John Labenski <jlaben...@...> writes:
>
> wxLua 2.8.10 introduced multiple inheritance in the C++ bindings which
> wxWidgets uses for the wxComboBox, wxChoice, and wxBitmapComboBox
> classes. However, I was not aware that C++ compilers shift the vtable
> of the second and higher base classes and therefore if you directly
> cast a void* pointer to an object to its second or higher base class
> you will segfault when you try to call one of its functions. I have
> finally come across a solution that does work for GCC and MSVC, but it
> is compiler implementation dependent. The only other compiler that I
> think we may care about is the Intel one, which I cannot test. The
> bottom line is that the code will probably not even compile or
> immediately segfault when called if GCC or MSVC change their code
> generation (unlikely), which is fine since we can fix it then, for the
> foreseeable future this will work even though it may upset C++
> purists.
>
Maybe I do not undestand the problem clearly.
As you mentioned v-tables vary from compiler to compiler and sometime the
linker changes them as well. They are not only incompatible across compiler
they have been known to be incompatible from version to version. It is very
difficult and sometime impossible to past (void *) pointers and then cast them
properly. "dynamic_cast" tries to address this problem but does not always do
it across compilers. Since you know which class you want to access dynamic
cast seems to be the best choice.
// TestCasting->cpp
//
#include "stdafx.h"
class A { public: int x; };
class B { public: int y; };
class AB : public A, public B { public: int z; };
void f(void *p)
{
AB* ab = (AB *)p;
A* a = dynamic_cast<A*>(ab);
B* b = dynamic_cast<B*>(ab);
printf("%x\n", p); // 347e28
printf("%x\n", ab); // 347e28
printf("%x\n", a); // 347e28
printf("%x\n", b); // 347e2c
}
int _tmain(int argc, _TCHAR* argv[])
{
AB *ab = new AB() ;
f((void *)ab);
return 0;
}
If you knew which class is being past to procedure f typing the argument would
save you the first cast and provide some checking redundancy.
Andre
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