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7c673cae FG |
1 | /*! |
2 | @file | |
3 | Defines `boost::hana::back`. | |
4 | ||
b32b8144 | 5 | @copyright Louis Dionne 2013-2017 |
7c673cae FG |
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_BACK_HPP | |
11 | #define BOOST_HANA_BACK_HPP | |
12 | ||
13 | #include <boost/hana/fwd/back.hpp> | |
14 | ||
15 | #include <boost/hana/at.hpp> | |
16 | #include <boost/hana/concept/iterable.hpp> | |
17 | #include <boost/hana/config.hpp> | |
18 | #include <boost/hana/core/dispatch.hpp> | |
19 | #include <boost/hana/length.hpp> | |
20 | ||
21 | #include <cstddef> | |
22 | ||
23 | ||
24 | BOOST_HANA_NAMESPACE_BEGIN | |
25 | //! @cond | |
26 | template <typename Xs> | |
27 | constexpr decltype(auto) back_t::operator()(Xs&& xs) const { | |
28 | using It = typename hana::tag_of<Xs>::type; | |
29 | using Back = BOOST_HANA_DISPATCH_IF(back_impl<It>, | |
30 | hana::Iterable<It>::value | |
31 | ); | |
32 | ||
33 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
34 | static_assert(hana::Iterable<It>::value, | |
35 | "hana::back(xs) requires 'xs' to be an Iterable"); | |
36 | #endif | |
37 | ||
38 | return Back::apply(static_cast<Xs&&>(xs)); | |
39 | } | |
40 | //! @endcond | |
41 | ||
42 | template <typename It, bool condition> | |
43 | struct back_impl<It, when<condition>> : default_ { | |
44 | template <typename Xs> | |
45 | static constexpr decltype(auto) apply(Xs&& xs) { | |
46 | constexpr std::size_t len = decltype(hana::length(xs))::value; | |
47 | static_assert(len > 0, "hana::back(xs) requires 'xs' to be non-empty"); | |
48 | return hana::at_c<len - 1>(static_cast<Xs&&>(xs)); | |
49 | } | |
50 | }; | |
51 | BOOST_HANA_NAMESPACE_END | |
52 | ||
53 | #endif // !BOOST_HANA_BACK_HPP |