// (C) Copyright David Abrahams 2002.

 // (C) Copyright Jeremy Siek    2002.

 // (C) Copyright Thomas Witt    2002.

 // 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_ENABLE_IF_23022003THW_HPP

 #define BOOST_ENABLE_IF_23022003THW_HPP

 #include <boost/detail/workaround.hpp>

 #include <boost/mpl/identity.hpp>

 #include <boost/iterator/detail/config_def.hpp>

 //

 // Boost iterators uses its own enable_if cause we need

 // special semantics for deficient compilers.

 // 23/02/03 thw

 //

 namespace boost

 {

   namespace iterators

   {

     //

     // Base machinery for all kinds of enable if

     //

     template<bool>

     struct enabled

     {

       template<typename T>

       struct base

       {

         typedef T type;

       };

     };

     //

     // For compilers that don't support "Substitution Failure Is Not An Error"

     // enable_if falls back to always enabled. See comments

     // on operator implementation for consequences.

     //

     template<>

     struct enabled<false>

     {

       template<typename T>

       struct base

       {

 #ifdef BOOST_NO_SFINAE

         typedef T type;

         // This way to do it would give a nice error message containing

         // invalid overload, but has the big disadvantage that

         // there is no reference to user code in the error message.

         //

         // struct invalid_overload;

         // typedef invalid_overload type;

         //

 #endif

       };

     };

     template <class Cond,

               class Return>

     struct enable_if

 # if !defined(BOOST_NO_SFINAE) && !defined(BOOST_NO_IS_CONVERTIBLE)

       : enabled<(Cond::value)>::template base<Return>

 # else

       : mpl::identity<Return>

 # endif 

     {

 # if BOOST_WORKAROUND(BOOST_MSVC, < 1300)

         typedef Return type;

 # endif 

     };

   } // namespace iterators

 } // namespace boost

 #include <boost/iterator/detail/config_undef.hpp>

 #endif // BOOST_ENABLE_IF_23022003THW_HPP