| // Copyright (c) 2012 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| // This file contains utility functions and classes that help the |
| // implementation, and management of the Callback objects. |
| |
| #ifndef BASE_CALLBACK_INTERNAL_H_ |
| #define BASE_CALLBACK_INTERNAL_H_ |
| |
| #include <stddef.h> |
| #include <map> |
| #include <memory> |
| #include <type_traits> |
| #include <vector> |
| |
| #include "base/base_export.h" |
| #include "base/macros.h" |
| #include "base/memory/ref_counted.h" |
| #include "base/memory/scoped_ptr.h" |
| #include "base/template_util.h" |
| |
| namespace base { |
| namespace internal { |
| class CallbackBase; |
| |
| // BindStateBase is used to provide an opaque handle that the Callback |
| // class can use to represent a function object with bound arguments. It |
| // behaves as an existential type that is used by a corresponding |
| // DoInvoke function to perform the function execution. This allows |
| // us to shield the Callback class from the types of the bound argument via |
| // "type erasure." |
| // At the base level, the only task is to add reference counting data. Don't use |
| // RefCountedThreadSafe since it requires the destructor to be a virtual method. |
| // Creating a vtable for every BindState template instantiation results in a lot |
| // of bloat. Its only task is to call the destructor which can be done with a |
| // function pointer. |
| class BindStateBase { |
| protected: |
| explicit BindStateBase(void (*destructor)(BindStateBase*)) |
| : ref_count_(0), destructor_(destructor) {} |
| ~BindStateBase() = default; |
| |
| private: |
| friend class scoped_refptr<BindStateBase>; |
| friend class CallbackBase; |
| |
| void AddRef(); |
| void Release(); |
| |
| std::atomic<int32_t> ref_count_; |
| |
| // Pointer to a function that will properly destroy |this|. |
| void (*destructor_)(BindStateBase*); |
| |
| DISALLOW_COPY_AND_ASSIGN(BindStateBase); |
| }; |
| |
| // Holds the Callback methods that don't require specialization to reduce |
| // template bloat. |
| class BASE_EXPORT CallbackBase { |
| public: |
| CallbackBase(const CallbackBase& c); |
| CallbackBase& operator=(const CallbackBase& c); |
| |
| // Returns true if Callback is null (doesn't refer to anything). |
| bool is_null() const { return bind_state_.get() == NULL; } |
| |
| // Returns the Callback into an uninitialized state. |
| void Reset(); |
| |
| protected: |
| // In C++, it is safe to cast function pointers to function pointers of |
| // another type. It is not okay to use void*. We create a InvokeFuncStorage |
| // that that can store our function pointer, and then cast it back to |
| // the original type on usage. |
| using InvokeFuncStorage = void(*)(); |
| |
| // Returns true if this callback equals |other|. |other| may be null. |
| bool Equals(const CallbackBase& other) const; |
| |
| // Allow initializing of |bind_state_| via the constructor to avoid default |
| // initialization of the scoped_refptr. We do not also initialize |
| // |polymorphic_invoke_| here because doing a normal assignment in the |
| // derived Callback templates makes for much nicer compiler errors. |
| explicit CallbackBase(BindStateBase* bind_state); |
| |
| // Force the destructor to be instantiated inside this translation unit so |
| // that our subclasses will not get inlined versions. Avoids more template |
| // bloat. |
| ~CallbackBase(); |
| |
| scoped_refptr<BindStateBase> bind_state_; |
| InvokeFuncStorage polymorphic_invoke_; |
| }; |
| |
| // A helper template to determine if given type is non-const move-only-type, |
| // i.e. if a value of the given type should be passed via std::move() in a |
| // destructive way. Types are considered to be move-only if they have a |
| // sentinel MoveOnlyTypeForCPP03 member: a class typically gets this from using |
| // the DISALLOW_COPY_AND_ASSIGN_WITH_MOVE_FOR_BIND macro. |
| // It would be easy to generalize this trait to all move-only types... but this |
| // confuses template deduction in VS2013 with certain types such as |
| // std::unique_ptr. |
| // TODO(dcheng): Revisit this when Windows switches to VS2015 by default. |
| template <typename T> struct IsMoveOnlyType { |
| template <typename U> |
| static YesType Test(const typename U::MoveOnlyTypeForCPP03*); |
| |
| template <typename U> |
| static NoType Test(...); |
| |
| static const bool value = sizeof((Test<T>(0))) == sizeof(YesType) && |
| !std::is_const<T>::value; |
| }; |
| |
| // Specialization of IsMoveOnlyType so that std::unique_ptr is still considered |
| // move-only, even without the sentinel member. |
| template <typename T> |
| struct IsMoveOnlyType<std::unique_ptr<T>> : std::true_type {}; |
| |
| template <typename> |
| struct CallbackParamTraitsForMoveOnlyType; |
| |
| template <typename> |
