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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::is_empty`. | |
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_IS_EMPTY_HPP | |
11 | #define BOOST_HANA_FWD_IS_EMPTY_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Returns whether the iterable is empty. | |
19 | //! @ingroup group-Iterable | |
20 | //! | |
21 | //! Given an `Iterable` `xs`, `is_empty` returns whether `xs` contains | |
22 | //! no more elements. In other words, it returns whether trying to | |
23 | //! extract the tail of `xs` would be an error. In the current version | |
24 | //! of the library, `is_empty` must return an `IntegralConstant` holding | |
25 | //! a value convertible to `bool`. This is because only compile-time | |
26 | //! `Iterable`s are supported right now. | |
27 | //! | |
28 | //! | |
29 | //! Example | |
30 | //! ------- | |
31 | //! @include example/is_empty.cpp | |
32 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
33 | constexpr auto is_empty = [](auto const& xs) { | |
34 | return tag-dispatched; | |
35 | }; | |
36 | #else | |
37 | template <typename It, typename = void> | |
38 | struct is_empty_impl : is_empty_impl<It, when<true>> { }; | |
39 | ||
40 | struct is_empty_t { | |
41 | template <typename Xs> | |
42 | constexpr auto operator()(Xs const& xs) const; | |
43 | }; | |
44 | ||
45 | constexpr is_empty_t is_empty{}; | |
46 | #endif | |
47 | BOOST_HANA_NAMESPACE_END | |
48 | ||
49 | #endif // !BOOST_HANA_FWD_IS_EMPTY_HPP |