detail/variadic/take.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::detail::variadic::take`.
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_TAKE_HPP
0011 #define BOOST_HANA_DETAIL_VARIADIC_TAKE_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/detail/variadic/split_at.hpp>
0015 #include <boost/hana/functional/always.hpp>
0016 #include <boost/hana/functional/reverse_partial.hpp>
0017
0018 #include <cstddef>
0019
0020
0021 namespace boost { namespace hana { namespace detail { namespace variadic {
0022     struct take_impl2 {
0023         template <typename F, typename ...Xs>
0024         constexpr decltype(auto) operator()(F&& f, Xs&& ...xs) const {
0025             return static_cast<F&&>(f)(static_cast<Xs&&>(xs)...);
0026         }
0027     };
0028
0029     struct take_impl1 {
0030         template <typename ...Xs>
0031         constexpr auto operator()(Xs&& ...xs) const {
0032             return hana::always(
0033                 reverse_partial(take_impl2{},
0034                     static_cast<Xs&&>(xs)...)
0035             );
0036         }
0037     };
0038
0039     template <std::size_t n>
0040     struct take_t {
0041         template <typename ...Xs>
0042         constexpr decltype(auto) operator()(Xs&& ...xs) const {
0043             return variadic::split_at<n>(static_cast<Xs&&>(xs)...)(take_impl1{});
0044         }
0045     };
0046
0047     template <std::size_t n>
0048     BOOST_HANA_INLINE_VARIABLE constexpr take_t<n> take{};
0049 }} }} // end namespace boost::hana
0050
0051 #endif // !BOOST_HANA_DETAIL_VARIADIC_TAKE_HPP