1/*!
2@file
3Forward declares `boost::hana::is_empty`.
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_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
17namespace boost { namespace hana {
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 BOOST_HANA_INLINE_VARIABLE constexpr is_empty_t is_empty{};
46#endif
47}} // end namespace boost::hana
48
49#endif // !BOOST_HANA_FWD_IS_EMPTY_HPP
50

source code of boost/libs/hana/include/boost/hana/fwd/is_empty.hpp