1/*!
2@file
3Defines `boost::hana::eval_if`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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
26namespace boost { namespace hana {
27 //! @cond
28 template <typename Cond, typename Then, typename Else>
29 constexpr decltype(auto) eval_if_t::operator()(Cond&& cond, Then&& then_, Else&& else_) const {
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),
41 static_cast<Then&&>(then_),
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);
86 return eval_if_helper(hana::bool_<truth_value>{},
87 static_cast<Then&&>(t),
88 static_cast<Else&&>(e));
89 }
90 };
91}} // end namespace boost::hana
92
93#endif // !BOOST_HANA_EVAL_IF_HPP
94

source code of boost/libs/hana/include/boost/hana/eval_if.hpp