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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::if_`. | |
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_IF_HPP | |
11 | #define BOOST_HANA_IF_HPP | |
12 | ||
13 | #include <boost/hana/fwd/if.hpp> | |
14 | ||
15 | #include <boost/hana/concept/logical.hpp> | |
16 | #include <boost/hana/config.hpp> | |
17 | #include <boost/hana/core/dispatch.hpp> | |
18 | #include <boost/hana/eval_if.hpp> | |
19 | ||
20 | ||
21 | BOOST_HANA_NAMESPACE_BEGIN | |
22 | //! @cond | |
23 | template <typename Cond, typename Then, typename Else> | |
24 | constexpr decltype(auto) if_t::operator()(Cond&& cond, Then&& then, Else&& else_) const { | |
25 | using Bool = typename hana::tag_of<Cond>::type; | |
26 | using If = BOOST_HANA_DISPATCH_IF(if_impl<Bool>, | |
27 | hana::Logical<Bool>::value | |
28 | ); | |
29 | ||
30 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
31 | static_assert(hana::Logical<Bool>::value, | |
32 | "hana::if_(cond, then, else) requires 'cond' to be a Logical"); | |
33 | #endif | |
34 | ||
35 | return If::apply(static_cast<Cond&&>(cond), | |
36 | static_cast<Then&&>(then), | |
37 | static_cast<Else&&>(else_)); | |
38 | } | |
39 | //! @endcond | |
40 | ||
41 | namespace detail { | |
42 | template <typename T> | |
43 | struct hold { | |
44 | T value; | |
45 | constexpr T&& operator()() && { return static_cast<T&&>(value); } | |
46 | }; | |
47 | } | |
48 | ||
49 | template <typename L, bool condition> | |
50 | struct if_impl<L, when<condition>> : default_ { | |
51 | template <typename C, typename T, typename E> | |
52 | static constexpr auto apply(C&& c, T&& t, E&& e) { | |
53 | return hana::eval_if(static_cast<C&&>(c), | |
54 | detail::hold<T&&>{static_cast<T&&>(t)}, | |
55 | detail::hold<E&&>{static_cast<E&&>(e)} | |
56 | ); | |
57 | } | |
58 | }; | |
59 | BOOST_HANA_NAMESPACE_END | |
60 | ||
61 | #endif // !BOOST_HANA_IF_HPP |