On Sun, 07 Nov 2010 02:40:13 -0600, vicente.botet <vicente.botet@wanadoo.fr> wrote:
I have tried to add a value_type trait
template<typename Sequence> struct value_type { typedef typename Sequence::value_type type; };
changed the c_str to use the type trait
template<typename Sequence> struct c_str { typedef typename mpl::end<Sequence>::type iend; typedef typename mpl::begin<Sequence>::type i0; #define M0(z, n, data) \ typedef \ typename mpl::aux_::next_unless<BOOST_PP_CAT(i, n), iend>::type \ BOOST_PP_CAT(i, BOOST_PP_INC(n)); BOOST_PP_REPEAT(BOOST_MPL_LIMIT_STRING_SIZE, M0, ~) #undef M0
typedef c_str type; static typename value_type<Sequence>::type const value[BOOST_MPL_LIMIT_STRING_SIZE+1]; };
and defined the following specializations:
namespace boost { namespace mpl { template<typename T, T N> struct value_type<integral_c<T,N> > { typedef T type; }; template<typename T0,typename T1,typename T2,typename T3,typename T4, typename T5,typename T6,typename T7,typename T8,typename T9, typename T10,typename T11> struct value_type<vector12<T0,T1,T2,T3,T4,T5,T6,T7,T8,T9,T10,T11> > { typedef typename value_type<T0>::type type; }; } }
Then we are able to use c_str on any sequence providing the specialization. Of course compile checking that the sequence size is not greater than BOOST_MPL_LIMIT_STRING_SIZE should be convenient.
A more general approach would be something like this (untested): BOOST_MPL_HAS_XXX_TRAIT_DEF( value_type ) template< bool, typename X > struct value_type_impl { typedef typename X::value_type type; } template< typename X > struct value_type : value_type_impl< has_value_type<X>::value, X > { }; template< typename X > struct value_type_impl<false,X> { typedef typename value_type< typename front<X>::type >::type type; };
Is there a possibility to add this trait to Boost.MPL and the corresponding specialization and move the c_str function to a standalone file ?
If it's OK with Eric, I'd be happy to accept the patch. -- Aleksey Gurtovoy MetaCommunications Engineering