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References: <CAB_AMN7niBPRf4Mc6Tq=NXnbQacc7DKmM4btodcmKJ4TjOf8Bw@mail.gmail.com>	<20130928183852.GA12891@host2.jankratochvil.net>
Date: Sun, 29 Sep 2013 01:59:00 -0000
Message-ID: <CAB_AMN5ngrKWwwVwUBFQ1wS5fLiDCyDRO+qe9bOjPBJAMo-05Q@mail.gmail.com>
Subject: Re: Could GDB get offset of a field in virtual base class through NULL pointer
From: hex <heixia108@gmail.com>
To: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: gdb@sourceware.org
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X-SW-Source: 2013-09/txt/msg00032.txt.bz2

>> 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
>

Sorry, a mistake, I meant "GDB could print &(((B *)0)->b), but it could not
print &(((B *)0)->a)."

> 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.