| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::when` and `boost::hana::when_valid`. |
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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_CORE_WHEN_HPP |
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| 11 | #define BOOST_HANA_FWD_CORE_WHEN_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 { |
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| 17 | //! @ingroup group-core |
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| 18 | //! Enable a partial specialization only if a boolean condition is true. |
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| 19 | //! |
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| 20 | //! You might also want to take a look at `when_valid`, which provides |
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| 21 | //! similar functionality but enables a specialziation only when some |
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| 22 | //! expression is well-formed. |
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| 23 | //! |
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| 24 | //! > #### Rationale for using `when` instead of `std::enable_if` |
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| 25 | //! > `when` is used to control the priority of partial specializations |
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| 26 | //! > in a finer grained manner than what can be achieved with the usual |
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| 27 | //! > `typename Enable = void` and `std::enable_if` pattern. For example, |
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| 28 | //! > a partially specialized tag-dispatched method will have a higher |
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| 29 | //! > priority than an equivalent specialization that uses `when`. For |
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| 30 | //! > more details, see the tutorial section on [tag-dispatching][1]. |
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| 31 | //! |
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| 32 | //! |
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| 33 | //! Example |
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| 34 | //! ------- |
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| 35 | //! @include example/core/when.cpp |
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| 36 | //! |
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| 37 | //! [1]: @ref tutorial-core-tag_dispatching |
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| 38 | template <bool condition> |
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| 39 | struct when; |
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| 40 | |
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| 41 | namespace core_detail { |
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| 42 | template <typename ...> |
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| 43 | struct always_true { static constexpr bool value = true; }; |
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| 44 | } |
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| 45 | |
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| 46 | //! @ingroup group-core |
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| 47 | //! Variant of `when` allowing specializations to be enabled only if an |
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| 48 | //! expression is well-formed. |
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| 49 | //! |
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| 50 | //! `when_valid<...>` is always equivalent to `when<true>`. However, when |
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| 51 | //! used inside a partial specialization, SFINAE will cause the partial |
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| 52 | //! specialization to be ignored when the expression is ill-formed. |
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| 53 | //! |
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| 54 | //! |
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| 55 | //! Example |
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| 56 | //! ------- |
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| 57 | //! @include example/core/when_valid.cpp |
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| 58 | //! |
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| 59 | //! |
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| 60 | //! @bug |
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| 61 | //! Using `when_valid` seems to trigger ambiguous partial specializations |
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| 62 | //! on GCC. |
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| 63 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 64 | template <typename ...> |
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| 65 | using when_valid = when<true>; |
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| 66 | #else |
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| 67 | template <typename ...Dummy> |
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| 68 | using when_valid = when< |
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| 69 | core_detail::always_true<Dummy...>::value |
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| 70 | >; |
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| 71 | #endif |
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| 72 | }} // end namespace boost::hana |
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| 73 | |
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| 74 | #endif // !BOOST_HANA_FWD_CORE_WHEN_HPP |
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| 75 | |
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