/*! @file Defines `boost::hana::drop_back`. @copyright Louis Dionne 2013-2017 Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) */ #ifndef BOOST_HANA_DROP_BACK_HPP #define BOOST_HANA_DROP_BACK_HPP #include <boost/hana/fwd/drop_back.hpp> #include <boost/hana/at.hpp> #include <boost/hana/concept/integral_constant.hpp> #include <boost/hana/concept/sequence.hpp> #include <boost/hana/config.hpp> #include <boost/hana/core/dispatch.hpp> #include <boost/hana/core/make.hpp> #include <boost/hana/integral_constant.hpp> #include <boost/hana/length.hpp> #include <cstddef> #include <utility> namespace boost { namespace hana { //! @cond template <typename Xs, typename N> constexpr auto drop_back_t::operator()(Xs&& xs, N const& n) const { using S = typename hana::tag_of<Xs>::type; using DropBack = BOOST_HANA_DISPATCH_IF(drop_back_impl<S>, hana::Sequence<S>::value && hana::IntegralConstant<N>::value ); #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS static_assert(hana::Sequence<S>::value, "hana::drop_back(xs, n) requires 'xs' to be a Sequence"); static_assert(hana::IntegralConstant<N>::value, "hana::drop_back(xs, n) requires 'n' to be an IntegralConstant"); #endif static_assert(N::value >= 0, "hana::drop_back(xs, n) requires 'n' to be non-negative"); return DropBack::apply(static_cast<Xs&&>(xs), n); } template <typename Xs> constexpr auto drop_back_t::operator()(Xs&& xs) const { return (*this)(static_cast<Xs&&>(xs), hana::size_c<1>); } //! @endcond template <typename S, bool condition> struct drop_back_impl<S, when<condition>> : default_ { template <typename Xs, std::size_t ...n> static constexpr auto drop_back_helper(Xs&& xs, std::index_sequence<n...>) { return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs))...); } template <typename Xs, typename N> static constexpr auto apply(Xs&& xs, N const&) { constexpr std::size_t n = N::value; constexpr std::size_t len = decltype(hana::length(xs))::value; return drop_back_helper(static_cast<Xs&&>(xs), std::make_index_sequence<(n > len ? 0 : len - n)>{}); } }; }} // end namespace boost::hana #endif // !BOOST_HANA_DROP_BACK_HPP