[Bug c++/19605] New: Wrong member offset in inherited classes

[Bug c++/19605] New: Wrong member offset in inherited classes

[peter at os dot inf dot tu-dresden dot de]

gcc-4.0 returns incorrect class relative offsets for members

Following:

1. example program producing the error
2. console output for gcc 4.0 (incorrect) and gcc 3.2 (correct)

============================================================

#include <cstdio>
#include <iostream>


#define GET_MEMBER_PTR(type,member) 					\
  ({									\
    void *type::*p = (void *type::*) &type::member;			\
    &(((type *) 0)->*p);						\
  })

using std::endl;
using std::cout;

class A
{
public:
  unsigned a;
  unsigned b;
};

class B
{
public:
  unsigned c;
  unsigned d;
};

class C
{
public:
  unsigned e;
  unsigned f;
};

class D : public A, public B, public C
{
public:
  unsigned g;
  unsigned h;
};

int
main(void)
{

  cout << "a: " << GET_MEMBER_PTR(D, a) << endl;
  cout << "b: " << GET_MEMBER_PTR(D, b) << endl;
  cout << "c: " << GET_MEMBER_PTR(D, c) << endl;
  cout << "d: " << GET_MEMBER_PTR(D, d) << endl;
  cout << "e: " << GET_MEMBER_PTR(D, e) << endl;
  cout << "f: " << GET_MEMBER_PTR(D, f) << endl;
  cout << "g: " << GET_MEMBER_PTR(D, g) << endl;
  cout << "h: " << GET_MEMBER_PTR(D, h) << endl;

  return 0;
}

===================================================

peter@erwin:~/t5> /usr/local/gcc/head/bin/g++ -v
Using built-in specs.
Configured with: ../gcc/configure --prefix=/usr/local/gcc/head --enable-__cxa_atexit
Thread model: posix
gcc version 4.0.0 20050123 (experimental)
peter@erwin:~/t5> /usr/local/gcc/head/bin/g++ --static -o c.4.x c.cc
peter@erwin:~/t5> ./c.4.x
a: 0
b: 0x4
c: 0
d: 0x4
e: 0
f: 0x4
g: 0x18
h: 0x1c
peter@erwin:~/t5> g++-3.2  -v
Reading specs from /usr/lib/gcc-lib/i386-linux/3.2.3/specs
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,proto,objc,ada --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.2 --enable-shared --with-system-zlib
--enable-nls --without-included-gettext --enable-__cxa_atexit
--enable-clocale=gnu --enable-java-gc=boehm --enable-objc-gc i386-linux
Thread model: posix
gcc version 3.2.3
peter@erwin:~/t5> g++-3.2 -o c.3.x c.cc
peter@erwin:~/t5> ./c.3.x
a: 0
b: 0x4
c: 0x8
d: 0xc
e: 0x10
f: 0x14
g: 0x18
h: 0x1c

-- 
           Summary: Wrong member offset in inherited classes
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: peter at os dot inf dot tu-dresden dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19605

[Bug c++/19605] Wrong member offset in inherited classes

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-24 14:56 -------
    &(((type *) 0)->*p);

That is not offsetof any more.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19605

[Bug c++/19605] [4.0 Regression] Wrong member offset in inherited classes

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-24 15:02 -------
Someone else has to say if this is valid C++ but I think it is just undefined but I could be wrong.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
            Summary|Wrong member offset in      |[4.0 Regression] Wrong
                   |inherited classes           |member offset in inherited
                   |                            |classes
   Target Milestone|---                         |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19605

[Bug c++/19605] [4.0 Regression] Wrong member offset in inherited classes

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-24 15:04 -------
Also one note is that 3.4.0 produces the same as 3.2.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19605

[Bug c++/19605] [4.0 Regression] Wrong member offset in inherited classes

[fm3 at os dot inf dot tu-dresden dot de]

------- Additional Comments From fm3 at os dot inf dot tu-dresden dot de  2005-01-24 15:12 -------
added myself as cc  

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fm3 at os dot inf dot tu-
                   |                            |dresden dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19605

[Bug c++/19605] [4.0 Regression] Wrong member offset in inherited classes

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-01-26 04:12 -------
This is certainly undefined code: you are dereferencing the Null pointer. 
It's a different matter of QoI whether we want to support this anyway. 
It's basically a different way to say offsetof, an area in which I believe 
Nathan has considerable experience. 
 
W. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nathan at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19605

[Bug c++/19605] [4.0 Regression] Wrong member offset in inherited classes

[mmitchel at gcc dot gnu dot org]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2005-01-29 00:00 -------
This is undefined behavior, and it is not worthwhile to try to make this work. 
There are existing ways (offsetof and poiners-to-members) to accomplish what the
submitter wants to do.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19605