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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::eval_if`. | |
4 | ||
b32b8144 | 5 | @copyright Louis Dionne 2013-2017 |
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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_EVAL_IF_HPP | |
11 | #define BOOST_HANA_EVAL_IF_HPP | |
12 | ||
13 | #include <boost/hana/fwd/eval_if.hpp> | |
14 | ||
15 | #include <boost/hana/bool.hpp> | |
16 | #include <boost/hana/concept/constant.hpp> | |
17 | #include <boost/hana/concept/logical.hpp> | |
18 | #include <boost/hana/config.hpp> | |
19 | #include <boost/hana/core/dispatch.hpp> | |
20 | #include <boost/hana/eval.hpp> | |
21 | #include <boost/hana/if.hpp> | |
22 | ||
23 | #include <type_traits> | |
24 | ||
25 | ||
1e59de90 | 26 | namespace boost { namespace hana { |
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27 | //! @cond |
28 | template <typename Cond, typename Then, typename Else> | |
b32b8144 | 29 | constexpr decltype(auto) eval_if_t::operator()(Cond&& cond, Then&& then_, Else&& else_) const { |
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30 | using Bool = typename hana::tag_of<Cond>::type; |
31 | using EvalIf = BOOST_HANA_DISPATCH_IF(eval_if_impl<Bool>, | |
32 | hana::Logical<Bool>::value | |
33 | ); | |
34 | ||
35 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
36 | static_assert(hana::Logical<Bool>::value, | |
37 | "hana::eval_if(cond, then, else) requires 'cond' to be a Logical"); | |
38 | #endif | |
39 | ||
40 | return EvalIf::apply(static_cast<Cond&&>(cond), | |
b32b8144 | 41 | static_cast<Then&&>(then_), |
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42 | static_cast<Else&&>(else_)); |
43 | } | |
44 | //! @endcond | |
45 | ||
46 | template <typename L, bool condition> | |
47 | struct eval_if_impl<L, when<condition>> : default_ { | |
48 | template <typename ...Args> | |
49 | static constexpr auto apply(Args&& ...) = delete; | |
50 | }; | |
51 | ||
52 | ////////////////////////////////////////////////////////////////////////// | |
53 | // Model for arithmetic data types | |
54 | ////////////////////////////////////////////////////////////////////////// | |
55 | template <typename L> | |
56 | struct eval_if_impl<L, when<std::is_arithmetic<L>::value>> { | |
57 | template <typename Cond, typename T, typename E> | |
58 | static constexpr auto apply(Cond const& cond, T&& t, E&& e) { | |
59 | return cond ? hana::eval(static_cast<T&&>(t)) | |
60 | : hana::eval(static_cast<E&&>(e)); | |
61 | } | |
62 | }; | |
63 | ||
64 | ////////////////////////////////////////////////////////////////////////// | |
65 | // Model for Constants over a Logical | |
66 | ////////////////////////////////////////////////////////////////////////// | |
67 | template <typename C> | |
68 | struct eval_if_impl<C, when< | |
69 | hana::Constant<C>::value && | |
70 | Logical<typename C::value_type>::value | |
71 | >> { | |
72 | template <typename Then, typename Else> | |
73 | static constexpr decltype(auto) | |
74 | eval_if_helper(hana::true_, Then&& t, Else&&) | |
75 | { return hana::eval(static_cast<Then&&>(t)); } | |
76 | ||
77 | template <typename Then, typename Else> | |
78 | static constexpr decltype(auto) | |
79 | eval_if_helper(hana::false_, Then&&, Else&& e) | |
80 | { return hana::eval(static_cast<Else&&>(e)); } | |
81 | ||
82 | template <typename Cond, typename Then, typename Else> | |
83 | static constexpr decltype(auto) apply(Cond const&, Then&& t, Else&& e) { | |
84 | constexpr auto cond = hana::value<Cond>(); | |
85 | constexpr bool truth_value = hana::if_(cond, true, false); | |
b32b8144 | 86 | return eval_if_helper(hana::bool_<truth_value>{}, |
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87 | static_cast<Then&&>(t), |
88 | static_cast<Else&&>(e)); | |
89 | } | |
90 | }; | |
1e59de90 | 91 | }} // end namespace boost::hana |
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92 | |
93 | #endif // !BOOST_HANA_EVAL_IF_HPP |