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{};
}