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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::back`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
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_FWD_BACK_HPP | |
11 | #define BOOST_HANA_FWD_BACK_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Returns the last element of a non-empty and finite iterable. | |
19 | //! @ingroup group-Iterable | |
20 | //! | |
21 | //! Given a non-empty and finite iterable `xs` with a linearization of | |
22 | //! `[x1, ..., xN]`, `back(xs)` is equal to `xN`. Equivalently, `back(xs)` | |
23 | //! must be equivalent to `at_c<N-1>(xs)`, and that regardless of the | |
24 | //! value category of `xs` (`back` must respect the reference semantics | |
25 | //! of `at`). | |
26 | //! | |
27 | //! | |
28 | //! Example | |
29 | //! ------- | |
30 | //! @include example/back.cpp | |
31 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
32 | constexpr auto back = [](auto&& xs) -> decltype(auto) { | |
33 | return tag-dispatched; | |
34 | }; | |
35 | #else | |
36 | template <typename It, typename = void> | |
37 | struct back_impl : back_impl<It, when<true>> { }; | |
38 | ||
39 | struct back_t { | |
40 | template <typename Xs> | |
41 | constexpr decltype(auto) operator()(Xs&& xs) const; | |
42 | }; | |
43 | ||
44 | constexpr back_t back{}; | |
45 | #endif | |
46 | BOOST_HANA_NAMESPACE_END | |
47 | ||
48 | #endif // !BOOST_HANA_FWD_BACK_HPP |