/*!
|
@file
|
Defines `boost::hana::detail::type_at`.
|
|
@copyright Louis Dionne 2013-2017
|
Distributed under the Boost Software License, Version 1.0.
|
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
|
*/
|
|
#ifndef BOOST_HANA_DETAIL_TYPE_AT_HPP
|
#define BOOST_HANA_DETAIL_TYPE_AT_HPP
|
|
#include <boost/hana/config.hpp>
|
|
#include <cstddef>
|
#include <utility>
|
|
|
// If possible, use an intrinsic provided by Clang
|
#if defined(__has_builtin)
|
# if __has_builtin(__type_pack_element)
|
# define BOOST_HANA_USE_TYPE_PACK_ELEMENT_INTRINSIC
|
# endif
|
#endif
|
|
BOOST_HANA_NAMESPACE_BEGIN namespace detail {
|
namespace td {
|
template <std::size_t I, typename T>
|
struct elt { using type = T; };
|
|
template <typename Indices, typename ...T>
|
struct indexer;
|
|
template <std::size_t ...I, typename ...T>
|
struct indexer<std::index_sequence<I...>, T...>
|
: elt<I, T>...
|
{ };
|
|
template <std::size_t I, typename T>
|
elt<I, T> get_elt(elt<I, T> const&);
|
}
|
|
//! @ingroup group-details
|
//! Classic MPL-style metafunction returning the nth element of a type
|
//! parameter pack.
|
template <std::size_t n, typename ...T>
|
struct type_at {
|
#if defined(BOOST_HANA_USE_TYPE_PACK_ELEMENT_INTRINSIC)
|
using type = __type_pack_element<n, T...>;
|
#else
|
using Indexer = td::indexer<std::make_index_sequence<sizeof...(T)>, T...>;
|
using type = typename decltype(td::get_elt<n>(Indexer{}))::type;
|
#endif
|
};
|
} BOOST_HANA_NAMESPACE_END
|
|
#endif // !BOOST_HANA_DETAIL_TYPE_AT_HPP
|
