| 1 | /*! |
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| 2 | @file |
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| 3 | Defines `BOOST_HANA_DISPATCH_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_DETAIL_DISPATCH_IF_HPP |
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| 11 | #define BOOST_HANA_DETAIL_DISPATCH_IF_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 | #include <type_traits> |
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| 16 | |
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| 17 | |
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| 18 | namespace boost { namespace hana { |
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| 19 | struct deleted_implementation { |
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| 20 | template <typename ...T> |
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| 21 | static constexpr auto apply(T&& ...) = delete; |
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| 22 | }; |
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| 23 | |
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| 24 | //! @ingroup group-details |
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| 25 | //! Dispatch to the given implementation method only when a condition is |
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| 26 | //! satisfied. |
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| 27 | //! |
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| 28 | //! If the condition is satisfied, this macro is equivalent to the type |
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| 29 | //! `IMPL`. Otherwise, it is equivalent to a type with a deleted static |
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| 30 | //! function named `apply`. When a tag-dispatching error happens, the |
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| 31 | //! condition should be false and the deleted static function `apply` |
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| 32 | //! will prevent the compiler from generating too much garbage. |
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| 33 | //! |
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| 34 | //! @note |
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| 35 | //! When `BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS` is defined, the |
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| 36 | //! condition is always ignored and this macro expands to the |
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| 37 | //! implementation only. |
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| 38 | //! |
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| 39 | //! @remark |
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| 40 | //! This must be implemented as a macro, because we don't want the |
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| 41 | //! condition to be evaluated at all when |
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| 42 | //! `BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS` is defined. |
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| 43 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS |
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| 44 | #define BOOST_HANA_DISPATCH_IF(IMPL, ...) \ |
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| 45 | ::std::conditional_t< \ |
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| 46 | (__VA_ARGS__), \ |
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| 47 | IMPL, \ |
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| 48 | ::boost::hana::deleted_implementation \ |
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| 49 | > \ |
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| 50 | /**/ |
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| 51 | #else |
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| 52 | #define BOOST_HANA_DISPATCH_IF(IMPL, ...) IMPL |
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| 53 | #endif |
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| 54 | }} // end namespace boost::hana |
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| 55 | |
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| 56 | #endif // !BOOST_HANA_DETAIL_DISPATCH_IF_HPP |
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| 57 | |
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