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Use __type_pack_element in mp_at(_c)

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Peter Dimov
2017-03-25 20:32:40 +02:00
parent aa5f2e4a5f
commit 3cd3a916a8
2 changed files with 26 additions and 9 deletions
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27
include/boost/mp11/algorithm.hpp
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@@ -264,20 +264,29 @@ template<class N> using mp_iota = detail::mp_from_sequence<make_integer_sequence
namespace detail
{
template<class L, class I> struct mp_at_impl
template<class L, std::size_t I> struct mp_at_c_impl;
#if defined(BOOST_MP11_HAS_TYPE_PACK_ELEMENT)
template<template<class...> class L, class... T, std::size_t I> struct mp_at_c_impl<L<T...>, I>
{
static_assert( I::value >= 0, "mp_at<L, I>: I must not be negative" );
using _map = mp_transform<mp_list, mp_iota<mp_size<L>>, L>;
using type = mp_second<mp_map_find<_map, mp_size_t<I::value>>>;
using type = __type_pack_element<I, T...>;
};
#else
template<class L, std::size_t I> struct mp_at_c_impl
{
using _map = mp_transform<mp_list, mp_iota<mp_size<L>>, L>;
using type = mp_second<mp_map_find<_map, mp_size_t<I>>>;
};
#endif
} // namespace detail
template<class L, std::size_t I> using mp_at_c = typename detail::mp_at_impl<L, mp_size_t<I>>::type;
template<class L, class I> using mp_at = typename detail::mp_at_impl<L, I>::type;
template<class L, std::size_t I> using mp_at_c = typename detail::mp_at_c_impl<L, I>::type;
template<class L, class I> using mp_at = typename detail::mp_at_c_impl<L, std::size_t{ I::value }>::type;
// mp_take(_c)<L, N>
namespace detail

8
include/boost/mp11/detail/config.hpp
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@@ -29,4 +29,12 @@
# endif
#endif
#if defined(__has_builtin)
# if __has_builtin(__type_pack_element)
# define BOOST_MP11_HAS_TYPE_PACK_ELEMENT
# endif
#endif
#endif // #ifndef BOOST_MP11_DETAIL_CONFIG_HPP_INCLUDED