projects / ceph.git / blame
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|---|---|---|
| 7c673cae FG |
1 | /*! |
| 2 | @file | |
| 3 | Forward declares `boost::hana::when` and `boost::hana::when_valid`. | |
| 4 | ||
| 5 | @copyright Louis Dionne 2013-2016 | |
| 6 | Distributed under the Boost Software License, Version 1.0. | |
| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
| 8 | */ | |
| 9 | ||
| 10 | #ifndef BOOST_HANA_FWD_CORE_WHEN_HPP | |
| 11 | #define BOOST_HANA_FWD_CORE_WHEN_HPP | |
| 12 | ||
| 13 | #include <boost/hana/config.hpp> | |
| 14 | ||
| 15 | ||
| 16 | BOOST_HANA_NAMESPACE_BEGIN | |
| 17 | //! @ingroup group-core | |
| 18 | //! Enable a partial specialization only if a boolean condition is true. | |
| 19 | //! | |
| 20 | //! You might also want to take a look at `when_valid`, which provides | |
| 21 | //! similar functionality but enables a specialziation only when some | |
| 22 | //! expression is well-formed. | |
| 23 | //! | |
| 24 | //! > #### Rationale for using `when` instead of `std::enable_if` | |
| 25 | //! > `when` is used to control the priority of partial specializations | |
| 26 | //! > in a finer grained manner than what can be achieved with the usual | |
| 27 | //! > `typename Enable = void` and `std::enable_if` pattern. For example, | |
| 28 | //! > a partially specialized tag-dispatched method will have a higher | |
| 29 | //! > priority than an equivalent specialization that uses `when`. For | |
| 30 | //! > more details, see the tutorial section on [tag-dispatching][1]. | |
| 31 | //! | |
| 32 | //! | |
| 33 | //! Example | |
| 34 | //! ------- | |
| 35 | //! @include example/core/when.cpp | |
| 36 | //! | |
| 37 | //! [1]: @ref tutorial-core-tag_dispatching | |
| 38 | template <bool condition> | |
| 39 | struct when; | |
| 40 | ||
| 41 | namespace core_detail { | |
| 42 | template <typename ...> | |
| 43 | struct always_true { static constexpr bool value = true; }; | |
| 44 | } | |
| 45 | ||
| 46 | //! @ingroup group-core | |
| 47 | //! Variant of `when` allowing specializations to be enabled only if an | |
| 48 | //! expression is well-formed. | |
| 49 | //! | |
| 50 | //! `when_valid<...>` is always equivalent to `when<true>`. However, when | |
| 51 | //! used inside a partial specialization, SFINAE will cause the partial | |
| 52 | //! specialization to be ignored when the expression is ill-formed. | |
| 53 | //! | |
| 54 | //! | |
| 55 | //! Example | |
| 56 | //! ------- | |
| 57 | //! @include example/core/when_valid.cpp | |
| 58 | //! | |
| 59 | //! | |
| 60 | //! @bug | |
| 61 | //! Using `when_valid` seems to trigger ambiguous partial specializations | |
| 62 | //! on GCC. | |
| 63 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
| 64 | template <typename ...> | |
| 65 | using when_valid = when<true>; | |
| 66 | #else | |
| 67 | template <typename ...Dummy> | |
| 68 | using when_valid = when< | |
| 69 | core_detail::always_true<Dummy...>::value | |
| 70 | >; | |
| 71 | #endif | |
| 72 | BOOST_HANA_NAMESPACE_END | |
| 73 | ||
| 74 | #endif // !BOOST_HANA_FWD_CORE_WHEN_HPP |