1 /*! 2 @file 3 Forward declares `boost::hana::detail::nested_than`. 4 5 @copyright Louis Dionne 2013-2017 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_DETAIL_NESTED_THAN_FWD_HPP 11 #define BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP 12 13 #include <boost/hana/config.hpp> 14 15 16 BOOST_HANA_NAMESPACE_BEGIN namespace detail { 17 template <typename Algorithm> 18 struct nested_than_t { 19 template <typename X> 20 constexpr decltype(auto) operator()(X&& x) const; 21 }; 22 23 //! @ingroup group-details 24 //! Provides a `.than` static constexpr function object. 25 //! 26 //! When creating a binary function object of type `Algo` whose signature 27 //! is `A x B -> Return`, `nested_than<Algo>` can be used as a base class 28 //! of `Algo`. Doing so will provide a static constexpr member called 29 //! `than`, which has the following signature: 30 //! @code 31 //! B -> A -> Return 32 //! @endcode 33 //! 34 //! Note that the function object `Algo` must be default-constructible, 35 //! since it will be called as `Algo{}(arguments...)`. 36 //! 37 //! @note 38 //! This function object is especially useful because it takes care of 39 //! avoiding ODR violations caused by the nested static constexpr member. 40 template <typename Algorithm> 41 struct nested_than { static constexpr nested_than_t<Algorithm> than{}; }; 42 43 template <typename Algorithm> 44 constexpr nested_than_t<Algorithm> nested_than<Algorithm>::than; 45 } BOOST_HANA_NAMESPACE_END 46 47 #endif // !BOOST_HANA_DETAIL_NESTED_THAN_FWD_HPP 48