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 12 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;
 
-  CallbackBase(InvokeFuncStorage polymorphic_invoke,
-               scoped_refptr<BindStateBase>* bind_state);
+  // 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_;
 };
 
@@ skipping 98 matching lines @@
 template <typename T>
 scoped_ptr<T> CallbackForward(scoped_array<T>& p) { return p.Pass(); }
 
 template <typename T>
 scoped_ptr<T> CallbackForward(scoped_ptr_malloc<T>& p) { return p.Pass(); }
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_