// (C) Copyright David Abrahams 2004.
// (C) Copyright Jonathan Turkanis 2004.
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt.

#ifndef BOOST_IOSTREAMS_DEFAULT_ETRACTABLE_HPP_INCLUDED
#define BOOST_IOSTREAMS_DEFAULT_ETRACTABLE_HPP_INCLUDED

#include <iosfwd>
#include <boost/type_traits/remove_cv.hpp>
#include <boost/mpl/bool.hpp>
#include <boost/detail/workaround.hpp>

namespace boost { namespace io { 

// This namespace ensures that ADL doesn't mess things up.
namespace default_extractable_ {

  // a type returned from operator++ when no increment is found in the
  // type's own namespace
  struct tag { };
  
  // any soaks up implicit conversions and makes the following
  // operator<< less-preferred than any other such operator that
  // might be found via ADL.
  struct any { template <class T> any(T const&); };

  // This is a last-resort operator>> for when none other is found
  template<typename Ch, typename Tr>
  tag operator>>(std::basic_istream<Ch, Tr>&, const any&);

  // two check overloads help us identify which operator<< was picked
  char (& check(tag) )[2];
  
  template<typename Ch, typename Tr>
  char check(std::basic_istream<Ch, Tr>&);
  
  template<typename T, typename Ch, typename Tr>
  struct impl {
      static typename add_reference< typename remove_cv<T>::type >::type t;
      static std::basic_istream<Ch, Tr>& in;

      BOOST_STATIC_CONSTANT(
          bool, value = sizeof(default_extractable_::check(in >> t)) == 1
      );
  };

} // End namespace default_extractable_.

template< class T, 
          typename Ch = char, 
          typename Tr = std::char_traits<Ch> >
struct is_default_extractable
    : mpl::bool_< 
          ::boost::io::default_extractable_::impl<T, Ch, Tr>::value 
      >
    { };

} } // End namespace boost::io.

#endif // BOOST_IOSTREAMS_DEFAULT_ETRACTABLE_HPP_INCLUDED