boost/hana/unpack.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::unpack`.
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_UNPACK_HPP
0011 #define BOOST_HANA_UNPACK_HPP
0012
0013 #include <boost/hana/fwd/unpack.hpp>
0014
0015 #include <boost/hana/accessors.hpp>
0016 #include <boost/hana/at.hpp>
0017 #include <boost/hana/concept/foldable.hpp>
0018 #include <boost/hana/concept/iterable.hpp>
0019 #include <boost/hana/concept/struct.hpp>
0020 #include <boost/hana/config.hpp>
0021 #include <boost/hana/core/dispatch.hpp>
0022 #include <boost/hana/first.hpp>
0023 #include <boost/hana/functional/partial.hpp>
0024 #include <boost/hana/fwd/fold_left.hpp>
0025 #include <boost/hana/length.hpp>
0026 #include <boost/hana/pair.hpp>
0027 #include <boost/hana/second.hpp>
0028
0029 #include <cstddef>
0030 #include <utility>
0031
0032
0033 namespace boost { namespace hana {
0034     //! @cond
0035     template <typename Xs, typename F>
0036     constexpr decltype(auto) unpack_t::operator()(Xs&& xs, F&& f) const {
0037         using S = typename hana::tag_of<Xs>::type;
0038         using Unpack = BOOST_HANA_DISPATCH_IF(unpack_impl<S>,
0039             hana::Foldable<S>::value
0040         );
0041
0042     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0043         static_assert(hana::Foldable<S>::value,
0044         "hana::unpack(xs, f) requires 'xs' to be Foldable");
0045     #endif
0046
0047         return Unpack::apply(static_cast<Xs&&>(xs), static_cast<F&&>(f));
0048     }
0049     //! @endcond
0050
0051     template <typename T, bool condition>
0052     struct unpack_impl<T, when<condition>> : default_ {
0053         template <typename Xs, typename F>
0054         static constexpr decltype(auto) apply(Xs&& xs, F&& f) {
0055             return hana::fold_left(static_cast<Xs&&>(xs),
0056                                    static_cast<F&&>(f),
0057                                    hana::partial)();
0058         }
0059     };
0060
0061     template <typename It>
0062     struct unpack_impl<It, when<
0063         hana::Iterable<It>::value && !is_default<length_impl<It>>::value
0064     >> {
0065         template <typename Xs, typename F, std::size_t ...i>
0066         static constexpr decltype(auto)
0067         unpack_helper(Xs&& xs, F&& f, std::index_sequence<i...>) {
0068             return static_cast<F&&>(f)(hana::at_c<i>(static_cast<Xs&&>(xs))...);
0069         }
0070
0071         template <typename Xs, typename F>
0072         static constexpr decltype(auto) apply(Xs&& xs, F&& f) {
0073             constexpr std::size_t N = decltype(hana::length(xs))::value;
0074             return unpack_helper(static_cast<Xs&&>(xs), static_cast<F&&>(f),
0075                                  std::make_index_sequence<N>{});
0076         }
0077     };
0078
0079     template <typename T, std::size_t N>
0080     struct unpack_impl<T[N]> {
0081         template <typename Xs, typename F, std::size_t ...i>
0082         static constexpr decltype(auto)
0083         unpack_helper(Xs&& xs, F&& f, std::index_sequence<i...>) {
0084             return static_cast<F&&>(f)(static_cast<Xs&&>(xs)[i]...);
0085         }
0086
0087         template <typename Xs, typename F>
0088         static constexpr decltype(auto) apply(Xs&& xs, F&& f) {
0089             return unpack_impl::unpack_helper(static_cast<Xs&&>(xs),
0090                                               static_cast<F&&>(f),
0091                                               std::make_index_sequence<N>{});
0092         }
0093     };
0094
0095     //////////////////////////////////////////////////////////////////////////
0096     // Model for Products
0097     //////////////////////////////////////////////////////////////////////////
0098     template <typename T>
0099     struct unpack_impl<T, when<hana::Product<T>::value>> {
0100         template <typename P, typename F>
0101         static constexpr decltype(auto) apply(P&& p, F&& f) {
0102             return static_cast<F&&>(f)(
0103                 hana::first(static_cast<P&&>(p)),
0104                 hana::second(static_cast<P&&>(p))
0105             );
0106         }
0107     };
0108
0109     //////////////////////////////////////////////////////////////////////////
0110     // Model for Structs
0111     //////////////////////////////////////////////////////////////////////////
0112     namespace struct_detail {
0113         // This is equivalent to `demux`, except that `demux` can't forward
0114         // the `udt` because it does not know the `g`s are accessors. Hence,
0115         // this can result in faster code.
0116         struct almost_demux {
0117             template <typename F, typename Udt, typename ...Members>
0118             constexpr decltype(auto)
0119             operator()(F&& f, Udt&& udt, Members&& ...g) const {
0120                 return static_cast<F&&>(f)(hana::make_pair(
0121                     hana::first(static_cast<Members&&>(g)),
0122                     hana::second(static_cast<Members&&>(g))
0123                                                 (static_cast<Udt&&>(udt))
0124                 )...);
0125             }
0126         };
0127     }
0128
0129     template <typename S>
0130     struct unpack_impl<S, when<hana::Struct<S>::value>> {
0131         template <typename Udt, typename F>
0132         static constexpr decltype(auto) apply(Udt&& udt, F&& f) {
0133             return hana::unpack(hana::accessors<S>(),
0134                 hana::partial(struct_detail::almost_demux{},
0135                               static_cast<F&&>(f),
0136                               static_cast<Udt&&>(udt)));
0137         }
0138     };
0139 }} // end namespace boost::hana
0140
0141 #endif // !BOOST_HANA_UNPACK_HPP