detail/variadic/drop_into.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::detail::variadic::drop_into`.
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_DROP_INTO_HPP
0011 #define BOOST_HANA_DETAIL_VARIADIC_DROP_INTO_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 F, typename = std::make_index_sequence<n>>
0021     struct dropper;
0022
0023     template <std::size_t n, typename F, std::size_t ...ignore>
0024     struct dropper<n, F, std::index_sequence<ignore...>> {
0025         F f;
0026
0027         template <typename ...Rest>
0028         constexpr auto go(decltype(ignore, (void*)0)..., Rest ...rest) const
0029         { return f(*rest...); }
0030
0031         template <typename ...Xs>
0032         constexpr auto operator()(Xs ...xs) const
0033         { return go(&xs...); }
0034     };
0035
0036     template <std::size_t n>
0037     struct make_dropper {
0038         template <typename F>
0039         constexpr auto operator()(F f) const
0040         { return dropper<n, decltype(f)>{f}; }
0041     };
0042
0043     template <std::size_t n>
0044     BOOST_HANA_INLINE_VARIABLE constexpr make_dropper<n> drop_into{};
0045 }} }} // end namespace boost::hana
0046
0047 #endif // !BOOST_HANA_DETAIL_VARIADIC_DROP_INTO_HPP