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Date: Sat, 28 Sep 2013 16:23:00 -0000
Message-ID: <CAB_AMN7niBPRf4Mc6Tq=NXnbQacc7DKmM4btodcmKJ4TjOf8Bw@mail.gmail.com>
Subject: Could GDB get offset of a field in virtual base class through NULL pointer
From: hex <heixia108@gmail.com>
To: gdb@sourceware.org
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X-SW-Source: 2013-09/txt/msg00030.txt.bz2

Hi All,

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).
I debugged GDB, and found it try to visit the virtual table of (B *)0,
then it failed.  It is reasonable. But I think whether GDB could
support this case, it could just regard the NULL pointer as a special
case: it could get offset using TYPE_BASECLASS_BITPOS().  But the
TYPE_BASECLASS_BITPOS() returns 0 now because we have not set a value
for virtual base class. Then problem is whether we could store an
offset for virtual base class to support above case.  But I do not
know how to get this offset from sections of its binary. Could you
give some suggestions please? Thank you!