Callback API Change: is_null, Reset, and Equals

base/callback.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
 $var MAX_ARITY = 6
 
 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
@@ skipping 198 matching lines @@
 //  - 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 {
 
+namespace internal {
+
+// Holds the methods that don't require specialization to reduce template bloat.
+class CallbackBase {
+ public:
+  // Returns true if Callback is null (doesn't refer to anything).
+  bool is_null() const {
+    return invoker_storage_.get() == NULL;
+  }
+
+  // Returns the Callback into an uninitalized state.
+  void Reset() {
+    invoker_storage_ = NULL;
+    polymorphic_invoke_ = NULL;
+  }
+
+  bool Equals(const CallbackBase& other) const {
+    return invoker_storage_.get() == other.invoker_storage_.get() &&
+           polymorphic_invoke_ == other.polymorphic_invoke_;
+  }
+
+ 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.
+  typedef void(*InvokeFuncStorage)(void);
+
+  CallbackBase(InvokeFuncStorage polymorphic_invoke,
+               scoped_refptr<InvokerStorageBase>* invoker_storage)
+      : polymorphic_invoke_(polymorphic_invoke) {
+    if (invoker_storage) {
+      invoker_storage_.swap(*invoker_storage);
+    }
+  }
+
+  scoped_refptr<InvokerStorageBase> invoker_storage_;
+  InvokeFuncStorage polymorphic_invoke_;
+};
+
+}  // namespace internal
+
+
 // 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-$(MAX_ARITY) parameters. Note that
 // even though the template typelist grows, the specialization still
 // only has one type: the function signature.
 template <typename Sig>
 class Callback;
 
-
 $range ARITY 0..MAX_ARITY
 $for ARITY [[
 $range ARG 1..ARITY
 
 $if ARITY == 0 [[
 template <typename R>
-class Callback<R(void)> {
+class Callback<R(void)> : public internal::CallbackBase {
 ]] $else [[
 template <typename R, $for ARG , [[typename A$(ARG)]]>
-class Callback<R($for ARG , [[A$(ARG)]])> {
+class Callback<R($for ARG , [[A$(ARG)]])> : public internal::CallbackBase {
 ]]
 
  public:
   typedef R(*PolymorphicInvoke)(internal::InvokerStorageBase*[[]]
 $if ARITY != 0 [[, ]]
 $for ARG ,
                                 [[const A$(ARG)&]]);
 
-  Callback() : polymorphic_invoke_(NULL) { }
+  Callback() : CallbackBase(NULL, NULL) { }
 
   // We pass InvokerStorageHolder by const ref to avoid incurring an
   // unnecessary 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.
   //
   // Note that this constructor CANNOT be explicit, and that Bind() CANNOT
   // return the exact Callback<> type.  See base/bind.h for details.
   template <typename T>
   Callback(const internal::InvokerStorageHolder<T>& invoker_holder)
-      : polymorphic_invoke_(&T::FunctionTraits::DoInvoke) {
-    invoker_storage_.swap(invoker_holder.invoker_storage_);
+      : CallbackBase(
+          reinterpret_cast<InvokeFuncStorage>(&T::FunctionTraits::DoInvoke),
+          &invoker_holder.invoker_storage_) {
   }
 
-
-$if ARITY == 0 [[
-  R Run(void) const {
-]] $else [[
   R Run($for ARG ,
         [[const A$(ARG)& a$(ARG)]]) const {
-]]
+    PolymorphicInvoke f =
+        reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
 
-    return polymorphic_invoke_(invoker_storage_.get()[[]]
+    return f(invoker_storage_.get()[[]]
 $if ARITY != 0 [[, ]]
 $for ARG ,
-                               [[a$(ARG)]]);
+             [[a$(ARG)]]);
   }
-
- private:
-  scoped_refptr<internal::InvokerStorageBase> invoker_storage_;
-  PolymorphicInvoke polymorphic_invoke_;
 };
 
 
 ]]  $$ for ARITY
 
 // 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