Unified callback system.

base/callback.h

-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// This file was GENERATED by command:
+//     pump.py callback.h.pump
+// DO NOT EDIT BY HAND!!!
+
+
+// Copyright (c) 2011 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.
 
 #ifndef BASE_CALLBACK_H_
 #define BASE_CALLBACK_H_
 #pragma once
 
-#include "base/tuple.h"
-#include "base/raw_scoped_refptr_mismatch_checker.h"
-
-// Callback --------------------------------------------------------------------
-//
-// A Callback is like a Task but with unbound parameters. It is basically an
-// object-oriented function pointer.
-//
-// Callbacks are designed to work with Tuples.  A set of helper functions and
-// classes is provided to hide the Tuple details from the consumer.  Client
-// code will generally work with the CallbackRunner base class, which merely
-// provides a Run method and is returned by the New* functions. This allows
-// users to not care which type of class implements the callback, only that it
-// has a certain number and type of arguments.
-//
-// The implementation of this is done by CallbackImpl, which inherits
-// CallbackStorage to store the data. This allows the storage of the data
-// (requiring the class type T) to be hidden from users, who will want to call
-// this regardless of the implementor's type T.
-//
-// Note that callbacks currently have no facility for cancelling or abandoning
-// them. We currently handle this at a higher level for cases where this is
-// necessary. The pointer in a callback must remain valid until the callback
-// is made.
-//
-// Like Task, the callback executor is responsible for deleting the callback
-// pointer once the callback has executed.
-//
-// Example client usage:
-//   void Object::DoStuff(int, string);
-//   Callback2<int, string>::Type* callback =
-//       NewCallback(obj, &Object::DoStuff);
-//   callback->Run(5, string("hello"));
-//   delete callback;
-// or, equivalently, using tuples directly:
-//   CallbackRunner<Tuple2<int, string> >* callback =
-//       NewCallback(obj, &Object::DoStuff);
-//   callback->RunWithParams(MakeTuple(5, string("hello")));
-//
-// There is also a 0-args version that returns a value.  Example:
-//   int Object::GetNextInt();
-//   CallbackWithReturnValue<int>::Type* callback =
-//       NewCallbackWithReturnValue(obj, &Object::GetNextInt);
-//   int next_int = callback->Run();
-//   delete callback;
-
-// Base for all Callbacks that handles storage of the pointers.
-template <class T, typename Method>
-class CallbackStorage {
- public:
-  CallbackStorage(T* obj, Method meth) : obj_(obj), meth_(meth) {
-  }
-
- protected:
-  T* obj_;
-  Method meth_;
-};
-
-// Interface that is exposed to the consumer, that does the actual calling
-// of the method.
-template <typename Params>
-class CallbackRunner {
- public:
-  typedef Params TupleType;
-
-  virtual ~CallbackRunner() {}
-  virtual void RunWithParams(const Params& params) = 0;
-
-  // Convenience functions so callers don't have to deal with Tuples.
-  inline void Run() {
-    RunWithParams(Tuple0());
-  }
-
-  template <typename Arg1>
-  inline void Run(const Arg1& a) {
-    RunWithParams(Params(a));
-  }
-
-  template <typename Arg1, typename Arg2>
-  inline void Run(const Arg1& a, const Arg2& b) {
-    RunWithParams(Params(a, b));
-  }
-
-  template <typename Arg1, typename Arg2, typename Arg3>
-  inline void Run(const Arg1& a, const Arg2& b, const Arg3& c) {
-    RunWithParams(Params(a, b, c));
-  }
-
-  template <typename Arg1, typename Arg2, typename Arg3, typename Arg4>
-  inline void Run(const Arg1& a, const Arg2& b, const Arg3& c, const Arg4& d) {
-    RunWithParams(Params(a, b, c, d));
-  }
-
-  template <typename Arg1, typename Arg2, typename Arg3,
-            typename Arg4, typename Arg5>
-  inline void Run(const Arg1& a, const Arg2& b, const Arg3& c,
-                  const Arg4& d, const Arg5& e) {
-    RunWithParams(Params(a, b, c, d, e));
-  }
-};
-
-template <class T, typename Method, typename Params>
-class CallbackImpl : public CallbackStorage<T, Method>,
-                     public CallbackRunner<Params> {
- public:
-  CallbackImpl(T* obj, Method meth) : CallbackStorage<T, Method>(obj, meth) {
-  }
-  virtual void RunWithParams(const Params& params) {
-    // use "this->" to force C++ to look inside our templatized base class; see
-    // Effective C++, 3rd Ed, item 43, p210 for details.
