For types that support Pass(), e.g. scoped_ptr and friends, use Passed in PostTaskAndReplyWithResul…

base/callback_internal.h

 // Copyright (c) 2012 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 148 matching lines @@
 // used by the Callback/Bind system for a set of movable-but-not-copyable
 // types.  It is needed because forwarding a movable-but-not-copyable
 // argument to another function requires us to invoke the proper move
 // operator to create a rvalue version of the type.  The supported types are
 // whitelisted below as overloads of the CallbackForward() function. The
 // default template compiles out to be a no-op.
 //
 // In C++11, std::forward would replace all uses of this function.  However, it
 // is impossible to implement a general std::forward with C++11 due to a lack
 // of rvalue references.
+//
+// In addition to Callback/Bind, this is used by PostTaskAndReplyWithResult to
+// simulate std::forward() and forward the result of one Callback as a
+// parameter to another callback. This is to support Callbacks that return
+// the movable-but-not-copyable types whitelisted above.
 template <typename T>
 T& CallbackForward(T& t) { return t; }
 
 template <typename T>
 scoped_ptr<T> CallbackForward(scoped_ptr<T>& p) { return p.Pass(); }
 
 template <typename T>
 scoped_array<T> CallbackForward(scoped_array<T>& p) { return p.Pass(); }
 
 template <typename T>
 scoped_ptr_malloc<T> CallbackForward(scoped_ptr_malloc<T>& p) {
   return p.Pass();
 }
 
 template <typename T>
 ScopedVector<T> CallbackForward(ScopedVector<T>& p) { return p.Pass(); }
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_