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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::detail::CanonicalConstant`. | |
4 | ||
b32b8144 | 5 | @copyright Louis Dionne 2013-2017 |
7c673cae FG |
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_DETAIL_CANONICAL_CONSTANT_HPP | |
11 | #define BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | ||
15 | ||
1e59de90 | 16 | namespace boost { namespace hana { namespace detail { |
7c673cae FG |
17 | //! @ingroup group-details |
18 | //! Tag representing a canonical `Constant`. | |
19 | //! | |
20 | //! This is an implementation detail used to provide many models for | |
21 | //! stuff like `Monoid`, `Group`, etc. To create a `CanonicalConstant`, | |
22 | //! simply create an object with a nested `hana_tag` equal to the proper | |
23 | //! specialization of `CanonicalConstant<T>`, and then also provide a | |
24 | //! `constexpr` static member `::%value` holding the value of the constant. | |
25 | template <typename T> | |
26 | struct CanonicalConstant { | |
27 | using value_type = T; | |
28 | }; | |
1e59de90 | 29 | } }} // end namespace boost::hana |
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30 | |
31 | ||
32 | #include <boost/hana/concept/constant.hpp> | |
33 | #include <boost/hana/concept/integral_constant.hpp> | |
34 | #include <boost/hana/core/to.hpp> | |
35 | #include <boost/hana/core/when.hpp> | |
36 | ||
37 | #include <type_traits> | |
38 | ||
39 | ||
1e59de90 | 40 | namespace boost { namespace hana { |
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41 | ////////////////////////////////////////////////////////////////////////// |
42 | // Constant | |
43 | ////////////////////////////////////////////////////////////////////////// | |
44 | template <typename T> | |
45 | struct value_impl<detail::CanonicalConstant<T>> { | |
46 | template <typename X> | |
47 | static constexpr decltype(auto) apply() | |
48 | { return X::value; } | |
49 | }; | |
50 | ||
51 | namespace detail { | |
52 | template <typename T, typename X> | |
53 | struct canonical_constant { | |
54 | static constexpr auto value = hana::to<T>(hana::value<X>()); | |
55 | using hana_tag = detail::CanonicalConstant<T>; | |
56 | }; | |
57 | } | |
58 | ||
59 | template <typename T, typename C> | |
60 | struct to_impl<detail::CanonicalConstant<T>, C, when< | |
61 | hana::Constant<C>::value && | |
62 | is_convertible<typename C::value_type, T>::value | |
63 | >> | |
64 | : embedding<is_embedded<typename C::value_type, T>::value> | |
65 | { | |
66 | template <typename X> | |
67 | static constexpr detail::canonical_constant<T, X> apply(X const&) | |
68 | { return {}; } | |
69 | }; | |
70 | ||
71 | ////////////////////////////////////////////////////////////////////////// | |
72 | // IntegralConstant (when value_type is integral) | |
73 | ////////////////////////////////////////////////////////////////////////// | |
74 | template <typename T> | |
75 | struct IntegralConstant<detail::CanonicalConstant<T>> { | |
76 | static constexpr bool value = std::is_integral<T>::value; | |
77 | }; | |
1e59de90 | 78 | }} // end namespace boost::hana |
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79 | |
80 | #endif // !BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP |