Re: What should the call stack look like when calling a C++ virtual function?

[Paul Pluzhnikov]

"Dev1024" <lmemmott@yahoo.com> writes:

> Having been a long time user of Microsoft Visual C++ .Net 2003 (MSVC),
> I've recently been using g++ on a project.  I've found a difference
> between the two compilers when calling a virtual function that has me
> puzzled.

The immediate question is "why do you care?"

> The issue is when calling virtual functions.  In MSVC the parameters
> are pushed on to the stack as expected and then the address of the
> function is loaded into EAX.  Finally "call EAX" is issues to call the
> virtual function.

That is incorrect: you are missing a step: loading "this" into %esi.

> In g++ (Cygwin g++ v3.4.4) the parameters are pushed
> onto the stack as expected, but then a pointer (32 bit pointer in my
> case) to the vtable is pushed on the stack as well

That is also incorrect: it's the "this" parameter that is pushed.
However, since g++ puts the vtable at the start of any class with
virtual functions, the "this" and &vtbl are the same thing.

> - Why does g++ load a pointer to the vtable on the stack after pushing
> on the params?

It doesn't. It simply passes all parameters (including "this")
on the stack.

> - Is there a compiler option that can be used to avoid putting a
> pointer to the vtable on the stack?  (I looked but couldn't find one.)

These are implementation details, best left alone.
Are you trying to link together MSVC and g++ compiled code?

Cheers,
-- 
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