Revert 114494 - Remove BindStateHolder and have Bind() return a Callback<> object directly.

base/callback_internal.h

 // 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.
 
 // 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_
 #pragma once
@@ skipping 11 matching lines @@
 // 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."
 class BindStateBase : public RefCountedThreadSafe<BindStateBase> {
  protected:
   friend class RefCountedThreadSafe<BindStateBase>;
   virtual ~BindStateBase() {}
 };
 
+// This structure exists purely to pass the returned |bind_state_| from
+// Bind() to Callback while avoiding an extra AddRef/Release() pair.
+//
+// To do this, the constructor of Callback<> must take a const-ref.  The
+// reference must be to a const object otherwise the compiler will emit a
+// warning about taking a reference to a temporary.
+//
+// Unfortunately, this means that the internal |bind_state_| field must
+// be made mutable.
+template <typename T>
+struct BindStateHolder {
+  explicit BindStateHolder(T* bind_state)
+      : bind_state_(bind_state) {
+  }
+
+  mutable scoped_refptr<BindStateBase> bind_state_;
+};
+
+template <typename T>
+BindStateHolder<T> MakeBindStateHolder(T* o) {
+  return BindStateHolder<T>(o);
+}
+
 // Holds the Callback methods that don't require specialization to reduce
 // template bloat.
 class BASE_EXPORT CallbackBase {
  public:
   // Returns true if Callback is null (doesn't refer to anything).
   bool is_null() const;
 
   // 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.
   typedef void(*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);
+  CallbackBase(InvokeFuncStorage polymorphic_invoke,
+               scoped_refptr<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_;
 };
 
@@ skipping 40 matching lines @@
 template <typename T>
 struct CallbackParamTraits<T[]> {
   typedef const T* ForwardType;
   typedef const T* StorageType;
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_