1#ifndef BOOST_TYPE_TRAITS_COMMON_TYPE_HPP_INCLUDED
2#define BOOST_TYPE_TRAITS_COMMON_TYPE_HPP_INCLUDED
3
4//
5// Copyright 2015 Peter Dimov
6//
7// Distributed under the Boost Software License, Version 1.0.
8// See accompanying file LICENSE_1_0.txt or copy at
9// http://www.boost.org/LICENSE_1_0.txt
10//
11
12#include <boost/config.hpp>
13#include <boost/type_traits/decay.hpp>
14#include <boost/type_traits/declval.hpp>
15
16#if defined(BOOST_NO_CXX11_DECLTYPE)
17#include <boost/type_traits/detail/common_type_impl.hpp>
18#endif
19
20#if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES) && !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
21#include <boost/type_traits/detail/mp_defer.hpp>
22#endif
23
24namespace boost
25{
26
27// variadic common_type
28
29#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
30
31template<class... T> struct common_type
32{
33};
34
35#if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)
36
37template<class... T> using common_type_t = typename common_type<T...>::type;
38
39namespace type_traits_detail
40{
41
42template<class T1, class T2, class... T> using common_type_fold = common_type_t<common_type_t<T1, T2>, T...>;
43
44} // namespace type_traits_detail
45
46template<class T1, class T2, class... T>
47struct common_type<T1, T2, T...>: type_traits_detail::mp_defer<type_traits_detail::common_type_fold, T1, T2, T...>
48{
49};
50
51#else
52
53template<class T1, class T2, class... T>
54struct common_type<T1, T2, T...>: common_type<typename common_type<T1, T2>::type, T...>
55{
56};
57
58#endif // !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)
59
60#else
61
62template<
63 class T1 = void, class T2 = void, class T3 = void,
64 class T4 = void, class T5 = void, class T6 = void,
65 class T7 = void, class T8 = void, class T9 = void
66>
67struct common_type: common_type<typename common_type<T1, T2>::type, T3, T4, T5, T6, T7, T8, T9>
68{
69};
70
71#endif // !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
72
73// one argument
74
75template<class T> struct common_type<T>: boost::decay<T>
76{
77};
78
79// two arguments
80
81namespace type_traits_detail
82{
83
84// binary common_type
85
86#if !defined(BOOST_NO_CXX11_DECLTYPE)
87
88#if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES) && !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
89
90#if !defined(BOOST_MSVC) || BOOST_MSVC > 1800
91
92// internal compiler error on msvc-12.0
93
94template<class T1, class T2> using builtin_common_type = typename boost::decay<decltype( boost::declval<bool>()? boost::declval<T1>(): boost::declval<T2>() )>::type;
95
96template<class T1, class T2> struct common_type_impl: mp_defer<builtin_common_type, T1, T2>
97{
98};
99
100#else
101
102template<class T1, class T2> using builtin_common_type = decltype( boost::declval<bool>()? boost::declval<T1>(): boost::declval<T2>() );
103
104template<class T1, class T2> struct common_type_impl_2: mp_defer<builtin_common_type, T1, T2>
105{
106};
107
108template<class T1, class T2> using decay_common_type = typename boost::decay<typename common_type_impl_2<T1, T2>::type>::type;
109
110template<class T1, class T2> struct common_type_impl: mp_defer<decay_common_type, T1, T2>
111{
112};
113
114#endif // !defined(BOOST_MSVC) || BOOST_MSVC > 1800
115
116#else
117
118template<class T1, class T2> struct common_type_impl: boost::decay<decltype( boost::declval<bool>()? boost::declval<T1>(): boost::declval<T2>() )>
119{
120};
121
122#endif // #if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES) && !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
123
124#endif // #if !defined(BOOST_NO_CXX11_DECLTYPE)
125
126// decay helper
127
128template<class T1, class T2, class T1d = typename boost::decay<T1>::type, class T2d = typename boost::decay<T2>::type> struct common_type_decay_helper: boost::common_type<T1d, T2d>
129{
130};
131
132template<class T1, class T2> struct common_type_decay_helper<T1, T2, T1, T2>: common_type_impl<T1, T2>
133{
134};
135
136} // type_traits_detail
137
138template<class T1, class T2> struct common_type<T1, T2>: type_traits_detail::common_type_decay_helper<T1, T2>
139{
140};
141
142} // namespace boost
143
144#endif // #ifndef BOOST_TYPE_TRAITS_COMMON_TYPE_HPP_INCLUDED
145

source code of boost/boost/type_traits/common_type.hpp