hana/fwd/difference.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::difference`.
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_DIFFERENCE_HPP
0011 #define BOOST_HANA_FWD_DIFFERENCE_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     // Note: This function is documented per datatype/concept only.
0019     //! @cond
0020     template <typename S, typename = void>
0021     struct difference_impl : difference_impl<S, when<true>> { };
0022     //! @endcond
0023
0024     struct difference_t {
0025         template <typename Xs, typename Ys>
0026         constexpr auto operator()(Xs&&, Ys&&) const;
0027     };
0028
0029     BOOST_HANA_INLINE_VARIABLE constexpr difference_t difference{};
0030 }} // end namespace boost::hana
0031
0032 #endif // !BOOST_HANA_FWD_DIFFERENCE_HPP