| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::detail::nested_to`. |
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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_TO_FWD_HPP |
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| 11 | #define BOOST_HANA_DETAIL_NESTED_TO_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_to_t { |
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| 19 | template <typename X> |
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| 20 | constexpr decltype(auto) operator()(X&& x) const; |
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| 21 | }; |
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| 22 | |
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| 23 | //! @ingroup group-details |
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| 24 | //! Provides a `.to` static constexpr function object. |
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| 25 | //! |
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| 26 | //! When creating a binary function object of type `Algo` whose signature |
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| 27 | //! is `Object x Object -> Return`, `nested_to<Algo>` can be used as a base |
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| 28 | //! class of `Algo`. Doing so will provide a static constexpr member called |
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| 29 | //! `to`, which has the following signature: |
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| 30 | //! @code |
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| 31 | //! Object -> Object -> Return |
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| 32 | //! @endcode |
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| 33 | //! |
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| 34 | //! Note that the function object `Algo` must be default-constructible, |
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| 35 | //! since the algorithm will be called as `Algo{}(arguments...)`. |
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| 36 | //! |
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| 37 | //! @note |
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| 38 | //! This function object is especially useful because it takes care of |
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| 39 | //! avoiding ODR violations caused by the nested static constexpr member. |
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| 40 | template <typename Algorithm> |
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| 41 | struct nested_to { static constexpr nested_to_t<Algorithm> to{}; }; |
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| 42 | |
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| 43 | template <typename Algorithm> |
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| 44 | constexpr nested_to_t<Algorithm> nested_to<Algorithm>::to; |
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| 45 | } }} // end namespace boost::hana |
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| 46 | |
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| 47 | #endif // !BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP |
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| 48 | |
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