boost/hana/drop_back.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::drop_back`.
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_DROP_BACK_HPP
0011 #define BOOST_HANA_DROP_BACK_HPP
0012
0013 #include <boost/hana/fwd/drop_back.hpp>
0014
0015 #include <boost/hana/at.hpp>
0016 #include <boost/hana/concept/integral_constant.hpp>
0017 #include <boost/hana/concept/sequence.hpp>
0018 #include <boost/hana/config.hpp>
0019 #include <boost/hana/core/dispatch.hpp>
0020 #include <boost/hana/core/make.hpp>
0021 #include <boost/hana/integral_constant.hpp>
0022 #include <boost/hana/length.hpp>
0023
0024 #include <cstddef>
0025 #include <utility>
0026
0027
0028 namespace boost { namespace hana {
0029     //! @cond
0030     template <typename Xs, typename N>
0031     constexpr auto drop_back_t::operator()(Xs&& xs, N const& n) const {
0032         using S = typename hana::tag_of<Xs>::type;
0033         using DropBack = BOOST_HANA_DISPATCH_IF(drop_back_impl<S>,
0034             hana::Sequence<S>::value &&
0035             hana::IntegralConstant<N>::value
0036         );
0037
0038     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0039         static_assert(hana::Sequence<S>::value,
0040         "hana::drop_back(xs, n) requires 'xs' to be a Sequence");
0041
0042         static_assert(hana::IntegralConstant<N>::value,
0043         "hana::drop_back(xs, n) requires 'n' to be an IntegralConstant");
0044     #endif
0045
0046         static_assert(N::value >= 0,
0047         "hana::drop_back(xs, n) requires 'n' to be non-negative");
0048
0049         return DropBack::apply(static_cast<Xs&&>(xs), n);
0050     }
0051
0052     template <typename Xs>
0053     constexpr auto drop_back_t::operator()(Xs&& xs) const {
0054         return (*this)(static_cast<Xs&&>(xs), hana::size_c<1>);
0055     }
0056     //! @endcond
0057
0058     template <typename S, bool condition>
0059     struct drop_back_impl<S, when<condition>> : default_ {
0060         template <typename Xs, std::size_t ...n>
0061         static constexpr auto drop_back_helper(Xs&& xs, std::index_sequence<n...>) {
0062             return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs))...);
0063         }
0064
0065         template <typename Xs, typename N>
0066         static constexpr auto apply(Xs&& xs, N const&) {
0067             constexpr std::size_t n = N::value;
0068             constexpr std::size_t len = decltype(hana::length(xs))::value;
0069             return drop_back_helper(static_cast<Xs&&>(xs),
0070                                     std::make_index_sequence<(n > len ? 0 : len - n)>{});
0071         }
0072     };
0073 }} // end namespace boost::hana
0074
0075 #endif // !BOOST_HANA_DROP_BACK_HPP