boost/iterator/detail/enable_if.hpp

   1 // (C) Copyright David Abrahams 2002.
   2 // (C) Copyright Jeremy Siek    2002.
   3 // (C) Copyright Thomas Witt    2002.
   4 // Distributed under the Boost Software License, Version 1.0. (See
   5 // accompanying file LICENSE_1_0.txt or copy at
   6 // http://www.boost.org/LICENSE_1_0.txt)
   7 #ifndef BOOST_ENABLE_IF_23022003THW_HPP
   8 #define BOOST_ENABLE_IF_23022003THW_HPP
   9
  10 #include <boost/detail/workaround.hpp>
  11 #include <boost/mpl/identity.hpp>
  12
  13 #include <boost/iterator/detail/config_def.hpp>
  14
  15 //
  16 // Boost iterators uses its own enable_if cause we need
  17 // special semantics for deficient compilers.
  18 // 23/02/03 thw
  19 //
  20
  21 namespace boost
  22 {
  23
  24   namespace iterators
  25   {
  26     //
  27     // Base machinery for all kinds of enable if
  28     //
  29     template<bool>
  30     struct enabled
  31     {
  32       template<typename T>
  33       struct base
  34       {
  35         typedef T type;
  36       };
  37     };
  38
  39     //
  40     // For compilers that don't support "Substitution Failure Is Not An Error"
  41     // enable_if falls back to always enabled. See comments
  42     // on operator implementation for consequences.
  43     //
  44     template<>
  45     struct enabled<false>
  46     {
  47       template<typename T>
  48       struct base
  49       {
  50 #ifdef BOOST_NO_SFINAE
  51
  52         typedef T type;
  53
  54         // This way to do it would give a nice error message containing
  55         // invalid overload, but has the big disadvantage that
  56         // there is no reference to user code in the error message.
  57         //
  58         // struct invalid_overload;
  59         // typedef invalid_overload type;
  60         //
  61 #endif
  62       };
  63     };
  64
  65
  66     template <class Cond,
  67               class Return>
  68     struct enable_if
  69 # if !defined(BOOST_NO_SFINAE) && !defined(BOOST_NO_IS_CONVERTIBLE)
  70       : enabled<(Cond::value)>::template base<Return>
  71 # else
  72       : mpl::identity<Return>
  73 # endif
  74     {
  75 # if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
  76         typedef Return type;
  77 # endif
  78     };
  79
  80   } // namespace iterators
  81
  82 } // namespace boost
  83
  84 #include <boost/iterator/detail/config_undef.hpp>
  85
  86 #endif // BOOST_ENABLE_IF_23022003THW_HPP