Redo r113722 - Add Pass(), which implements move semantics, to scoped_ptr, scoped_array....

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
 
 #include <stddef.h>
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
 
 namespace base {
 namespace internal {
 
 // BindStateBase is used to provide an opaque handle that the Callback
 // class can use to represent a function object with bound arguments.  It
 // 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."
@@ skipping 97 matching lines @@
   typedef const T* StorageType;
 };
 
 // See comment for CallbackParamTraits<T[n]>.
 template <typename T>
 struct CallbackParamTraits<T[]> {
   typedef const T* ForwardType;
   typedef const T* StorageType;
 };
 
+// Parameter traits for movable-but-not-copyable scopers.
+//
+// Callback<>/Bind() understands movable-but-not-copyable semantics where
+// the type cannot be copied but can still have its state destructively
+// transferred (aka. moved) to another instance of the same type by calling a
+// helper function.  When used with Bind(), this signifies transferal of the
+// object's state to the target function.
+//
+// For these types, the ForwardType must not be a const reference, or a
+// reference.  A const reference is inappropriate, and would break const
+// correctness, because we are implementing a destructive move.  A non-const
+// reference cannot be used with temporaries which means the result of a
+// function or a cast would not be usable with Callback<> or Bind().
+//
+// TODO(ajwong): We might be able to use SFINAE to search for the existence of
+// a Pass() function in the type and avoid the whitelist in CallbackParamTraits
+// and CallbackForward.
+template <typename T>
+struct CallbackParamTraits<scoped_ptr<T> > {
+  typedef scoped_ptr<T> ForwardType;
+  typedef scoped_ptr<T> StorageType;
+};
+
+template <typename T>
+struct CallbackParamTraits<scoped_array<T> > {
+  typedef scoped_array<T> ForwardType;
+  typedef scoped_array<T> StorageType;
+};
+
+template <typename T>
+struct CallbackParamTraits<scoped_ptr_malloc<T> > {
+  typedef scoped_ptr_malloc<T> ForwardType;
+  typedef scoped_ptr_malloc<T> StorageType;
+};
+
+// CallbackForward() is a very limited simulation of C++11's std::forward()
+// 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.
+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_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_