1/*!
2@file
3Defines `boost::hana::unpack`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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
33namespace boost { namespace hana {
34 //! @cond
35 template <typename Xs, typename F>
36 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>
54 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)
67 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>
72 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)
83 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>
88 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>
101 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)
119 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>
132 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}} // end namespace boost::hana
140
141#endif // !BOOST_HANA_UNPACK_HPP
142

source code of boost/libs/hana/include/boost/hana/unpack.hpp