1 // Copyright (c) 2012 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 // This file contains utility functions and classes that help the 6 // implementation, and management of the Callback objects. 7 8 #ifndef BASE_CALLBACK_INTERNAL_H_ 9 #define BASE_CALLBACK_INTERNAL_H_ 10 11 #include <stddef.h> 12 #include <map> 13 #include <memory> 14 #include <type_traits> 15 #include <vector> 16 17 #include "base/atomic_ref_count.h" 18 #include "base/base_export.h" 19 #include "base/macros.h" 20 #include "base/memory/ref_counted.h" 21 #include "base/memory/scoped_ptr.h" 22 #include "base/template_util.h" 23 24 namespace base { 25 namespace internal { 26 class CallbackBase; 27 28 // BindStateBase is used to provide an opaque handle that the Callback 29 // class can use to represent a function object with bound arguments. It 30 // behaves as an existential type that is used by a corresponding 31 // DoInvoke function to perform the function execution. This allows 32 // us to shield the Callback class from the types of the bound argument via 33 // "type erasure." 34 // At the base level, the only task is to add reference counting data. Don't use 35 // RefCountedThreadSafe since it requires the destructor to be a virtual method. 36 // Creating a vtable for every BindState template instantiation results in a lot 37 // of bloat. Its only task is to call the destructor which can be done with a 38 // function pointer. 39 class BindStateBase { 40 protected: BindStateBase(void (* destructor)(BindStateBase *))41 explicit BindStateBase(void (*destructor)(BindStateBase*)) 42 : ref_count_(0), destructor_(destructor) {} 43 ~BindStateBase() = default; 44 45 private: 46 friend class scoped_refptr<BindStateBase>; 47 friend class CallbackBase; 48 49 void AddRef(); 50 void Release(); 51 52 AtomicRefCount ref_count_; 53 54 // Pointer to a function that will properly destroy |this|. 55 void (*destructor_)(BindStateBase*); 56 57 DISALLOW_COPY_AND_ASSIGN(BindStateBase); 58 }; 59 60 // Holds the Callback methods that don't require specialization to reduce 61 // template bloat. 62 class BASE_EXPORT CallbackBase { 63 public: 64 CallbackBase(const CallbackBase& c); 65 CallbackBase& operator=(const CallbackBase& c); 66 67 // Returns true if Callback is null (doesn't refer to anything). is_null()68 bool is_null() const { return bind_state_.get() == NULL; } 69 70 // Returns the Callback into an uninitialized state. 71 void Reset(); 72 73 protected: 74 // In C++, it is safe to cast function pointers to function pointers of 75 // another type. It is not okay to use void*. We create a InvokeFuncStorage 76 // that that can store our function pointer, and then cast it back to 77 // the original type on usage. 78 using InvokeFuncStorage = void(*)(); 79 80 // Returns true if this callback equals |other|. |other| may be null. 81 bool Equals(const CallbackBase& other) const; 82 83 // Allow initializing of |bind_state_| via the constructor to avoid default 84 // initialization of the scoped_refptr. We do not also initialize 85 // |polymorphic_invoke_| here because doing a normal assignment in the 86 // derived Callback templates makes for much nicer compiler errors. 87 explicit CallbackBase(BindStateBase* bind_state); 88 89 // Force the destructor to be instantiated inside this translation unit so 90 // that our subclasses will not get inlined versions. Avoids more template 91 // bloat. 92 ~CallbackBase(); 93 94 scoped_refptr<BindStateBase> bind_state_; 95 InvokeFuncStorage polymorphic_invoke_; 96 }; 97 98 // A helper template to determine if given type is non-const move-only-type, 99 // i.e. if a value of the given type should be passed via std::move() in a 100 // destructive way. Types are considered to be move-only if they have a 101 // sentinel MoveOnlyTypeForCPP03 member: a class typically gets this from using 102 // the DISALLOW_COPY_AND_ASSIGN_WITH_MOVE_FOR_BIND macro. 103 // It would be easy to generalize this trait to all move-only types... but this 104 // confuses template deduction in VS2013 with certain types such as 105 // std::unique_ptr. 106 // TODO(dcheng): Revisit this when Windows switches to VS2015 by default. 107 template <typename T> struct IsMoveOnlyType { 108 template <typename U> 109 static YesType Test(const typename U::MoveOnlyTypeForCPP03*); 110 111 template <typename U> 112 static NoType Test(...); 113 114 static const bool value = sizeof((Test<T>(0))) == sizeof(YesType) && 115 !is_const<T>::value; 116 }; 117 118 // Specialization of IsMoveOnlyType so that std::unique_ptr is still considered 119 // move-only, even without the sentinel member. 120 template <typename T> 121 struct IsMoveOnlyType<std::unique_ptr<T>> : std::true_type {}; 122 123 template <typename> 124 struct CallbackParamTraitsForMoveOnlyType; 125 126 template <typename> 127 struct CallbackParamTraitsForNonMoveOnlyType; 128 129 // TODO(tzik): Use a default parameter once MSVS supports variadic templates 130 // with default values. 