-    DispatchToMethod(this->obj_, this->meth_, params);
-  }
-};
-
-// 0-arg implementation
-struct Callback0 {
-  typedef CallbackRunner<Tuple0> Type;
-};
-
-template <class T>
-typename Callback0::Type* NewCallback(T* object, void (T::*method)()) {
-  return new CallbackImpl<T, void (T::*)(), Tuple0 >(object, method);
-}
-
-// 1-arg implementation
-template <typename Arg1>
-struct Callback1 {
-  typedef CallbackRunner<Tuple1<Arg1> > Type;
-};
-
-template <class T, typename Arg1>
-typename Callback1<Arg1>::Type* NewCallback(T* object,
-                                            void (T::*method)(Arg1)) {
-  return new CallbackImpl<T, void (T::*)(Arg1), Tuple1<Arg1> >(object, method);
-}
-
-// 2-arg implementation
-template <typename Arg1, typename Arg2>
-struct Callback2 {
-  typedef CallbackRunner<Tuple2<Arg1, Arg2> > Type;
-};
-
-template <class T, typename Arg1, typename Arg2>
-typename Callback2<Arg1, Arg2>::Type* NewCallback(
-    T* object,
-    void (T::*method)(Arg1, Arg2)) {
-  return new CallbackImpl<T, void (T::*)(Arg1, Arg2),
-      Tuple2<Arg1, Arg2> >(object, method);
-}
-
-// 3-arg implementation
-template <typename Arg1, typename Arg2, typename Arg3>
-struct Callback3 {
-  typedef CallbackRunner<Tuple3<Arg1, Arg2, Arg3> > Type;
-};
-
-template <class T, typename Arg1, typename Arg2, typename Arg3>
-typename Callback3<Arg1, Arg2, Arg3>::Type* NewCallback(
-    T* object,
-    void (T::*method)(Arg1, Arg2, Arg3)) {
-  return new CallbackImpl<T,  void (T::*)(Arg1, Arg2, Arg3),
-      Tuple3<Arg1, Arg2, Arg3> >(object, method);
-}
-
-// 4-arg implementation
-template <typename Arg1, typename Arg2, typename Arg3, typename Arg4>
-struct Callback4 {
-  typedef CallbackRunner<Tuple4<Arg1, Arg2, Arg3, Arg4> > Type;
-};
-
-template <class T, typename Arg1, typename Arg2, typename Arg3, typename Arg4>
-typename Callback4<Arg1, Arg2, Arg3, Arg4>::Type* NewCallback(
-    T* object,
-    void (T::*method)(Arg1, Arg2, Arg3, Arg4)) {
-  return new CallbackImpl<T, void (T::*)(Arg1, Arg2, Arg3, Arg4),
-      Tuple4<Arg1, Arg2, Arg3, Arg4> >(object, method);
-}
-
-// 5-arg implementation
-template <typename Arg1, typename Arg2, typename Arg3,
-          typename Arg4, typename Arg5>
-struct Callback5 {
-  typedef CallbackRunner<Tuple5<Arg1, Arg2, Arg3, Arg4, Arg5> > Type;
-};
-
-template <class T, typename Arg1, typename Arg2,
-          typename Arg3, typename Arg4, typename Arg5>
-typename Callback5<Arg1, Arg2, Arg3, Arg4, Arg5>::Type* NewCallback(
-    T* object,
-    void (T::*method)(Arg1, Arg2, Arg3, Arg4, Arg5)) {
-  return new CallbackImpl<T, void (T::*)(Arg1, Arg2, Arg3, Arg4, Arg5),
-      Tuple5<Arg1, Arg2, Arg3, Arg4, Arg5> >(object, method);
-}
-
-// An UnboundMethod is a wrapper for a method where the actual object is
-// provided at Run dispatch time.
