ceph/src/boost/boost/hana/detail/type_at.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::detail::type_at`.
   4
   5 @copyright Louis Dionne 2013-2017
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_DETAIL_TYPE_AT_HPP
  11 #define BOOST_HANA_DETAIL_TYPE_AT_HPP
  12
  13 #include <boost/hana/config.hpp>
  14
  15 #include <cstddef>
  16 #include <utility>
  17
  18
  19 // If possible, use an intrinsic provided by Clang
  20 #if defined(__has_builtin)
  21 #   if __has_builtin(__type_pack_element)
  22 #       define BOOST_HANA_USE_TYPE_PACK_ELEMENT_INTRINSIC
  23 #   endif
  24 #endif
  25
  26 namespace boost { namespace hana { namespace detail {
  27     namespace td {
  28         template <std::size_t I, typename T>
  29         struct elt { using type = T; };
  30
  31         template <typename Indices, typename ...T>
  32         struct indexer;
  33
  34         template <std::size_t ...I, typename ...T>
  35         struct indexer<std::index_sequence<I...>, T...>
  36             : elt<I, T>...
  37         { };
  38
  39         template <std::size_t I, typename T>
  40         elt<I, T> get_elt(elt<I, T> const&);
  41     }
  42
  43     //! @ingroup group-details
  44     //! Classic MPL-style metafunction returning the nth element of a type
  45     //! parameter pack.
  46     template <std::size_t n, typename ...T>
  47     struct type_at {
  48 #if defined(BOOST_HANA_USE_TYPE_PACK_ELEMENT_INTRINSIC)
  49         using type = __type_pack_element<n, T...>;
  50 #else
  51         using Indexer = td::indexer<std::make_index_sequence<sizeof...(T)>, T...>;
  52         using type = typename decltype(td::get_elt<n>(Indexer{}))::type;
  53 #endif
  54     };
  55 } }} // end namespace boost::hana
  56
  57 #endif // !BOOST_HANA_DETAIL_TYPE_AT_HPP