Re: [patch] libstdc++/65473 Make <ciso646> define libstdc++ version macros.

[Martin Sebor <msebor at gmail dot com>]

On 09/03/2015 04:58 AM, Jonathan Wakely wrote:
> This change would allow including <ciso646> to be used to check for
> __GLIBCXX__ and detect whether youre using libstdc++ or not. Howard
> Hinnant recommends including that header for libc++ because it has no
> other effects in C++.
>
> We could make every <cxxx> header include <bits/c++config.h> so that
> any of them can be used, but I can't be bothered doing that change!
> This makes it work for the one header that is recommended to be used,
> but of course that doesn't help people using older versions of
> libstdc++, who still need to include some other header.
>
> Is this worth doing?

I'd say it's worth doing consistently, in every header. Users are
told by others (e.g., on various discussion forums) to expect to
be able to check what C++ library implementation they're using by
including any C++ standard header and testing the known version
macros.

Martin

PS Out of curiosity I looked to see which headers don't include
c++config.g.

$ (for f in cassert ccomplex cctype cerrno cfenv cfloat cinttypes 
ciso646 climits clocale cmath csetjmp csignal cstdalign cstdarg cstdbool 
cstddef cstdint cstdio cstdlib cstring ctgmath ctime cuchar; do printf " 
 %-20s " "<$f>" && echo "#include <$f>" | ~/bin/gcc-5.1.0/bin/g++ -E 
-std=c++14 -xc++ - | grep -l "c++config\.h" | wc -l; done )
  <cassert>            0
  <ccomplex>           1
  <cctype>             1
  <cerrno>             0
  <cfenv>              1
  <cfloat>             0
  <cinttypes>          1
  <ciso646>            0
  <climits>            0
  <clocale>            1
  <cmath>              1
  <csetjmp>            1
  <csignal>            1
  <cstdalign>          1
  <cstdarg>            1
  <cstdbool>           1
  <cstddef>            1
  <cstdint>            1
  <cstdio>             1
  <cstdlib>            1
  <cstring>            1
  <ctgmath>            1
  <ctime>              1
  <cuchar>             <stdin>:1:18: fatal error: cuchar: No such file 
or directory
compilation terminated.
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