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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: Sat, 28 Sep 2013 20:38:52 +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>
On Sat, 28 Sep 2013 18:23:22 +0200, hex wrote:
> I defined two classes as following:
> // test.cpp
> class A{
> public:
> int a;
> };
> class B: public virtual A{
> public:
> int b;
> };
>
> GDB could print &(((B *)0)->a), but it could not print &(((B *)0)->a).
^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^
Those two expressions are the same and they really od not work:
(gdb) p &(((B *)0)->a)
Cannot access memory at address 0x0
> it could just regard the NULL pointer as a special
> case: it could get offset using TYPE_BASECLASS_BITPOS().
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;
}
Jan Kratochvil