Need help in debugging a stack corruption

[Robert Shearman]

Stefan Dösinger wrote:

>IDirectDraw7Vtbl IDirectDraw7_Vtbl = {
>	/* IUnknown */
>	...
>	/* IDirectDraw7 */
>	...
>	IDirectDrawImpl_SetCooperativeLevel,
>	IDirectDrawImpl_SetDisplayMode,
>	...
>}
>
>IDirectDraw4Vtbl IDirectDraw4_Vtbl = {
>	/* IUnknown */
>	...
>	/* IDirectDraw4 */
>	...
>	( void * ) IDirectDrawImpl_SetCooperativeLevel,
>	( void * )	IDirectDrawImpl_SetDisplayMode,
>	...
>}
>  
>

You can't do this. In order to access the object that the vtable is part 
of you need to subtract an offset from the vtable pointer. If you use 
the same function for two vtables then which offset do you use? The 
answer: both or neither - it is impossible. Therefore, even though the 
implementation of SetCooperativeLevel may be the same for the second 
vtable, you still need to implement another function for it, even if it 
just forwards to the true implementation.

You should never cast function pointers in vtables. The risks are just 
too great.

-- 
Rob Shearman