hana/detail/nested_than_fwd.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::detail::nested_than`.
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_DETAIL_NESTED_THAN_FWD_HPP
0011 #define BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP
0012
0013 #include <boost/hana/config.hpp>
0014
0015
0016 namespace boost { namespace hana { namespace detail {
0017     template <typename Algorithm>
0018     struct nested_than_t {
0019         template <typename X>
0020         constexpr decltype(auto) operator()(X&& x) const;
0021     };
0022
0023     //! @ingroup group-details
0024     //! Provides a `.than` static constexpr function object.
0025     //!
0026     //! When creating a binary function object of type `Algo` whose signature
0027     //! is `A x B -> Return`, `nested_than<Algo>` can be used as a base class
0028     //! of `Algo`. Doing so will provide a static constexpr member called
0029     //! `than`, which has the following signature:
0030     //! @code
0031     //!     B -> A -> Return
0032     //! @endcode
0033     //!
0034     //! Note that the function object `Algo` must be default-constructible,
0035     //! since it will be called as `Algo{}(arguments...)`.
0036     //!
0037     //! @note
0038     //! This function object is especially useful because it takes care of
0039     //! avoiding ODR violations caused by the nested static constexpr member.
0040     template <typename Algorithm>
0041     struct nested_than { static constexpr nested_than_t<Algorithm> than{}; };
0042
0043     template <typename Algorithm>
0044     constexpr nested_than_t<Algorithm> nested_than<Algorithm>::than;
0045 } }} // end namespace boost::hana
0046
0047 #endif // !BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP