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 13 #include "base/base_export.h" 14 #include "base/memory/ref_counted.h" 15 #include "base/memory/scoped_ptr.h" 16 17 template <typename T> 18 class ScopedVector; 19 20 namespace base { 21 namespace internal { 22 23 // BindStateBase is used to provide an opaque handle that the Callback 24 // class can use to represent a function object with bound arguments. It 25 // behaves as an existential type that is used by a corresponding 26 // DoInvoke function to perform the function execution. This allows 27 // us to shield the Callback class from the types of the bound argument via 28 // "type erasure." 29 class BindStateBase : public RefCountedThreadSafe<BindStateBase> { 30 protected: 31 friend class RefCountedThreadSafe<BindStateBase>; 32 virtual ~BindStateBase() {} 33 }; 34 35 // Holds the Callback methods that don't require specialization to reduce 36 // template bloat. 37 class BASE_EXPORT CallbackBase { 38 public: 39 // Returns true if Callback is null (doesn't refer to anything). 40 bool is_null() const; 41 42 // Returns the Callback into an uninitialized state. 43 void Reset(); 44 45 protected: 46 // In C++, it is safe to cast function pointers to function pointers of 47 // another type. It is not okay to use void*. We create a InvokeFuncStorage 48 // that that can store our function pointer, and then cast it back to 49 // the original type on usage. 50 typedef void(*InvokeFuncStorage)(void); 51 52 // Returns true if this callback equals |other|. |other| may be null. 53 bool Equals(const CallbackBase& other) const; 54 55 // Allow initializing of |bind_state_| via the constructor to avoid default 56 // initialization of the scoped_refptr. We do not also initialize 57 // |polymorphic_invoke_| here because doing a normal assignment in the 58 // derived Callback templates makes for much nicer compiler errors. 59 explicit CallbackBase(BindStateBase* bind_state); 60 61 // Force the destructor to be instantiated inside this translation unit so 62 // that our subclasses will not get inlined versions. Avoids more template 63 // bloat. 64 ~CallbackBase(); 65 66 scoped_refptr<BindStateBase> bind_state_; 67 InvokeFuncStorage polymorphic_invoke_; 68 }; 69 70 // A helper template to determine if given type is non-const move-only-type, 71 // i.e. if a value of the given type should be passed via .Pass() in a 72 // destructive way. 73 template <typename T> struct IsMoveOnlyType { 74 template <typename U> 75 static YesType Test(const typename U::MoveOnlyTypeForCPP03*); 76 77 template <typename U> 78 static NoType Test(...); 79 80 static const bool value = sizeof(Test<T>(0)) == sizeof(YesType) && 81 !is_const<T>::value; 82 }; 83 84 // This is a typetraits object that's used to take an argument type, and 85 // extract a suitable type for storing and forwarding arguments. 86 // 87 // In particular, it strips off references, and converts arrays to 88 // pointers for storage; and it avoids accidentally trying to create a 89 // "reference of a reference" if the argument is a reference type. 90 // 91 // This array type becomes an issue for storage because we are passing bound 92 // parameters by const reference. In this case, we end up passing an actual 93 // array type in the initializer list which C++ does not allow. This will 94 // break passing of C-string literals. 95 template <typename T, bool is_move_only = IsMoveOnlyType<T>::value> 96 struct CallbackParamTraits { 97 typedef const T& ForwardType; 98 typedef T StorageType; 99 }; 100 101 // The Storage should almost be impossible to trigger unless someone manually 102 // specifies type of the bind parameters. However, in case they do, 103 // this will guard against us accidentally storing a reference parameter. 104 // 105 // The ForwardType should only be used for unbound arguments. 106 template <typename T> 107 struct CallbackParamTraits<T&, false> { 108 typedef T& ForwardType; 109 typedef T StorageType; 110 }; 111 112 // Note that for array types, we implicitly add a const in the conversion. This 113 // means that it is not possible to bind array arguments to functions that take 114 // a non-const pointer. Trying to specialize the template based on a "const 115 // T[n]" does not seem to match correctly, so we are stuck with this 116 // restriction. 117 template <typename T, size_t n> 118 struct CallbackParamTraits<T[n], false> { 119 typedef const T* ForwardType; 120 typedef const T* StorageType; 121 }; 122 123 // See comment for CallbackParamTraits<T[n]>. 124 template <typename T> 125 struct CallbackParamTraits<T[], false> { 126 typedef const T* ForwardType; 127 typedef const T* StorageType; 128 }; 129 130 // Parameter traits for movable-but-not-copyable scopers. 131 // 132 // Callback<>/Bind() understands movable-but-not-copyable semantics where 133 // the type cannot be copied but can still have its state destructively 134 // transferred (aka. moved) to another instance of the same type by calling a 135 // helper function. When used with Bind(), this signifies transferal of the 136 // object's state to the target function. 137 // 138 // For these types, the ForwardType must not be a const reference, or a 139 // reference. A const reference is inappropriate, and would break const 140 // correctness, because we are implementing a destructive move. A non-const 141 // reference cannot be used with temporaries which means the result of a 142 // function or a cast would not be usable with Callback<> or Bind(). 143 template <typename T> 144 struct CallbackParamTraits<T, true> { 145 typedef T ForwardType; 146 typedef T StorageType; 147 }; 148 149 // CallbackForward() is a very limited simulation of C++11's std::forward() 150 // used by the Callback/Bind system for a set of movable-but-not-copyable 151 // types. It is needed because forwarding a movable-but-not-copyable 152 // argument to another function requires us to invoke the proper move 153 // operator to create a rvalue version of the type. The supported types are 154 // whitelisted below as overloads of the CallbackForward() function. The 155 // default template compiles out to be a no-op. 156 // 157 // In C++11, std::forward would replace all uses of this function. However, it 158 // is impossible to implement a general std::forward with C++11 due to a lack 159 // of rvalue references. 160 // 161 // In addition to Callback/Bind, this is used by PostTaskAndReplyWithResult to 162 // simulate std::forward() and forward the result of one Callback as a 163 // parameter to another callback. This is to support Callbacks that return 164 // the movable-but-not-copyable types whitelisted above. 165 template <typename T> 166 typename enable_if<!IsMoveOnlyType<T>::value, T>::type& CallbackForward(T& t) { 167 return t; 168 } 169 170 template <typename T> 171 typename enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward(T& t) { 172 return t.Pass(); 173 } 174 175 } // namespace internal 176 } // namespace base 177 178 #endif // BASE_CALLBACK_INTERNAL_H_ 179
