Accessing COM Objects by Using C

Any COM interface method can be called from a C program. There are two things to remember when calling an interface method from C:

• The first parameter of the method always refers to the object that has been created and that invokes the method (the this argument).
• Each method in the interface is referenced through a pointer to the object's vtable.

The following example creates a surface associated with a DirectDraw object by calling the IDirectDraw2::CreateSurface method with the C programming language:

ret = lpDD->lpVtbl->CreateSurface (lpDD, &ddsd, &lpDDS, 
      NULL); 
 

The lpDD parameter references the DirectDraw object associated with the new surface. Incidentally, this method fills a surface-description structure (&ddsd) and returns a pointer to the new surface (&lpDDS).

To call the IDirectDraw2::CreateSurface method, first dereference the DirectDraw object's vtable, and then dereference the method from the vtable. The first parameter supplied in the method is a reference to the DirectDraw object that has been created and which invokes the method.

To illustrate the difference between calling a COM object method in C and C++, the same method in C++ is shown below (C++ implicitly dereferences the lpVtbl parameter and passes the this pointer):

ret = lpDD->CreateSurface(&ddsd, &lpDDS, NULL)