Question

is the order of overloaded methods in the vtable always the same across win32 compilers?

Problem:
I have “interfaces” (pure virtual classes with no data members). They can be used via pointer from different compilers (the client gets the pointer by calling a standard c dll factory method). This works fine across different compiler (e.g. client written with borland, interface dll written with Visual C++) except for one method.
This method is overloaded with the same return value but different parameter. There are 4 versions of this method. The same call to this method returns different results depending on the compiler that compiled the client. A quick look a the assembler code showed me that there seems to be a different offset into the vtable (I’m not really good at reading assembler).

Now I don’t know – did I find the cause or is borland just handling the vtable different to visual studio and everything is correct and I have to search elsewhere.

best regards and thank you for your answers

Tobias

Answer 1

There are two possible causes: either the client compiler is choosing a different overload than you expect, or the different compilers are putting the overloads in different vtable entries.

What parameters are you passing/expecting? Could overload resolution be the problem?

If it’s the vtable entries then you could try renaming the overloads.

Have you tried using whatever COM mechanism is available on your target compilers when declaring the interface — e.g. using the interface keyword in MSVC, and giving your interface class a GUID. COM interfaces are supposed to have the functions in the vtable in the order declared, which is therefore common between compilers if they share the same header.