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From "Travis Vitek" <Travis.Vi...@roguewave.com>
Subject RE: svn commit: r669735 - in /stdcxx/branches/4.3.x: include/rw/_meta_other.h include/type_traits tests/utilities/20.meta.trans.other.cpp
Date Mon, 30 Jun 2008 16:02:24 GMT
 

Martin Sebor wrote:
>
>I searched library headers and sources for how we define unions and
>with the exception of limits_bits.cpp we always follow this rule.
>Unless there is a reason not to make this change to aligned_union,
>I think we should change both limits_bits.cpp and aligned_union to
>always define the member with the more strict alignment requirement
>first, just for peace of mind.
>
>Is there any reason/advantage to having the char buffer first?
>

If the first member is used to define the alignment, then you have to
know (at compile time) which of the union members has the strictest
alignment requirement so that it can be put first.

This problem comes up in the definition of __rw_aligned_buffer. On most
implementations the members are ordered according to the scheme you
mentioned previously, but it is very possible for them to be out of
order.

    union {
  #ifndef _RWSTD_NO_LONG_DOUBLE
        long double _C_pad;
  #else
        double      _C_pad;
  #endif   // _RWSTD_NO_LONG_DOUBLE
        void       *_C_void_pad;
        void      (*_C_pfn_pad)();
        char        _C_data [sizeof (_TypeT)];
    } _C_buf;

Travis

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