Add scoped_ptr<>::PassAs<>().

base/memory/scoped_ptr.h

Index: base/memory/scoped_ptr.h
diff --git a/base/memory/scoped_ptr.h b/base/memory/scoped_ptr.h
index 7133a3407d0a185158e1923ab961bc50f5e462e4..77a203992038f07687baad78a0b37ea55b21ba97 100644
--- a/base/memory/scoped_ptr.h
+++ b/base/memory/scoped_ptr.h
@@ -67,6 +67,22 @@
 // the Pass() function to signify a destructive transfer of state. CreateFoo()
 // is different though because we are constructing a temporary on the return
 // line and thus can avoid needing to call Pass().
+//
+// Pass() properly handles upcast in assignment, i.e. you can assign
+// scoped_ptr<Child> to scoped_ptr<Parent>:
+//
+//   scoped_ptr<Foo> foo(new Foo());
+//   scoped_ptr<FooParent> parent = foo.Pass();
+//
+// PassAs<>() should be used to upcast return value in return statement:
+//
+//   scoped_ptr<Foo> CreateFoo() {
+//     scoped_ptr<FooChild> result(new FooChild());
+//     return result.PassAs<Foo>();
+//   }
+//
+// Note that PassAs<>() is implemented only for scoped_ptr, but not for
+// scoped_array. This is because casting array pointers may not be safe.
 
 #ifndef BASE_MEMORY_SCOPED_PTR_H_
 #define BASE_MEMORY_SCOPED_PTR_H_
@@ -182,6 +198,11 @@ class scoped_ptr {
     return retVal;
   }
 
+  template <typename PassAsType>
+  scoped_ptr<PassAsType> PassAs() {

[darin (slow to review), 2012/01/24 21:17:53]

Had you considered putting this in base/move.h alongside the definition of Pass?
 I'm not sure how to do it, so maybe it isn't even possible.

[Sergey Ulanov, 2012/01/24 21:55:18]

Yes, I considered it, but decided against it because it would be unsafe to allow
PassAs<>() for scoped_array. Specifically upcasting array pointers isn't safe
when sizes of parent and child classes differ.

+    return scoped_ptr<PassAsType>(release());
+  }
+
  private:
   C* ptr_;