131 // http://connect.microsoft.com/VisualStudio/feedbackdetail/view/957801/compilation-error-with-variadic-templates 132 // 133 // This is a typetraits object that's used to take an argument type, and 134 // extract a suitable type for storing and forwarding arguments. 135 // 136 // In particular, it strips off references, and converts arrays to 137 // pointers for storage; and it avoids accidentally trying to create a 138 // "reference of a reference" if the argument is a reference type. 139 // 140 // This array type becomes an issue for storage because we are passing bound 141 // parameters by const reference. In this case, we end up passing an actual 142 // array type in the initializer list which C++ does not allow. This will 143 // break passing of C-string literals. 144 template <typename T> 145 struct CallbackParamTraits 146 : std::conditional<IsMoveOnlyType<T>::value, 147 CallbackParamTraitsForMoveOnlyType<T>, 148 CallbackParamTraitsForNonMoveOnlyType<T>>::type { 149 }; 150 151 template <typename T> 152 struct CallbackParamTraitsForNonMoveOnlyType { 153 using ForwardType = const T&; 154 using StorageType = T; 155 }; 156 157 // The Storage should almost be impossible to trigger unless someone manually 158 // specifies type of the bind parameters. However, in case they do, 159 // this will guard against us accidentally storing a reference parameter. 160 // 161 // The ForwardType should only be used for unbound arguments. 162 template <typename T> 163 struct CallbackParamTraitsForNonMoveOnlyType<T&> { 164 using ForwardType = T&; 165 using StorageType = T; 166 }; 167 168 // Note that for array types, we implicitly add a const in the conversion. This 169 // means that it is not possible to bind array arguments to functions that take 170 // a non-const pointer. Trying to specialize the template based on a "const 171 // T[n]" does not seem to match correctly, so we are stuck with this 172 // restriction. 173 template <typename T, size_t n> 174 struct CallbackParamTraitsForNonMoveOnlyType<T[n]> { 175 using ForwardType = const T*; 176 using StorageType = const T*; 177 }; 178 179 // See comment for CallbackParamTraits<T[n]>. 180 template <typename T> 181 struct CallbackParamTraitsForNonMoveOnlyType<T[]> { 182 using ForwardType = const T*; 183 using StorageType = const T*; 184 }; 185 186 // Parameter traits for movable-but-not-copyable scopers. 187 // 188 // Callback<>/Bind() understands movable-but-not-copyable semantics where 189 // the type cannot be copied but can still have its state destructively 190 // transferred (aka. moved) to another instance of the same type by calling a 191 // helper function. When used with Bind(), this signifies transferal of the 192 // object's state to the target function. 193 // 194 // For these types, the ForwardType must not be a const reference, or a 195 // reference. A const reference is inappropriate, and would break const 196 // correctness, because we are implementing a destructive move. A non-const 197 // reference cannot be used with temporaries which means the result of a 198 // function or a cast would not be usable with Callback<> or Bind(). 199 template <typename T> 200 struct CallbackParamTraitsForMoveOnlyType { 201 using ForwardType = T; 202 using StorageType = T; 203 }; 204 205 // CallbackForward() is a very limited simulation of C++11's std::forward() 206 // used by the Callback/Bind system for a set of movable-but-not-copyable 207 // types. It is needed because forwarding a movable-but-not-copyable 208 // argument to another function requires us to invoke the proper move 209 // operator to create a rvalue version of the type. The supported types are 210 // whitelisted below as overloads of the CallbackForward() function. The 211 // default template compiles out to be a no-op. 212 // 213 // In C++11, std::forward would replace all uses of this function. However, it 214 // is impossible to implement a general std::forward without C++11 due to a lack 215 // of rvalue references. 216 // 217 // In addition to Callback/Bind, this is used by PostTaskAndReplyWithResult to 218 // simulate std::forward() and forward the result of one Callback as a 219 // parameter to another callback. This is to support Callbacks that return 220 // the movable-but-not-copyable types whitelisted above. 221 template <typename T> 222 typename std::enable_if<!IsMoveOnlyType<T>::value, T>::type& CallbackForward( 223 T& t) { 224 return t; 225 } 226 227 template <typename T> 228 typename std::enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward( 229 T& t) { 230 return std::move(t); 231 } 232 233 } // namespace internal 234 } // namespace base 235 236 #endif // BASE_CALLBACK_INTERNAL_H_ 237