1 /*! 2 @file 3 Defines `boost::hana::detail::variadic::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_VARIADIC_AT_HPP 11 #define BOOST_HANA_DETAIL_VARIADIC_AT_HPP 12 13 #include <boost/hana/config.hpp> 14 15 #include <cstddef> 16 #include <utility> 17 18 19 BOOST_HANA_NAMESPACE_BEGIN namespace detail { namespace variadic { 20 template <std::size_t n, typename = std::make_index_sequence<n>> 21 struct at_type; 22 23 template <std::size_t n, std::size_t ...ignore> 24 struct at_type<n, std::index_sequence<ignore...>> { 25 private: 26 template <typename Nth> 27 static constexpr auto go(decltype(ignore, (void*)0)..., Nth nth, ...) 28 { return nth; } 29 30 public: 31 template <typename ...Xs> operator ()detail::variadic::at_type32 constexpr auto operator()(Xs ...xs) const 33 { return *go(&xs...); } 34 }; 35 36 template <std::size_t n> 37 constexpr at_type<n> at{}; 38 }} BOOST_HANA_NAMESPACE_END 39 40 #endif // !BOOST_HANA_DETAIL_VARIADIC_AT_HPP 41