
>this a multiple inheritance problem? I.e. does the object implement both 
>IWineD3DSurface and IWineD3DResource and not have one directly 
>inheriting from the other?

No, a single inheritance issue. Let me try to explain...

Resource is defined as having functions A,B,C and its vtbl is {a,b,c}
Surface is defined as having functions A,B,C,D,E,F and its vtbl is
{a,b,c,d,e,f}. Note 'a' is an entrypoint to surface not resource, since that
wasn't possible (see another thread a long time ago!)...

So in surface_A I want to call resource_a, and code it as expected, ie
Resource_a((resource) iface, parms)... But the entrypoint, regardless of the
casting, is got from the first entry in the vtbl, which (again, regardless
of the casting) is surfaces a function...

Better?
Jason