1 /*! 2 @file 3 Defines `boost::hana::unpack`. 4 5 @copyright Louis Dionne 2013-2017 6 Distributed under the Boost Software License, Version 1.0. 7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) 8 */ 9 10 #ifndef BOOST_HANA_UNPACK_HPP 11 #define BOOST_HANA_UNPACK_HPP 12 13 #include <boost/hana/fwd/unpack.hpp> 14 15 #include <boost/hana/accessors.hpp> 16 #include <boost/hana/at.hpp> 17 #include <boost/hana/concept/foldable.hpp> 18 #include <boost/hana/concept/iterable.hpp> 19 #include <boost/hana/concept/struct.hpp> 20 #include <boost/hana/config.hpp> 21 #include <boost/hana/core/dispatch.hpp> 22 #include <boost/hana/first.hpp> 23 #include <boost/hana/functional/partial.hpp> 24 #include <boost/hana/fwd/fold_left.hpp> 25 #include <boost/hana/length.hpp> 26 #include <boost/hana/pair.hpp> 27 #include <boost/hana/second.hpp> 28 29 #include <cstddef> 30 #include <utility> 31 32 33 BOOST_HANA_NAMESPACE_BEGIN 34 //! @cond 35 template <typename Xs, typename F> operator ()(Xs && xs,F && f) const36 constexpr decltype(auto) unpack_t::operator()(Xs&& xs, F&& f) const { 37 using S = typename hana::tag_of<Xs>::type; 38 using Unpack = BOOST_HANA_DISPATCH_IF(unpack_impl<S>, 39 hana::Foldable<S>::value 40 ); 41 42 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS 43 static_assert(hana::Foldable<S>::value, 44 "hana::unpack(xs, f) requires 'xs' to be Foldable"); 45 #endif 46 47 return Unpack::apply(static_cast<Xs&&>(xs), static_cast<F&&>(f)); 48 } 49 //! @endcond 50 51 template <typename T, bool condition> 52 struct unpack_impl<T, when<condition>> : default_ { 53 template <typename Xs, typename F> applyunpack_impl54 static constexpr decltype(auto) apply(Xs&& xs, F&& f) { 55 return hana::fold_left(static_cast<Xs&&>(xs), 56 static_cast<F&&>(f), 57 hana::partial)(); 58 } 59 }; 60 61 template <typename It> 62 struct unpack_impl<It, when< 63 hana::Iterable<It>::value && !is_default<length_impl<It>>::value 64 >> { 65 template <typename Xs, typename F, std::size_t ...i> 66 static constexpr decltype(auto) unpack_helperunpack_impl67 unpack_helper(Xs&& xs, F&& f, std::index_sequence<i...>) { 68 return static_cast<F&&>(f)(hana::at_c<i>(static_cast<Xs&&>(xs))...); 69 } 70 71 template <typename Xs, typename F> applyunpack_impl72 static constexpr decltype(auto) apply(Xs&& xs, F&& f) { 73 constexpr std::size_t N = decltype(hana::length(xs))::value; 74 return unpack_helper(static_cast<Xs&&>(xs), static_cast<F&&>(f), 75 std::make_index_sequence<N>{}); 76 } 77 }; 78 79 template <typename T, std::size_t N> 80 struct unpack_impl<T[N]> { 81 template <typename Xs, typename F, std::size_t ...i> 82 static constexpr decltype(auto) unpack_helperunpack_impl83 unpack_helper(Xs&& xs, F&& f, std::index_sequence<i...>) { 84 return static_cast<F&&>(f)(static_cast<Xs&&>(xs)[i]...); 85 } 86 87 template <typename Xs, typename F> applyunpack_impl88 static constexpr decltype(auto) apply(Xs&& xs, F&& f) { 89 return unpack_impl::unpack_helper(static_cast<Xs&&>(xs), 90 static_cast<F&&>(f), 91 std::make_index_sequence<N>{}); 92 } 93 }; 94 95 ////////////////////////////////////////////////////////////////////////// 96 // Model for Products 97 ////////////////////////////////////////////////////////////////////////// 98 template <typename T> 99 struct unpack_impl<T, when<hana::Product<T>::value>> { 100 template <typename P, typename F> applyunpack_impl101 static constexpr decltype(auto) apply(P&& p, F&& f) { 102 return static_cast<F&&>(f)( 103 hana::first(static_cast<P&&>(p)), 104 hana::second(static_cast<P&&>(p)) 105 ); 106 } 107 }; 108 109 ////////////////////////////////////////////////////////////////////////// 110 // Model for Structs 111 ////////////////////////////////////////////////////////////////////////// 112 namespace struct_detail { 113 // This is equivalent to `demux`, except that `demux` can't forward 114 // the `udt` because it does not know the `g`s are accessors. Hence, 115 // this can result in faster code. 116 struct almost_demux { 117 template <typename F, typename Udt, typename ...Members> 118 constexpr decltype(auto) operator ()struct_detail::almost_demux119 operator()(F&& f, Udt&& udt, Members&& ...g) const { 120 return static_cast<F&&>(f)(hana::make_pair( 121 hana::first(static_cast<Members&&>(g)), 122 hana::second(static_cast<Members&&>(g)) 123 (static_cast<Udt&&>(udt)) 124 )...); 125 } 126 }; 127 } 128 129 template <typename S> 130 struct unpack_impl<S, when<hana::Struct<S>::value>> { 131 template <typename Udt, typename F> applyunpack_impl132 static constexpr decltype(auto) apply(Udt&& udt, F&& f) { 133 return hana::unpack(hana::accessors<S>(), 134 hana::partial(struct_detail::almost_demux{}, 135 static_cast<F&&>(f), 136 static_cast<Udt&&>(udt))); 137 } 138 }; 139 BOOST_HANA_NAMESPACE_END 140 141 #endif // !BOOST_HANA_UNPACK_HPP 142