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