-template <class T, class Method, class Params>
-class UnboundMethod {
- public:
-  UnboundMethod(Method m, const Params& p) : m_(m), p_(p) {
-    COMPILE_ASSERT(
-        (base::internal::ParamsUseScopedRefptrCorrectly<Params>::value),
-        badunboundmethodparams);
-  }
-  void Run(T* obj) const {
-    DispatchToMethod(obj, m_, p_);
-  }
- private:
-  Method m_;
-  Params p_;
-};
-
-// Return value implementation with no args.
-template <typename ReturnValue>
-struct CallbackWithReturnValue {
-  class Type {
-   public:
-    virtual ~Type() {}
-    virtual ReturnValue Run() = 0;
-  };
-};
-
-template <class T, typename Method, typename ReturnValue>
-class CallbackWithReturnValueImpl
-    : public CallbackStorage<T, Method>,
-      public CallbackWithReturnValue<ReturnValue>::Type {
- public:
-  CallbackWithReturnValueImpl(T* obj, Method meth)
-      : CallbackStorage<T, Method>(obj, meth) {}
-
-  virtual ReturnValue Run() {
-    return (this->obj_->*(this->meth_))();
-  }
-
- protected:
-  virtual ~CallbackWithReturnValueImpl() {}
-};
-
-template <class T, typename ReturnValue>
-typename CallbackWithReturnValue<ReturnValue>::Type*
-NewCallbackWithReturnValue(T* object, ReturnValue (T::*method)()) {
-  return new CallbackWithReturnValueImpl<T, ReturnValue (T::*)(), ReturnValue>(
-      object, method);
-}
+#include "base/callback_helpers.h"
+#include "base/callback_old.h"
+
+// New, super-duper, unified Callback system.  This will eventually replace
+// NewRunnableMethod, NewRunnableFunction, CreateFunctor, and CreateCallback
+// systems currently in the Chromium code base.
+//
+// WHAT IS THIS:
+//
+// The templated Callback class is a generalized funciton object. Together
+// with the Bind() function in bind.h, they provide a type-safe method for
+// performing currying of arguments, and createing a "closure."
+//
+// In programing languages, a closure is a first-class function where all its
+// parameters have been bound (usually via currying).  Closures are well
+// suited for representing, and passing around a unit of delayed execution.
+// They are used in Chromium code to schedule tasks on different MessageLoops.
+//
+// EXAMPLE USAGE:
+//
+// /* Binding a normal function. */
+// int Return5() { return 5; }
+// base::Callback<int(int)> func_cb = base::Bind(&Return5);
+// LOG(INFO) << func_cb.Run(5);  // Prints 5.
+//
+// void PrintHi() { LOG(INFO) << "hi."; }
+// base::Closure void_func_cb = base::Bind(&PrintHi);
+// LOG(INFO) << void_func_cb.Run();  // Prints: hi.
+//
+// /* Binding a class method. */
+// class Ref : public RefCountedThreadSafe<Ref> {
+//  public:
+//   int Foo() { return 3; }
+//   void PrintBye() { LOG(INFO) << "bye."; }
+// };
+// scoped_refptr<Ref> ref = new Ref();
+// base::Callback<int(void)> ref_cb = base::Bind(&Ref::Foo, ref.get());
+// LOG(INFO) << ref_cb.Run();  // Prints out 3.
+//
+// base::Closure void_ref_cb = base::Bind(&Ref::PrintBye, ref.get());
+// void_ref_cb.Run();  // Prints: bye.
+//
+// /* Binding a class method in a non-refcounted class.
+//  *
+//  * WARNING: You must be sure the referee outlives the callback!
+//  *          This is particularly important if you post a closure to a
+//  *          MessageLoop because then it becomes hard to know what the
+//  *          lifetime of the referee needs to be.
