Re: offsetof() macro, C++, and GCC 3.4.2

At Joseph Koshy's request, this is now filed as misc/72307.

 -Patrick

On Sun, 2004-10-03 at 20:02 -0500, Patrick Hartling wrote:
> The offsetof() macro defined in stddef.h evaluates to code that can
> violate the C++ standard depending upon the context of its use.  I have
> attached code that demonstrates the problem.  GCC 3.4.2 in the base
> system refuses to compile the code, giving this error message:
> 
> % g++ -c const-exp.cpp
> const-exp.cpp:14: error: a casts to a type other than an integral or
> enumeration type cannot appear in a constant-expression
> const-exp.cpp:14: error: '->' cannot appear in a constant-expression
> const-exp.cpp:14: error: `&' cannot appear in a constant-expression
> 
> The gcc34 port (gcc-3.4.2_20040827) builds it just fine.  The
> difference, as far as I can tell, is that gcc34 from the Ports
> Collection gets offsetof() from its own stddef.h, which has C++-friendly
> code:
> 
> /* Offset of member MEMBER in a struct of type TYPE. */
> #ifndef __cplusplus
> #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
> #else
> /* The cast to "char &" below avoids problems with user-defined
>    "operator &", which can appear in a POD type.  */
> #define offsetof(TYPE, MEMBER)                                  \
>   (__offsetof__ (reinterpret_cast <size_t>                      \
>                  (&reinterpret_cast <const volatile char &>     \
>                   (static_cast<TYPE *> (0)->MEMBER))))
> #endif /* C++ */
> 
> Would fixing this problem be as simple as using the above code
> in /usr/include/stddef.h?
> 
> The attached code was adapted from some very similar code in
> Boost.Python (from the file boost/python/object/instance.hpp), which
> does not compile with the base system GCC 3.4.2.  There may be other
> parts of Boost 1.31.0--and the upcoming Boost 1.32.0--that are affected
> by this offsetof() problem.
> 
>  -Patrick
> 
> 
-- 
Patrick L. Hartling                     | Research Assistant, VRAC
http://www.137.org/patrick/             | 2274 Howe Hall Room 2624
PGP: http://tinyurl.com/2oum9           | http://www.vrac.iastate.edu/