/*! @file Forward declares `boost::hana::detail::nested_by`. @copyright Louis Dionne 2013-2017 Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) */ #ifndef BOOST_HANA_DETAIL_NESTED_BY_FWD_HPP #define BOOST_HANA_DETAIL_NESTED_BY_FWD_HPP #include <boost/hana/config.hpp> namespace boost { namespace hana { namespace detail { template <typename Algorithm> struct nested_by_t { template <typename Predicate, typename Object> constexpr decltype(auto) operator()(Predicate&& predicate, Object&& object) const; template <typename Predicate> constexpr decltype(auto) operator()(Predicate&& predicate) const; }; //! @ingroup group-details //! Provides a `.by` static constexpr function object. //! //! When creating a binary function object of type `Algorithm` whose //! signature is `Object x Predicate -> Return`, `nested_by<Algorithm>` //! can be used as a base class to `Algorithm`. Doing so will provide a //! static constexpr member called `by`, which has the two following //! signatures: //! @code //! Predicate x Object -> Return //! Predicate -> (Object -> Return) //! @endcode //! //! In other words, `nested_by` is a `curry`ed and `flip`ped version of //! `Algorithm`. Note that the function object `Algorithm` must be //! default-constructible, since the algorithm will be called as //! `Algorithm{}(arguments...)`. //! //! @note //! This function object is especially useful because it takes care of //! avoiding ODR violations caused by the nested static constexpr member. template <typename Algorithm> struct nested_by { static constexpr nested_by_t<Algorithm> by{}; }; template <typename Algorithm> constexpr nested_by_t<Algorithm> nested_by<Algorithm>::by; } }} // end namespace boost::hana #endif // !BOOST_HANA_DETAIL_NESTED_BY_FWD_HPP