PR 31426: TR1 and the Experimental C++0x Mode

[Doug Gregor <dgregor at cs dot indiana dot edu>]

Hello all,

I've filed PR 31426, which concerns the interaction between TR1 and  
the experimental C++0x mode. The program in question is:

  #include <tr1/tuple>

  int main()
  {
   std::tr1::tuple<int, double> x(17, 3.14);
   return 0;
  }

Currently, this compiles in the default (C++98) mode but fails in the  
experimental C++0x mode. I don't think this is what we want. If the  
user includes a tr1/ header specifically, s/he should get these  
facilities in the std::tr1 namespace, regardless of what mode the C++  
compiler is in. The current situation makes a lot of code break when - 
std=c++0x is turned on.

I can see two easy solutions:

	(1) Namespace alias: in each of the tr1/ headers, include the header  
<tr1/import.h> shown below:

#ifndef _TR1_IMPORT
#define _TR1_IMPORT 1

#ifdef __GXX_EXPERIMENTAL_CXX0X__
namespace std {
  namespace tr1 = __cxx200x;
}
#endif

#endif


	(2) Strong using without the __cxx200x namespace:  make the tr1/  
headers actually put everything into namespace std::tr1. When we're  
in C++0x mode, have each of the C++0x headers (or C++98 headers that  
have been extended by C++0x), do this:

#ifdef __GXX_EXPERIMENTAL_CXX0X__
#  include <tr1/the_equivalent_tr1_header>
#  include <bits/import_from_tr1.h>
#endif

	where bits/import_from_tr1.h is:

#ifndef _IMPORT_FROM_TR1
#define _IMPORT_FROM_TR1 1

namespace std
{
  using namespace tr1 __attribute__ ((strong));
}

#endif

	Cheers,
	Doug