5. It never deletes data unsafely, but sometimes fails to delete
   garbage. If one were to destroy the last reference to a doubly linked
   list, the list elements would still be referring to each other, and
   so would not be deleted. In general, the reference counting scheme
   fails to reclaim "cyclic" data structures.

   For this reason, STL offers weak_ptr<T>, which allows the pointed object to
   be deleted. So one might use a shared_ptr<node> for the NEXT list
   node and a weak_ptr<node> for the PREVIOUS node.