On 6/20/06, Nicolas Fleury <nidoizo@yahoo.com> wrote:
Is there something like a is_specialization in Boost.Traits?
From your example, I believe you mean "is instantation of" not "is specialization of". If so, I do agree that such a metafunction would be useful, as I find myself rolling my own similar metafunctions every once in a while. However, your example incorrectly assumes that char and int are different sizes ( sizeof(int) can yield 1 in a compliant C++ implementation ). Instead, prefer using type_traits's yes/no types or use array reference types or something similar.
Another simple implementation would be to just use partial template specialization: template< typename Type, template< typename > class Template > struct is_instantiation_of : false_type { }; template< typename Arg, template< typename > class Template > struct is_instantiation_of< Template< Arg >, Template > : true_type { }; ///////////////////////// // And a more modular form ///////////////////////// #define BOOST_MAX_INSTANTIATION_OF_PARAMS 10 #define BOOST_DETAIL_IS_INSTANTIATION_OF( z, num_params, dummy ) \ template< typename Type \ , template< BOOST_PP_ENUM_PARAMS_Z \ ( z, num_params, typename BOOST_PP_INTERCEPT ) > \ class Template > \ struct BOOST_PP_CAT( is_instantiation_of, num_params ) \ : false_type \ { \ }; \ \ template< BOOST_PP_ENUM_PARAMS_Z( z, num_params, typename Arg ) \ , template< BOOST_PP_ENUM_PARAMS_Z \ ( z, num_params, typename BOOST_PP_INTERCEPT ) > \ class Template > \ struct BOOST_PP_CAT( is_instantiation_of, num_params ) \ < Template< BOOST_PP_ENUM_PARAMS_Z( z, num_params, Arg ) > \ , Template > \ : true_type \ { \ }; BOOST_PP_REPEAT_FROM_TO ( 1, BOOST_PP_INC( BOOST_MAX_INSTANTIATION_OF_PARAMS ) , BOOST_DETAIL_IS_INSTANTIATION_OF , BOOST_PP_NIL ) #undef BOOST_DETAIL_IS_INSTANTIATION_OF /////////////////////////// /* To account for templates which have non-type template parameters, the best you could probably do is make a metafunction-generator macro which goes on a per-template basis:*/ /////////////////////////// #define BOOST_GENERATE_IS_INSTANTIATION_OF( meta_name, template_name, params ) \ template< typename Type > \ struct meta_name \ : false_type \ { \ }; \ \ template< BOOST_SEQ_FOR_EACH_I \ ( BOOST_DETAIL_GEN_IS_INST_PARAM, BOOST_PP_NIL, params ) > \ struct meta_name< template_name< BOOST_PP_ENUM_PARAMS \ ( BOOST_PP_SEQ_SIZE( params ), Arg ) > > \ : true_type \ { \ }; #define BOOST_DETAIL_GEN_IS_INST_PARAM( r, dummy, index, kind ) \ kind BOOST_PP_CAT( Arg, index ) ///////////////////////////// // Usage would be: ///////////////////////////// BOOST_GENERATE_IS_INSTANTIATION_OF ( is_boost_array, array, (typename)(::std::size_t) ) // Then use somewhere else bool const value = ::boost::is_boost_array< ::boost::array< int, 10 >
::value;
/* The above macro would have a limitation being that a comma cannot appear directly inside one of the sequence elements of the last argument (so, i.e. the type can't come from a metafunction invocation which takes more than one argument explicitly unless additional measures are taken). */ -- -Matt Calabrese