Remove BindStateHolder and have Bind() return a Callback<> object directly.

base/callback.h.pump

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 $$ See comment for MAX_ARITY in base/bind.h.pump.
 $var MAX_ARITY = 7
 
@@ skipping 116 matching lines @@
 // HOW THE IMPLEMENTATION WORKS:
 //
 // There are three main components to the system:
 //   1) The Callback classes.
 //   2) The Bind() functions.
 //   3) The arguments wrappers (e.g., 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 BindStateHolder<> object.  In the context of
-// the constructor, the static type of this BindStateHolder<> object
-// uniquely identifies the function it is representing, all its bound
-// parameters, and a DoInvoke() that is capable of invoking the target.
+// constructor that takes an BindState<>*.  In the context of the constructor,
+// the static type of this BindState<> pointer uniquely identifies the
+// function it is representing, all its bound parameters, and a Run() method
+// that is capable of invoking the target.
 //
-// Callback's constructor is takes the BindStateHolder<> 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 BindStateHolder<> to a
-// BindStateBase. This is safe as long as this BindStateBase pointer
-// is only used with the stored DoInvoke() pointer.
+// Callback's constructor takes the BindState<>* that has the full static type
+// and erases the target function type as well as the types of the bound
+// parameters.  It does this by storing a pointer to the specific Run()
+// function, and upcasting the state of BindState<>* to a
+// BindStateBase*. This is safe as long as this BindStateBase pointer
+// is only used with the stored Run() pointer.
 //
-// To create BindStateHolder<> objects, we use the Bind() functions.
-// These functions, along with a set of internal templates, are reponsible for
+// To BindState<> objects are created inside the Bind() functions.
+// These functions, along with a set of internal templates, are responsible 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
+//  - Creating the BindState storing the bound parameters
 //  - Performing compile-time asserts to avoid error-prone behavior
-//  - Returning an BindStateHolder<> 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.
+//  - Returning an Callback<> with an arity matching the number of unbound
+//    parameters and that knows the correct refcounting semantics for the
+//    target object if we are binding a 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
 // (e.g., Unretained(), and ConstRef()).  These are simple container templates
 // that are passed by value, and wrap a pointer to argument.  See the
@@ skipping 67 matching lines @@
 //
 // 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.
 //
 // If you are thinking of forward declaring Callback in your own header file,
 // please include "base/callback_forward.h" instead.
 template <typename Sig>
 class Callback;
 
+namespace internal {
+template <typename Runnable, typename RunType, typename BoundArgsType>
+struct BindState;
+}  // namespace internal
+
 
 $range ARITY 0..MAX_ARITY
 $for ARITY [[
 $range ARG 1..ARITY
 
 $if ARITY == 0 [[
 template <typename R>
 class Callback<R(void)> : public internal::CallbackBase {
 ]] $else [[
 template <typename R, $for ARG , [[typename A$(ARG)]]>
 class Callback<R($for ARG , [[A$(ARG)]])> : public internal::CallbackBase {
 ]]
 
  public:
   typedef R(RunType)($for ARG , [[A$(ARG)]]);
 
-  Callback() : CallbackBase(NULL, NULL) { }
+  Callback() : CallbackBase(NULL) { }
 
-  // We pass BindStateHolder 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::BindStateHolder<T>& bind_state_holder)
-      : CallbackBase(NULL, &bind_state_holder.bind_state_) {
-    // Force the assignment to a location variable of PolymorphicInvoke
+  template <typename Runnable, typename RunType, typename BoundArgsType>
+  Callback(internal::BindState<Runnable, RunType, BoundArgsType>* bind_state)
+      : CallbackBase(bind_state) {
+
+    // Force the assignment to a local variable of PolymorphicInvoke
     // so the compiler will typecheck that the passed in Run() method has
     // the correct type.
-    PolymorphicInvoke invoke_func = &T::InvokerType::Run;
+    PolymorphicInvoke invoke_func =
+        &internal::BindState<Runnable, RunType, BoundArgsType>
+            ::InvokerType::Run;
     polymorphic_invoke_ = reinterpret_cast<InvokeFuncStorage>(invoke_func);
   }
 
   bool Equals(const Callback& other) const {
     return CallbackBase::Equals(other);
   }
 
   R Run($for ARG ,
         [[typename internal::CallbackParamTraits<A$(ARG)>::ForwardType a$(ARG)]]) const {
     PolymorphicInvoke f =
@@ skipping 16 matching lines @@
 
 ]]  $$ 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