boost/hana/remove_at.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::remove_at` and `boost::hana::remove_at_c`.
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_REMOVE_AT_HPP
0011 #define BOOST_HANA_REMOVE_AT_HPP
0012
0013 #include <boost/hana/fwd/remove_at.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 remove_at_t::operator()(Xs&& xs, N const& n) const {
0032         using S = typename hana::tag_of<Xs>::type;
0033         using RemoveAt = BOOST_HANA_DISPATCH_IF(remove_at_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::remove_at(xs, n) requires 'xs' to be a Sequence");
0041
0042         static_assert(hana::IntegralConstant<N>::value,
0043         "hana::remove_at(xs, n) requires 'n' to be an IntegralConstant");
0044     #endif
0045
0046         static_assert(N::value >= 0,
0047         "hana::remove_at(xs, n) requires 'n' to be non-negative");
0048
0049         return RemoveAt::apply(static_cast<Xs&&>(xs), n);
0050     }
0051     //! @endcond
0052
0053     template <typename S, bool condition>
0054     struct remove_at_impl<S, when<condition>> : default_ {
0055         template <typename Xs, std::size_t ...before, std::size_t ...after>
0056         static constexpr auto
0057         remove_at_helper(Xs&& xs, std::index_sequence<before...>,
0058                                   std::index_sequence<after...>)
0059         {
0060             return hana::make<S>(
0061                 hana::at_c<before>(static_cast<Xs&&>(xs))...,
0062                 hana::at_c<after + sizeof...(before) + 1>(static_cast<Xs&&>(xs))...
0063             );
0064         }
0065
0066         template <typename Xs, typename N>
0067         static constexpr auto apply(Xs&& xs, N const&) {
0068             constexpr std::size_t n = N::value;
0069             constexpr std::size_t len = decltype(hana::length(xs))::value;
0070             static_assert(n < len,
0071             "hana::remove_at(xs, n) requires 'n' to be in the bounds of the sequence");
0072             return remove_at_helper(static_cast<Xs&&>(xs),
0073                                     std::make_index_sequence<n>{},
0074                                     std::make_index_sequence<len - n - 1>{});
0075         }
0076     };
0077
0078     template <std::size_t n>
0079     struct remove_at_c_t {
0080         template <typename Xs>
0081         constexpr decltype(auto) operator()(Xs&& xs) const
0082         { return hana::remove_at(static_cast<Xs&&>(xs), hana::size_c<n>); }
0083     };
0084 }} // end namespace boost::hana
0085
0086 #endif // !BOOST_HANA_REMOVE_AT_HPP