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/*!
@file
Forward declares `boost::hana::detail::nested_by`.
@copyright Louis Dionne 2013-2016
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>
BOOST_HANA_NAMESPACE_BEGIN 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;
} BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_DETAIL_NESTED_BY_FWD_HPP
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