| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::if_`. |
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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_FWD_IF_HPP |
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| 11 | #define BOOST_HANA_FWD_IF_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | #include <boost/hana/core/when.hpp> |
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| 15 | |
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| 16 | |
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| 17 | namespace boost { namespace hana { |
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| 18 | //! Conditionally return one of two values based on a condition. |
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| 19 | //! @ingroup group-Logical |
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| 20 | //! |
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| 21 | //! Specifically, `then` is returned iff `cond` is true-valued, and |
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| 22 | //! `else_` is returned otherwise. Note that some `Logical` models may |
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| 23 | //! allow `then` and `else_` to have different types, while others may |
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| 24 | //! require both values to have the same type. |
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| 25 | //! |
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| 26 | //! |
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| 27 | //! @param cond |
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| 28 | //! The condition determining which of the two values is returned. |
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| 29 | //! |
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| 30 | //! @param then |
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| 31 | //! The value returned when `cond` is true-valued. |
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| 32 | //! |
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| 33 | //! @param else_ |
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| 34 | //! The value returned when `cond` is false-valued. |
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| 35 | //! |
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| 36 | //! |
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| 37 | //! Example |
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| 38 | //! ------- |
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| 39 | //! @include example/if.cpp |
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| 40 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 41 | constexpr auto if_ = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) { |
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| 42 | return tag-dispatched; |
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| 43 | }; |
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| 44 | #else |
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| 45 | template <typename L, typename = void> |
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| 46 | struct if_impl : if_impl<L, when<true>> { }; |
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| 47 | |
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| 48 | struct if_t { |
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| 49 | template <typename Cond, typename Then, typename Else> |
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| 50 | constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const; |
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| 51 | }; |
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| 52 | |
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| 53 | BOOST_HANA_INLINE_VARIABLE constexpr if_t if_{}; |
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| 54 | #endif |
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| 55 | }} // end namespace boost::hana |
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| 56 | |
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| 57 | #endif // !BOOST_HANA_FWD_IF_HPP |
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| 58 | |
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