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1 | // Copyright (c) 2011-present, Facebook, Inc. All rights reserved. |
2 | // This source code is licensed under both the GPLv2 (found in the | |
3 | // COPYING file in the root directory) and Apache 2.0 License | |
4 | // (found in the LICENSE.Apache file in the root directory). | |
5 | ||
6 | #pragma once | |
7 | ||
8 | #include <type_traits> | |
9 | #include <utility> | |
10 | ||
11 | namespace folly { | |
12 | ||
13 | #if !defined(_MSC_VER) | |
14 | template <class T> | |
15 | struct is_trivially_copyable | |
16 | : std::integral_constant<bool, __has_trivial_copy(T)> {}; | |
17 | #else | |
18 | template <class T> | |
19 | using is_trivially_copyable = std::is_trivially_copyable<T>; | |
20 | #endif | |
21 | ||
22 | /*** | |
23 | * _t | |
24 | * | |
25 | * Instead of: | |
26 | * | |
27 | * using decayed = typename std::decay<T>::type; | |
28 | * | |
29 | * With the C++14 standard trait aliases, we could use: | |
30 | * | |
31 | * using decayed = std::decay_t<T>; | |
32 | * | |
33 | * Without them, we could use: | |
34 | * | |
35 | * using decayed = _t<std::decay<T>>; | |
36 | * | |
37 | * Also useful for any other library with template types having dependent | |
38 | * member types named `type`, like the standard trait types. | |
39 | */ | |
40 | template <typename T> | |
41 | using _t = typename T::type; | |
42 | ||
43 | /** | |
44 | * type_t | |
45 | * | |
46 | * A type alias for the first template type argument. `type_t` is useful for | |
47 | * controlling class-template and function-template partial specialization. | |
48 | * | |
49 | * Example: | |
50 | * | |
51 | * template <typename Value> | |
52 | * class Container { | |
53 | * public: | |
54 | * template <typename... Args> | |
55 | * Container( | |
56 | * type_t<in_place_t, decltype(Value(std::declval<Args>()...))>, | |
57 | * Args&&...); | |
58 | * }; | |
59 | * | |
60 | * void_t | |
61 | * | |
62 | * A type alias for `void`. `void_t` is useful for controling class-template | |
63 | * and function-template partial specialization. | |
64 | * | |
65 | * Example: | |
66 | * | |
67 | * // has_value_type<T>::value is true if T has a nested type `value_type` | |
68 | * template <class T, class = void> | |
69 | * struct has_value_type | |
70 | * : std::false_type {}; | |
71 | * | |
72 | * template <class T> | |
73 | * struct has_value_type<T, folly::void_t<typename T::value_type>> | |
74 | * : std::true_type {}; | |
75 | */ | |
76 | ||
77 | /** | |
78 | * There is a bug in libstdc++, libc++, and MSVC's STL that causes it to | |
79 | * ignore unused template parameter arguments in template aliases and does not | |
80 | * cause substitution failures. This defect has been recorded here: | |
81 | * http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558. | |
82 | * | |
83 | * This causes the implementation of std::void_t to be buggy, as it is likely | |
84 | * defined as something like the following: | |
85 | * | |
86 | * template <typename...> | |
87 | * using void_t = void; | |
88 | * | |
89 | * This causes the compiler to ignore all the template arguments and does not | |
90 | * help when one wants to cause substitution failures. Rather declarations | |
91 | * which have void_t in orthogonal specializations are treated as the same. | |
92 | * For example, assuming the possible `T` types are only allowed to have | |
93 | * either the alias `one` or `two` and never both or none: | |
94 | * | |
95 | * template <typename T, | |
96 | * typename std::void_t<std::decay_t<T>::one>* = nullptr> | |
97 | * void foo(T&&) {} | |
98 | * template <typename T, | |
99 | * typename std::void_t<std::decay_t<T>::two>* = nullptr> | |
100 | * void foo(T&&) {} | |
101 | * | |
102 | * The second foo() will be a redefinition because it conflicts with the first | |
103 | * one; void_t does not cause substitution failures - the template types are | |
104 | * just ignored. | |
105 | */ | |
106 | ||
107 | namespace traits_detail { | |
108 | template <class T, class...> | |
109 | struct type_t_ { | |
110 | using type = T; | |
111 | }; | |
112 | } // namespace traits_detail | |
113 | ||
114 | template <class T, class... Ts> | |
115 | using type_t = typename traits_detail::type_t_<T, Ts...>::type; | |
116 | template <class... Ts> | |
117 | using void_t = type_t<void, Ts...>; | |
118 | ||
119 | /** | |
120 | * A type trait to remove all const volatile and reference qualifiers on a | |
121 | * type T | |
122 | */ | |
123 | template <typename T> | |
124 | struct remove_cvref { | |
125 | using type = | |
126 | typename std::remove_cv<typename std::remove_reference<T>::type>::type; | |
127 | }; | |
128 | template <typename T> | |
129 | using remove_cvref_t = typename remove_cvref<T>::type; | |
130 | ||
131 | template <class T> | |
132 | struct IsNothrowSwappable | |
133 | : std::integral_constant< | |
134 | bool, | |
135 | std::is_nothrow_move_constructible<T>::value&& noexcept( | |
136 | std::swap(std::declval<T&>(), std::declval<T&>()))> {}; | |
137 | ||
138 | template <typename...> | |
139 | struct Conjunction : std::true_type {}; | |
140 | template <typename T> | |
141 | struct Conjunction<T> : T {}; | |
142 | template <typename T, typename... TList> | |
143 | struct Conjunction<T, TList...> | |
144 | : std::conditional<T::value, Conjunction<TList...>, T>::type {}; | |
145 | ||
146 | template <typename T> | |
147 | struct Negation : std::integral_constant<bool, !T::value> {}; | |
148 | ||
149 | template <std::size_t I> | |
150 | using index_constant = std::integral_constant<std::size_t, I>; | |
151 | ||
152 | } // namespace folly |