1 /* 2 * Copyright (c) 2021 The WebRTC project authors. All Rights Reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 11 // This implementation is borrowed from Chromium. 12 13 #ifndef RTC_BASE_CONTAINERS_INVOKE_H_ 14 #define RTC_BASE_CONTAINERS_INVOKE_H_ 15 16 #include <type_traits> 17 #include <utility> 18 19 namespace webrtc { 20 21 namespace invoke_internal { 22 23 // Helper struct and alias to deduce the class type from a member function 24 // pointer or member object pointer. 25 template <typename DecayedF> 26 struct member_pointer_class {}; 27 28 template <typename ReturnT, typename ClassT> 29 struct member_pointer_class<ReturnT ClassT::*> { 30 using type = ClassT; 31 }; 32 33 template <typename DecayedF> 34 using member_pointer_class_t = typename member_pointer_class<DecayedF>::type; 35 36 // Utility struct to detect specializations of std::reference_wrapper. 37 template <typename T> 38 struct is_reference_wrapper : std::false_type {}; 39 40 template <typename T> 41 struct is_reference_wrapper<std::reference_wrapper<T>> : std::true_type {}; 42 43 // Small helpers used below in invoke_internal::invoke to make the SFINAE more 44 // concise. 45 template <typename F> 46 const bool& IsMemFunPtr = 47 std::is_member_function_pointer<std::decay_t<F>>::value; 48 49 template <typename F> 50 const bool& IsMemObjPtr = std::is_member_object_pointer<std::decay_t<F>>::value; 51 52 template <typename F, 53 typename T, 54 typename MemPtrClass = member_pointer_class_t<std::decay_t<F>>> 55 const bool& IsMemPtrToBaseOf = 56 std::is_base_of<MemPtrClass, std::decay_t<T>>::value; 57 58 template <typename T> 59 const bool& IsRefWrapper = is_reference_wrapper<std::decay_t<T>>::value; 60 61 template <bool B> 62 using EnableIf = std::enable_if_t<B, bool>; 63 64 // Invokes a member function pointer on a reference to an object of a suitable 65 // type. Covers bullet 1 of the INVOKE definition. 66 // 67 // Reference: https://wg21.link/func.require#1.1 68 template <typename F, 69 typename T1, 70 typename... Args, 71 EnableIf<IsMemFunPtr<F> && IsMemPtrToBaseOf<F, T1>> = true> 72 constexpr decltype(auto) InvokeImpl(F&& f, T1&& t1, Args&&... args) { 73 return (std::forward<T1>(t1).*f)(std::forward<Args>(args)...); 74 } 75 76 // Invokes a member function pointer on a std::reference_wrapper to an object of 77 // a suitable type. Covers bullet 2 of the INVOKE definition. 78 // 79 // Reference: https://wg21.link/func.require#1.2 80 template <typename F, 81 typename T1, 82 typename... Args, 83 EnableIf<IsMemFunPtr<F> && IsRefWrapper<T1>> = true> 84 constexpr decltype(auto) InvokeImpl(F&& f, T1&& t1, Args&&... args) { 85 return (t1.get().*f)(std::forward<Args>(args)...); 86 } 87 88 // Invokes a member function pointer on a pointer-like type to an object of a 89 // suitable type. Covers bullet 3 of the INVOKE definition. 90 // 91 // Reference: https://wg21.link/func.require#1.3 92 template <typename F, 93 typename T1, 94 typename... Args, 95 EnableIf<IsMemFunPtr<F> && !IsMemPtrToBaseOf<F, T1> && 96 !IsRefWrapper<T1>> = true> 97 constexpr decltype(auto) InvokeImpl(F&& f, T1&& t1, Args&&... args) { 98 return ((*std::forward<T1>(t1)).*f)(std::forward<Args>(args)...); 99 } 100 101 // Invokes a member object pointer on a reference to an object of a suitable 102 // type. Covers bullet 4 of the INVOKE definition. 103 // 104 // Reference: https://wg21.link/func.require#1.4 105 template <typename F, 106 typename T1, 107 EnableIf<IsMemObjPtr<F> && IsMemPtrToBaseOf<F, T1>> = true> 108 constexpr decltype(auto) InvokeImpl(F&& f, T1&& t1) { 109 return std::forward<T1>(t1).*f; 110 } 111 112 // Invokes a member object pointer on a std::reference_wrapper to an object of 113 // a suitable type. Covers bullet 5 of the INVOKE definition. 114 // 115 // Reference: https://wg21.link/func.require#1.5 116 template <typename F, 117 typename T1, 118 EnableIf<IsMemObjPtr<F> && IsRefWrapper<T1>> = true> 119 constexpr decltype(auto) InvokeImpl(F&& f, T1&& t1) { 120 return t1.get().*f; 121 } 122 123 // Invokes a member object pointer on a pointer-like type to an object of a 124 // suitable type. Covers bullet 6 of the INVOKE definition. 125 // 126 // Reference: https://wg21.link/func.require#1.6 127 template <typename F, 128 typename T1, 129 EnableIf<IsMemObjPtr<F> && !IsMemPtrToBaseOf<F, T1> && 130 !IsRefWrapper<T1>> = true> 131 constexpr decltype(auto) InvokeImpl(F&& f, T1&& t1) { 132 return (*std::forward<T1>(t1)).*f; 133 } 134 135 // Invokes a regular function or function object. Covers bullet 7 of the INVOKE 136 // definition. 137 // 138 // Reference: https://wg21.link/func.require#1.7 139 template <typename F, typename... Args> 140 constexpr decltype(auto) InvokeImpl(F&& f, Args&&... args) { 141 return std::forward<F>(f)(std::forward<Args>(args)...); 142 } 143 144 } // namespace invoke_internal 145 146 // Implementation of C++17's std::invoke. This is not based on implementation 147 // referenced in original std::invoke proposal, but rather a manual 148 // implementation, so that it can be constexpr. 149 // 150 // References: 151 // - https://wg21.link/n4169#implementability 152 // - https://en.cppreference.com/w/cpp/utility/functional/invoke 153 // - https://wg21.link/func.invoke 154 template <typename F, typename... Args> 155 constexpr decltype(auto) invoke(F&& f, Args&&... args) { 156 return invoke_internal::InvokeImpl(std::forward<F>(f), 157 std::forward<Args>(args)...); 158 } 159 160 } // namespace webrtc 161 162 #endif // RTC_BASE_CONTAINERS_INVOKE_H_ 163