[Bug c++/11072] New: [3.4 regression] Implementation of offsetof macro

[Bug c++/11072] New: [3.4 regression] Implementation of offsetof macro

[bangerth@dealii.org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072

           Summary: [3.4 regression] Implementation of offsetof macro
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: critical
          Priority: P1
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bangerth@dealii.org
                CC: gcc-bugs@gcc.gnu.org
OtherBugsDependingO 10922
             nThis:

[Yet another problem with 10922. It is similar to 9881, which is why I CC:
you, Nathan.]

This fails on mainline:
-----------------------------
#include <cstddef>

template <typename> struct S {
    int i, j;
    static const unsigned int value = offsetof(S,j);
};
-----------------------------

g/x> /home/bangerth/bin/gcc-3.3-pre/bin/c++ -c y.cc

g/x> /home/bangerth/bin/gcc-3.4-pre/bin/c++ -c y.cc
y.cc:5: error: a cast to a type other than an integral or enumeration type
   cannot appear in a constant-expression


Now, in contrast to the claim in PR 10922, I can't find a place in the standard
that would mandate that the result of offsetof is an integer constant
expression. 18.1.5 only says that it can only be applied to POD types, which
certainly is the case here. However, it might be a QoI issue to really
let the result be an integral constant.

W.

PS: The expansion of offsetof yields here:
(reinterpret_cast <size_t> (&reinterpret_cast <char &>(static_cast <S *> (0)->j)));



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[Bug c++/11072] [3.4 regression] Implementation of offsetof macro

[rwgk@yahoo.com]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072



------- Additional Comments From rwgk@yahoo.com  2003-06-03 02:29 -------
ISO/IEC 9899:1999 (E) 

7.17 Common definitions <stddef.h>

3 The macros are

NULL

which expands to an implementation-defined null pointer 
constant; and offsetof(type, member-designator) which 
expands to an integer constant expression that has type
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
size_t, the value of which is the offset in bytes, to the
structure member (designated by member-designator), from 
the beginning of its structure (designated by type). The
type and member designator shall be such that given

static type t;

then the expression &(t.member-designator) evaluates to an 
address constant. (If the specified member is a bit-field,
the behavior is undefined.)




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[Bug c++/11072] [3.4 regression] Implementation of offsetof macro

[bangerth@ices.utexas.edu]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072



------- Additional Comments From bangerth@ices.utexas.edu  2003-06-03 13:48 -------
Subject: Re:  [3.4 regression] Implementation of offsetof
 macro


> ------- Additional Comments From rwgk@yahoo.com  2003-06-03 02:29 -------
> ISO/IEC 9899:1999 (E) 
> 
> 7.17 Common definitions <stddef.h>
> 
> 3 The macros are
> 
> NULL
> 
> which expands to an implementation-defined null pointer 
> constant; and offsetof(type, member-designator) which 
> expands to an integer constant expression that has type
>               ^^^^^^^^^^^^^^^^^^^^^^^^^^^

This provision is only in the C standard. The C++ standard has a different 
wording, which I believe is intentional given that 18.1.2 says
  The contents are the same as in the Standard C library header stddef.h, 
  with the following changes:
followed by definitions of NULL and offsetof.

W.

-------------------------------------------------------------------------
Wolfgang Bangerth              email:            bangerth@ices.utexas.edu
                               www: http://www.ices.utexas.edu/~bangerth/






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[Bug c++/11072] [3.4 regression] Implementation of offsetof macro

[gdr@gcc.gnu.org]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072


gdr@gcc.gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2003-06-03 16:45:03
               date|                            |





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[Bug c++/11072] [3.4 regression] Implementation of offsetof macro

[bangerth@dealii.org]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072



------- Additional Comments From bangerth@dealii.org  2003-06-03 20:00 -------
See also discussion about legality of the code here:
  http://gcc.gnu.org/ml/gcc-bugs/2003-06/msg00482.html



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[Bug c++/11072] [3.4 regression] Implementation of offsetof macro

[cvs-commit at gcc dot gnu dot org]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072



------- Additional Comments From cvs-commit at gcc dot gnu dot org  2003-07-02 14:30 -------
Subject: Bug 11072

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	nathan@gcc.gnu.org	2003-07-02 14:30:53

Modified files:
	gcc            : ChangeLog 
	gcc/ginclude   : stddef.h 
	gcc/testsuite  : ChangeLog 
	gcc/testsuite/g++.dg/other: offsetof2.C 
Added files:
	gcc/testsuite/g++.dg/other: offsetof5.C 

Log message:
	PR c++/11072
	* ginclude/stddef.h (offsetof): Remove cast to 'char &'. Explain why.
	testsuite:
	PR c++/11072
	* g++.dg/other/offsetof2.C: XFAIL.
	* g++.dg/other/offsetof5.C: New.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.300&r2=2.301
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ginclude/stddef.h.diff?cvsroot=gcc&r1=1.18&r2=1.19
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.2828&r2=1.2829
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/other/offsetof5.C.diff?cvsroot=gcc&r1=NONE&r2=1.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/other/offsetof2.C.diff?cvsroot=gcc&r1=1.1&r2=1.2

[Bug c++/11072] [3.4 regression] Implementation of offsetof macro

[nathan at gcc dot gnu dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072



------- Additional Comments From nathan at gcc dot gnu dot org  2003-07-02 14:40 -------
2003-07-02  Nathan Sidwell  <nathan@codesourcery.com>

	PR c++/11072
	* ginclude/stddef.h (offsetof): Remove cast to 'char &'. Explain why.

2003-07-02  Nathan Sidwell  <nathan@codesourcery.com>

	PR c++/11072
	* g++.dg/other/offsetof2.C: XFAIL.
	* g++.dg/other/offsetof5.C: New.

[Bug c++/11072] [3.4 regression] Implementation of offsetof macro

[nathan at gcc dot gnu dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11072


nathan at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


------- Additional Comments From nathan at gcc dot gnu dot org  2003-07-02 14:40 -------
2003-07-02  Nathan Sidwell  <nathan@codesourcery.com>

	PR c++/11072
	* ginclude/stddef.h (offsetof): Remove cast to 'char &'. Explain why.

2003-07-02  Nathan Sidwell  <nathan@codesourcery.com>

	PR c++/11072
	* g++.dg/other/offsetof2.C: XFAIL.
	* g++.dg/other/offsetof5.C: New.