| struct CallbackParamTraitsForNonMoveOnlyType; |
| |
| // TODO(tzik): Use a default parameter once MSVS supports variadic templates |
| // with default values. |
| // http://connect.microsoft.com/VisualStudio/feedbackdetail/view/957801/compilation-error-with-variadic-templates |
| // |
| // This is a typetraits object that's used to take an argument type, and |
| // extract a suitable type for storing and forwarding arguments. |
| // |
| // In particular, it strips off references, and converts arrays to |
| // pointers for storage; and it avoids accidentally trying to create a |
| // "reference of a reference" if the argument is a reference type. |
| // |
| // This array type becomes an issue for storage because we are passing bound |
| // parameters by const reference. In this case, we end up passing an actual |
| // array type in the initializer list which C++ does not allow. This will |
| // break passing of C-string literals. |
| template <typename T> |
| struct CallbackParamTraits |
| : std::conditional<IsMoveOnlyType<T>::value, |
| CallbackParamTraitsForMoveOnlyType<T>, |
| CallbackParamTraitsForNonMoveOnlyType<T>>::type { |
| }; |
| |
| template <typename T> |
| struct CallbackParamTraitsForNonMoveOnlyType { |
| using ForwardType = const T&; |
| using StorageType = T; |
| }; |
| |
| // The Storage should almost be impossible to trigger unless someone manually |
| // specifies type of the bind parameters. However, in case they do, |
| // this will guard against us accidentally storing a reference parameter. |
| // |
| // The ForwardType should only be used for unbound arguments. |
| template <typename T> |
| struct CallbackParamTraitsForNonMoveOnlyType<T&> { |
| using ForwardType = T&; |
| using StorageType = T; |
| }; |
| |
| // Note that for array types, we implicitly add a const in the conversion. This |
| // means that it is not possible to bind array arguments to functions that take |
| // a non-const pointer. Trying to specialize the template based on a "const |
| // T[n]" does not seem to match correctly, so we are stuck with this |
| // restriction. |
| template <typename T, size_t n> |
| struct CallbackParamTraitsForNonMoveOnlyType<T[n]> { |
| using ForwardType = const T*; |
| using StorageType = const T*; |
| }; |
| |
| // See comment for CallbackParamTraits<T[n]>. |
| template <typename T> |
| struct CallbackParamTraitsForNonMoveOnlyType<T[]> { |
| using ForwardType = const T*; |
| using StorageType = const T*; |
| }; |
| |
| // Parameter traits for movable-but-not-copyable scopers. |
| // |
| // Callback<>/Bind() understands movable-but-not-copyable semantics where |
| // the type cannot be copied but can still have its state destructively |
| // transferred (aka. moved) to another instance of the same type by calling a |
| // helper function. When used with Bind(), this signifies transferal of the |
| // object's state to the target function. |
| // |
| // For these types, the ForwardType must not be a const reference, or a |
| // reference. A const reference is inappropriate, and would break const |
| // correctness, because we are implementing a destructive move. A non-const |
| // reference cannot be used with temporaries which means the result of a |
| // function or a cast would not be usable with Callback<> or Bind(). |
| template <typename T> |
| struct CallbackParamTraitsForMoveOnlyType { |
| using ForwardType = T; |
| using StorageType = T; |
| }; |
| |
| // CallbackForward() is a very limited simulation of C++11's std::forward() |
| // used by the Callback/Bind system for a set of movable-but-not-copyable |
| // types. It is needed because forwarding a movable-but-not-copyable |
| // argument to another function requires us to invoke the proper move |
| // operator to create a rvalue version of the type. The supported types are |
| // whitelisted below as overloads of the CallbackForward() function. The |
| // default template compiles out to be a no-op. |
| // |
| // In C++11, std::forward would replace all uses of this function. However, it |
| // is impossible to implement a general std::forward without C++11 due to a lack |
| // of rvalue references. |
| // |
| // In addition to Callback/Bind, this is used by PostTaskAndReplyWithResult to |
| // simulate std::forward() and forward the result of one Callback as a |
| // parameter to another callback. This is to support Callbacks that return |
| // the movable-but-not-copyable types whitelisted above. |
| template <typename T> |
| typename std::enable_if<!IsMoveOnlyType<T>::value, T>::type& CallbackForward( |
| T& t) { |
| return t; |
| } |
| |
| template <typename T> |
| typename std::enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward( |
| T& t) { |
| return std::move(t); |
| } |
| |
| } // namespace internal |
| } // namespace base |
| |
| #endif // BASE_CALLBACK_INTERNAL_H_ |