detail/variadic/at.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::detail::variadic::at`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_DETAIL_VARIADIC_AT_HPP
0011 #define BOOST_HANA_DETAIL_VARIADIC_AT_HPP
0012
0013 #include <boost/hana/config.hpp>
0014
0015 #include <cstddef>
0016 #include <utility>
0017
0018
0019 namespace boost { namespace hana { namespace detail { namespace variadic {
0020     template <std::size_t n, typename = std::make_index_sequence<n>>
0021     struct at_type;
0022
0023     template <std::size_t n, std::size_t ...ignore>
0024     struct at_type<n, std::index_sequence<ignore...>> {
0025     private:
0026         template <typename Nth>
0027         static constexpr auto go(decltype(ignore, (void*)0)..., Nth nth, ...)
0028         { return nth; }
0029
0030     public:
0031         template <typename ...Xs>
0032         constexpr auto operator()(Xs ...xs) const
0033         { return *go(&xs...); }
0034     };
0035
0036     template <std::size_t n>
0037     BOOST_HANA_INLINE_VARIABLE constexpr at_type<n> at{};
0038 }} }} // end namespace boost::hana
0039
0040 #endif // !BOOST_HANA_DETAIL_VARIADIC_AT_HPP