+//  */
+// class NoRef {
+//  public:
+//   int Foo() { return 4; }
+//   void PrintWhy() { LOG(INFO) << "why???"; }
+// };
+// NoRef no_ref;
+// base::Callback<int(void)> base::no_ref_cb =
+//     base::Bind(&NoRef::Foo, base::Unretained(&no_ref));
+// LOG(INFO) << ref_cb.Run();  // Prints out 4.
+//
+// base::Closure void_no_ref_cb =
+//     base::Bind(&NoRef::PrintWhy, base::Unretained(no_ref));
+// void_no_ref_cb.Run();  // Prints: why???
+//
+// /* Binding a reference. */
+// int Identity(int n) { return n; }
+// int value = 1;
+// base::Callback<int(void)> bound_copy_cb = base::Bind(&Identity, value);
+// base::Callback<int(void)> bound_ref_cb =
+//     base::Bind(&Identity, base::ConstRef(value));
+// LOG(INFO) << bound_copy_cb.Run();  // Prints 1.
+// LOG(INFO) << bound_ref_cb.Run();  // Prints 1.
+// value = 2;
+// LOG(INFO) << bound_copy_cb.Run();  // Prints 1.
+// LOG(INFO) << bound_ref_cb.Run();  // Prints 2.
+//
+//
+// WHERE IS THIS DESIGN FROM:
+//
+// The design Callback and Bind is heavily influenced by C++'s
+// tr1::function/tr1::bind, and by the "Google Callback" system used inside
+// Google.
+//
+//
+// HOW THE IMPLEMENTATION WORKS:
+//
+// There are three main components to the system:
+//   1) The Callback classes.
+//   2) The Bind() functions.
+//   3) The arguments wrappers (eg., Unretained() and ConstRef()).
+//
+// The Callback classes represent a generic function pointer. Internally,
+// it stores a refcounted piece of state that represents the target function
+// and all its bound parameters.  Each Callback specialization has a templated
+// constructor that takes an InvokerStorageHolder<> object.  In the context of
+// the constructor, the static type of this InvokerStorageHolder<> object
+// uniquely identifies the function it is representing, all its bound
+// parameters, and a DoInvoke that is capable of invoking the target.
+//
+// Callback's constructor is takes the InvokerStorageHolder<> that has the
+// full static type and erases the target function type, and the bound
+// parameters.  It does this by storing a pointer to the specific DoInvoke
+// function, and upcasting the state of InvokerStorageHolder<> to a
+// InvokerStorageBase. This is safe as long as this InvokerStorageBase pointer
+// is only used with the stored DoInvoke pointer.
+//
+// To create InvokerStorageHolder<> objects, we use the Bind() functions.
+// These functions, along with a set of internal templates, are reponsible for
+//
+//  - Unwrapping the function signature into return type, and parameters
+//  - Determining the number of parameters that are bound
+//  - Creating the storage for the bound parameters
+//  - Performing compile-time asserts to avoid error-prone behavior
+//  - Returning an InvokerStorageHolder<> with an DoInvoke that has an arity
+//    matching the number of unbound parameters, and knows the correct
+//    refcounting semantics for the target object if we are binding a class
+//    method.
+//
+// The Bind functions do the above using type-inference, and template
+// specializations.
+//
+// By default Bind() will store copies of all bound parameters, and attempt
+// to refcount a target object if the function being bound is a class method.
+//
+// To change this behavior, we introduce a set of argument wrappers
+// (eg. Unretained(), and ConstRef()).  These are simple container templates
+// that are passed by value, and wrap a pointer to argument.
+//
+// ConstRef() allows Bind()'s storage to preserve copy-semantics even if we
+// wish to pass the invoked object a reference to the bound parameter.

[brettw, 2011/02/10 17:41:24]

These two paragraphs about ConstRef and Unretained don't really illuminate much
for me. I'd probably remove them and reference the documentation in
bind_helpers.h after the sentence above "...and wrap a pointer to argument."

[awong, 2011/02/12 09:44:45]

Done.

+//
+// Unretained() allows us to tag an object for different refcounting semantics.
+//
+// These types are passed to the Unwrap() functions, and the MaybeRefcount()
+// functions respectively to modify the behavior of Bind().  The Unwrap()
+// and MaybeRefcount() functions change behavior by doing partial
+// specialization based on whether or not a parameter is a wrapper type.
