| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::detail::nested_by`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_DETAIL_NESTED_BY_FWD_HPP |
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| 11 | #define BOOST_HANA_DETAIL_NESTED_BY_FWD_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | |
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| 15 | |
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| 16 | namespace boost { namespace hana { namespace detail { |
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| 17 | template <typename Algorithm> |
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| 18 | struct nested_by_t { |
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| 19 | template <typename Predicate, typename Object> |
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| 20 | constexpr decltype(auto) |
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| 21 | operator()(Predicate&& predicate, Object&& object) const; |
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| 22 | |
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| 23 | template <typename Predicate> |
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| 24 | constexpr decltype(auto) operator()(Predicate&& predicate) const; |
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| 25 | }; |
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| 26 | |
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| 27 | //! @ingroup group-details |
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| 28 | //! Provides a `.by` static constexpr function object. |
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| 29 | //! |
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| 30 | //! When creating a binary function object of type `Algorithm` whose |
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| 31 | //! signature is `Object x Predicate -> Return`, `nested_by<Algorithm>` |
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| 32 | //! can be used as a base class to `Algorithm`. Doing so will provide a |
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| 33 | //! static constexpr member called `by`, which has the two following |
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| 34 | //! signatures: |
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| 35 | //! @code |
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| 36 | //! Predicate x Object -> Return |
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| 37 | //! Predicate -> (Object -> Return) |
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| 38 | //! @endcode |
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| 39 | //! |
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| 40 | //! In other words, `nested_by` is a `curry`ed and `flip`ped version of |
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| 41 | //! `Algorithm`. Note that the function object `Algorithm` must be |
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| 42 | //! default-constructible, since the algorithm will be called as |
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| 43 | //! `Algorithm{}(arguments...)`. |
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| 44 | //! |
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| 45 | //! @note |
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| 46 | //! This function object is especially useful because it takes care of |
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| 47 | //! avoiding ODR violations caused by the nested static constexpr member. |
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| 48 | template <typename Algorithm> |
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| 49 | struct nested_by { static constexpr nested_by_t<Algorithm> by{}; }; |
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| 50 | |
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| 51 | template <typename Algorithm> |
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| 52 | constexpr nested_by_t<Algorithm> nested_by<Algorithm>::by; |
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| 53 | } }} // end namespace boost::hana |
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| 54 | |
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| 55 | #endif // !BOOST_HANA_DETAIL_NESTED_BY_FWD_HPP |
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| 56 | |
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