set_weak_flag() for objects?

What if I have an object like this:

class Server {
  private mixed id;

  void create() {
    id = this;
    werror("Created\n");
  }

  void destroy() {
    werror("Destructed\n");
  }
}

object ref = Server();
ref = 0;	// At this point, I want the object to "selfdestruct"

But the selfdestruct doesn't work because of the internal reference from id
to the class instance.
How can I mark the member id as a weak reference?
-- 
Stephen.