+//
+// ConstRef() is similar to tr1::cref.  Unretained() is specific to Chromium.
+//
+//
+// WHY NOT TR1 FUNCTION/BIND?
+//
+// Direct use of tr1::function and tr1::bind was considered, but ultimately
+// rejected because of the number of copy constructors invocations involved
+// in the binding of arguments during construction, and the forwarding of
+// arguments during invocation.  These copies will no longer be an issue in
+// C++0x because C++0x will support rvalue reference allowing for the compiler
+// to avoid these copies.  However, waiting for C++0x is not an option.
+//
+// Measured with valgrind on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), the
+// tr1::bind call itself will invoke a non-trivial copy constructor three times
+// for each bound parameter.  Also, each when passing a tr1::function, each
+// bound argument will be copied again.
+//
+// In addition to the copies taken at binding and invocation, copying a
+// tr1::function causes a copy to be made of all the bound parameters and
+// state.
+//
+// Furthermore, in Chromium, it is desirable for the Callback to take a
+// reference on a target object when representing a class method call.  This
+// is not supported by tr1.
+//
+// Lastly, tr1::function and tr1::bind has a more general and flexible API.
+// This includes things like argument reordering by use of
+// tr1::bind::placeholder, support for non-const reference parameters, and some
+// limited amount of subtyping of the tr1::function object (eg.,
+// tr1::function<int(int)> is convertable to tr1::function<void(int)>).
+//
+// These are not features that are required in Chromium. Some of them, such as
+// allowing for reference parameters, and subtyping of functions, may actually
+// because a source of errors. Removing support for these features actually
+// allows for a simpler implementation, and a terser Currying API.
+//
+//
+// WHY NOT GOOGLE CALLBACKS?
+//
+// The Google callback system also does not support refcounting.  Furthermore,
+// its implementation has a number of strange edge cases with respect to type
+// convesrion of its arguments.  In particular, the argument's constness must
+// at times match exactly the function signature, or the type-inference might
+// break.  Given the above, writing a custom solution was easier.
+//
+//
+// MISSING FUNCTIONALITY
+//  - Invoking the return of Bind.  Bind(&foo).Run() does not work;
+//  - Binding arrays to functions that take a non-const pointer.
+//    Example:
+//      void Foo(const char* ptr);
+//      void Bar(char* ptr);
+//      Bind(&Foo, "test");
+//      Bind(&Bar, "tesT");  // This fails because ptr is not const.
+
+namespace base {
+
+// First, we forward declare the Callback class template. This informs the
+// compiler that the template only has 1 type parameter which is the function
+// signature that the Callback is representing.
+//
+// After this, create template specializations for 0-6 parameters. Note that
+// even though the template typelist grows, the specialization still
+// only has one type: the function signature.
+//
+// Also, note that the templated constructor should *not* be explicit. This is
+// to allow for a natural assignment syntax from the result of Bind(), which
+// is not the same type as Callback().  See the description of Bind for
+// details.
+template <typename Sig>
+class Callback;
+
+template <typename R>
+class Callback<R(void)> {
+ public:
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*);
+
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  // We pass InvokerStorageHolder by const ref to avoid incurring an
+  // unnecssary AddRef/Unref pair even though we will modify the object.
+  // We cannot use a normal reference because the compiler will warn
+  // since this is often used on a return value, which is a temporary.
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+  R Run(void) const {
+    return polymorphic_invoke_(invoker_storage_.get());
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+template <typename R, typename A1>
+class Callback<R(A1)> {
+ public:
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*, const A1&);
+
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  // We pass InvokerStorageHolder by const ref to avoid incurring an
+  // unnecssary AddRef/Unref pair even though we will modify the object.
+  // We cannot use a normal reference because the compiler will warn
+  // since this is often used on a return value, which is a temporary.
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+  R Run(const A1& a1) const {
+    return polymorphic_invoke_(invoker_storage_.get(), a1);
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+template <typename R, typename A1, typename A2>
+class Callback<R(A1, A2)> {
+ public:
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*, const A1&,
+                                const A2&);
+
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  // We pass InvokerStorageHolder by const ref to avoid incurring an
+  // unnecssary AddRef/Unref pair even though we will modify the object.
