Re: Could GDB get offset of a field in virtual base class through NULL pointer

[Jan Kratochvil <jan.kratochvil@redhat.com>]

On Sun, 29 Sep 2013 03:59:54 +0200, hex wrote:
> > I do not see what it should do.  In the following case &(((B *)&OBJECT)->a)
> > prints once 12 and once 16 for different OBJECT so what it should print for 0?
> >
> > class X:public virtual A,public B {};
> > class C {
> > public:
> >         int c;
> > };
> > class Y:public virtual A,public C,public B {};
> > #include <iostream>
> > int main() {
> >         X x;
> >         Y y;
> >         std::cout << (char *)&(((B *)&x)->a)-(char *)&x << std::endl;
> >         std::cout << (char *)&(((B *)&y)->a)-(char *)&y << std::endl;
> > }
> >
> 
> If we use &(((B *)0)->a), we are likely to get offset of 'a' in class
> B. If GDB could
> support this specific case, we do not need a real object to get the offset.

This would apply if you had s/virtual A/A/.  But with the inheritance of
A being virtual the memory location of A inside the whole object instance is
"random", it does not depend on B but it depends on X or Y.  Specifically it
depends on virtual tables used for the specific instance, the virtual tables
specify the location of A.  This is what I am trying to show you in the
example above.

The same expression (char *)&(((B *)&OBJECT)->a) produces different result
depending on which OBJECT you pass there.  Therefore which result should
produce passing 0 instead of &OBJECT there?  It cannot be a single number.


Jan Kratochvil