hana/fwd/is_empty.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::is_empty`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_IS_EMPTY_HPP
0011 #define BOOST_HANA_FWD_IS_EMPTY_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Returns whether the iterable is empty.
0019     //! @ingroup group-Iterable
0020     //!
0021     //! Given an `Iterable` `xs`, `is_empty` returns whether `xs` contains
0022     //! no more elements. In other words, it returns whether trying to
0023     //! extract the tail of `xs` would be an error. In the current version
0024     //! of the library, `is_empty` must return an `IntegralConstant` holding
0025     //! a value convertible to `bool`. This is because only compile-time
0026     //! `Iterable`s are supported right now.
0027     //!
0028     //!
0029     //! Example
0030     //! -------
0031     //! @include example/is_empty.cpp
0032 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0033     constexpr auto is_empty = [](auto const& xs) {
0034         return tag-dispatched;
0035     };
0036 #else
0037     template <typename It, typename = void>
0038     struct is_empty_impl : is_empty_impl<It, when<true>> { };
0039
0040     struct is_empty_t {
0041         template <typename Xs>
0042         constexpr auto operator()(Xs const& xs) const;
0043     };
0044
0045     BOOST_HANA_INLINE_VARIABLE constexpr is_empty_t is_empty{};
0046 #endif
0047 }} // end namespace boost::hana
0048
0049 #endif // !BOOST_HANA_FWD_IS_EMPTY_HPP