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Re: Could GDB get offset of a field in virtual base class through NULL pointer
- From: Jan Kratochvil <jan dot kratochvil at redhat dot com>
- To: hex <heixia108 at gmail dot com>
- Cc: gdb at sourceware dot org
- Date: Mon, 30 Sep 2013 10:16:26 +0200
- Subject: Re: Could GDB get offset of a field in virtual base class through NULL pointer
- Authentication-results: sourceware.org; auth=none
- References: <CAB_AMN7niBPRf4Mc6Tq=NXnbQacc7DKmM4btodcmKJ4TjOf8Bw at mail dot gmail dot com> <20130928183852 dot GA12891 at host2 dot jankratochvil dot net> <CAB_AMN5ngrKWwwVwUBFQ1wS5fLiDCyDRO+qe9bOjPBJAMo-05Q at mail dot gmail dot 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