boost_1_45_0/libs/type_traits/doc/user_defined.qbk

[/ 
  Copyright 2007 John Maddock.
  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).
]

[section:user_defined User Defined Specializations]

Occationally the end user may need to provide their own specialization 
for one of the type traits - typically where intrinsic compiler support 
is required to implement a specific trait fully.  
These specializations should derive from boost::__true_type or boost::__false_type 
as appropriate:

   #include <boost/type_traits/is_pod.hpp>
   #include <boost/type_traits/is_class.hpp>
   #include <boost/type_traits/is_union.hpp>

   struct my_pod{};
   struct my_union
   {
      char c;
      int i;
   };

   namespace boost
   {
      template<>
      struct __is_pod<my_pod> : public __true_type{};
         
      template<>
      struct __is_pod<my_union> : public __true_type{};
      
      template<>
      struct __is_union<my_union> : public __true_type{};
      
      template<>
      struct __is_class<my_union> : public __false_type{};
   }

[endsect]