DX6 more compatibility.

[Stefan Dösinger]

> If I get it right, the correct fix is to add a thunk vtable for
> IDirectDraw4 that uses relaxed parameter checks on AddAttachedSurface.
Most likely yes

> [1] In fact, an API test is required. Strict or relaxed semantics might
also > depend on how the surface was created (from IDirectDraw4 or
> IDirectDraw7) instead of the interfaced used to add an attached surface.
> This would be strange IMO, as the idea about COM is that the interface
> describes semantics that work regardless how the object was created, but
> let's test that, nevertheless.
Yes, a test is needed, no way around that. Other IDirectDrawSurface methods
depend on how the object was created indeed, like GetDDInterface.

On a sidenode, I think it is impossible to QueryInterface
IDirectDrawSurface7 from IDirectDrawSurface < 7, or vice versa, but I am not
sure. I think there are tests for IDirectDrawX, more tests would never hurt.

> I wonder if  there is an easy way to do an API abstraction of some
> sort... not with a flag perse but like you can in C++ with the
> different
> instanciations depending upon the parms passed. Then instead of
> thunking
> , which is inherently buggy and tends to be slow,  you use the v7
> structure and return it without the v7 specific items at the end.
> That way you store one table instead of 2
I recommend to read how object orientation and inheritance are implemented
on assembler level by the C++ compiler. It's multiple vtables and thunking
in many places.

The C++ equivalent here would be to keep 2 vtables, which differ only in the
implementation of the AddAttachedSurface and Flip functions, and otherwise
use the same function pointers without thunks. C++ uses thunks e.g. in the
case of multiple inheritance. A vtable construct like IDirectDrawSurface3,
IDirectDrawSurface4, IDirectDrawSurface7 and IDirectDrawGammaControl on the
same object in the same time is not really expressable as far as I can see.

Thunks are not a pressing performance issue in general, but I want to avoid
them if its possible in a clean way, since the 0.5-1.0% performance gains
are where you gain your framerate differences that matter to the user.