[plt-scheme] references to deleted objects

We're using drscheme's class system to interface to a graphical system.

For example 

  (define o (make-object oval% dynapad))

creates an oval on the graphic surface.  The class oval% has a field
which holds a reference (scheme_make_cptr) to a C++ object which implements
the oval on the graphic surface.

The graphical objects are manipulated from drscheme by messages.
For example,

  (send o slide 10 20)
  (send o fill "red")

  (send o delete)

The last example deletes the oval from the graphic surface.
Of course the scheme object remains as long as there is a
reference to it and that leads to problems because messages
will continue to be sent to those references.  (For example,
the scheme object might be the destination of a hyperlink,
or it might be on a list of objects to be notified about
some event.)

Two solutions come to mind, but maybe there is something better.

1) When creating a reference to an instance of oval%, also create
   a reference from the instance back to the referrer (probably kept
   in a field in oval%), and have the delete method update referrers
   to remove their reference to the instance.

2) Add a "deleted" field to oval%, initialize it to #t, have the
   delete method set deleted to #f, and have the other oval% methods
   check deleted before trying to operate on the missing graphic object.

2.5) Add a "deleted" field and replace send with a send that checks
   deleted before actually sending the message.

Surely others have had to deal with references to deleted scheme objects.

Any suggestions?

Thanks,

Ron
stanonik at cogsci.ucsd.edu