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1 | /*! | |
2 | @file | |
3 | Forward declares `boost::hana::front`. | |
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_FRONT_HPP | |
11 | #define BOOST_HANA_FWD_FRONT_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 first element of a non-empty iterable. | |
19 | //! @ingroup group-Iterable | |
20 | //! | |
21 | //! Given a non-empty Iterable `xs` with a linearization of `[x1, ..., xN]`, | |
22 | //! `front(xs)` is equal to `x1`. If `xs` is empty, it is an error to | |
23 | //! use this function. Equivalently, `front(xs)` must be equivalent to | |
24 | //! `at_c<0>(xs)`, and that regardless of the value category of `xs` | |
25 | //! (`front` must respect the reference semantics of `at`). | |
26 | //! | |
27 | //! | |
28 | //! Example | |
29 | //! ------- | |
30 | //! @include example/front.cpp | |
31 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
32 | constexpr auto front = [](auto&& xs) -> decltype(auto) { | |
33 | return tag-dispatched; | |
34 | }; | |
35 | #else | |
36 | template <typename It, typename = void> | |
37 | struct front_impl : front_impl<It, when<true>> { }; | |
38 | ||
39 | struct front_t { | |
40 | template <typename Xs> | |
41 | constexpr decltype(auto) operator()(Xs&& xs) const; | |
42 | }; | |
43 | ||
44 | constexpr front_t front{}; | |
45 | #endif | |
46 | BOOST_HANA_NAMESPACE_END | |
47 | ||
48 | #endif // !BOOST_HANA_FWD_FRONT_HPP |