3 Defines `boost::hana::common` and `boost::hana::common_t`.
5 @copyright Louis Dionne 2013-2017
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)
10 #ifndef BOOST_HANA_CORE_COMMON_HPP
11 #define BOOST_HANA_CORE_COMMON_HPP
13 #include <boost/hana/fwd/core/common.hpp>
15 #include <boost/hana/concept/constant.hpp>
16 #include <boost/hana/config.hpp>
17 #include <boost/hana/core/when.hpp>
18 #include <boost/hana/detail/canonical_constant.hpp>
19 #include <boost/hana/detail/std_common_type.hpp>
20 #include <boost/hana/detail/void_t.hpp>
22 #include <type_traits>
25 BOOST_HANA_NAMESPACE_BEGIN
26 //////////////////////////////////////////////////////////////////////////
28 //////////////////////////////////////////////////////////////////////////
30 template <typename T, typename U, typename>
31 struct common : common<T, U, when<true>> { };
34 template <typename T, typename U, bool condition>
35 struct common<T, U, when<condition>>
36 : detail::std_common_type<T, U>
44 //////////////////////////////////////////////////////////////////////////
46 //////////////////////////////////////////////////////////////////////////
47 template <typename T, typename U, typename>
48 struct has_common : std::false_type { };
50 template <typename T, typename U>
51 struct has_common<T, U, detail::void_t<typename common<T, U>::type>>
55 //////////////////////////////////////////////////////////////////////////
56 // Provided common data types for Constants
57 //////////////////////////////////////////////////////////////////////////
58 namespace constant_detail {
60 //! This is an awful hack to avoid having
62 //! common<integral_constant_tag<int>, integral_constant_tag<long>>
64 //! CanonicalConstant<long>
66 template <typename A, typename B, typename C>
68 using type = detail::CanonicalConstant<C>;
71 template <template <typename ...> class A, typename T, typename U, typename C>
72 struct which<A<T>, A<U>, C> {
77 template <typename A, typename B>
78 struct common<A, B, when<
79 hana::Constant<A>::value &&
80 hana::Constant<B>::value &&
81 has_common<typename A::value_type, typename B::value_type>::value
83 using type = typename constant_detail::which<
85 typename common<typename A::value_type,
86 typename B::value_type>::type
90 template <typename A, typename B>
91 struct common<A, B, when<
92 hana::Constant<A>::value &&
93 !hana::Constant<B>::value &&
94 has_common<typename A::value_type, B>::value
96 using type = typename common<typename A::value_type, B>::type;
99 template <typename A, typename B>
100 struct common<A, B, when<
101 !hana::Constant<A>::value &&
102 hana::Constant<B>::value &&
103 has_common<A, typename B::value_type>::value
105 using type = typename common<A, typename B::value_type>::type;
107 BOOST_HANA_NAMESPACE_END
109 #endif // !BOOST_HANA_CORE_COMMON_HPP