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

-- This time for sure. --

Add Pass(), which implements move semantics, to scoped_ptr, scoped_array, and
scoped_ptr_malloc.

This modification to the scopers implements the "moveable but not copyable"
semantics that were introduced in C++11's unique_ptr<>.

With this, is now possible to use scopers as an argument type or a return type.
This signifies, in the type system, transfer of ownership into a function or out
of a function respectively.  Calling, or returning such a function MUST use the
temporary resulting from a function or explicit cast.

This distinction makes it possible to avoid the implicit ownership transfer
issues of auto_ptr, but still allow us to have compiler enforced ownership
transfer.

Also adds a Passed() helper that allows using a scoper with Bind().

Original Review URL: http://codereview.chromium.org/8774032

BUG=96118
TEST=new unittests

Review URL: http://codereview.chromium.org/9018037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@115607 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 59 insertions, 0 deletions

diff --git a/base/callback_internal.h b/base/callback_internal.h
index 4bb8aa9..21599bf 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -13,6 +13,7 @@
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
 
 namespace base {
 namespace internal {
@@ -110,6 +111,64 @@ struct CallbackParamTraits<T[]> {
   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