+  // We cannot use a normal reference because the compiler will warn
+  // since this is often used on a return value, which is a temporary.
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+  R Run(const A1& a1,
+        const A2& a2) const {
+    return polymorphic_invoke_(invoker_storage_.get(), a1,
+                               a2);
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+template <typename R, typename A1, typename A2, typename A3>
+class Callback<R(A1, A2, A3)> {
+ public:
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*, const A1&,
+                                const A2&,
+                                const A3&);
+
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  // We pass InvokerStorageHolder by const ref to avoid incurring an
+  // unnecssary AddRef/Unref pair even though we will modify the object.
+  // We cannot use a normal reference because the compiler will warn
+  // since this is often used on a return value, which is a temporary.
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+  R Run(const A1& a1,
+        const A2& a2,
+        const A3& a3) const {
+    return polymorphic_invoke_(invoker_storage_.get(), a1,
+                               a2,
+                               a3);
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+template <typename R, typename A1, typename A2, typename A3, typename A4>
+class Callback<R(A1, A2, A3, A4)> {
+ public:
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*, const A1&,
+                                const A2&,
+                                const A3&,
+                                const A4&);
+
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  // We pass InvokerStorageHolder by const ref to avoid incurring an
+  // unnecssary AddRef/Unref pair even though we will modify the object.
+  // We cannot use a normal reference because the compiler will warn
+  // since this is often used on a return value, which is a temporary.
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+  R Run(const A1& a1,
+        const A2& a2,
+        const A3& a3,
+        const A4& a4) const {
+    return polymorphic_invoke_(invoker_storage_.get(), a1,
+                               a2,
+                               a3,
+                               a4);
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+template <typename R, typename A1, typename A2, typename A3, typename A4,
+    typename A5>
+class Callback<R(A1, A2, A3, A4, A5)> {
+ public:
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*, const A1&,
+                                const A2&,
+                                const A3&,
+                                const A4&,
+                                const A5&);
+
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  // We pass InvokerStorageHolder by const ref to avoid incurring an
+  // unnecssary AddRef/Unref pair even though we will modify the object.
+  // We cannot use a normal reference because the compiler will warn
+  // since this is often used on a return value, which is a temporary.
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+  R Run(const A1& a1,
+        const A2& a2,
+        const A3& a3,
+        const A4& a4,
+        const A5& a5) const {
+    return polymorphic_invoke_(invoker_storage_.get(), a1,
+                               a2,
+                               a3,
+                               a4,
+                               a5);
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+template <typename R, typename A1, typename A2, typename A3, typename A4,
+    typename A5, typename A6>
+class Callback<R(A1, A2, A3, A4, A5, A6)> {
+ public:
+  typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*, const A1&,
+                                const A2&,
+                                const A3&,
+                                const A4&,
+                                const A5&,
+                                const A6&);
+
+  Callback() : polymorphic_invoke_(NULL) { }
+
+  // We pass InvokerStorageHolder by const ref to avoid incurring an
+  // unnecssary AddRef/Unref pair even though we will modify the object.
+  // We cannot use a normal reference because the compiler will warn
+  // since this is often used on a return value, which is a temporary.
+  template <typename T>
+  Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
+      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
+    invoker_storage_.swap(invoker_holder.invoker_storage_);
+  }
+
+  R Run(const A1& a1,
+        const A2& a2,
+        const A3& a3,
+        const A4& a4,
+        const A5& a5,
+        const A6& a6) const {
+    return polymorphic_invoke_(invoker_storage_.get(), a1,
+                               a2,
+                               a3,
+                               a4,
+                               a5,
+                               a6);
+  }
+
+ private:
+  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
+  PolymorphicInvoke polymorphic_invoke_;
+};
+
+
+// Syntactic sugar to make Callbacks<void(void)> easier to declare since it
+// will be used in a lot of APIs with delayed execution.
+typedef Callback<void(void)> Closure;
+
+}  // namespace base
 
 #endif  // BASE_CALLBACK_H