How does a reference-counting smart pointer’s reference counting work?

I’ve seen two different non-intrusive approaches to this:

1. The smart pointer allocates a small
   block of memory to contain the
   reference counter. Each copy of the
   smart pointer then receives a
   pointer to the actual object and a
   pointer to the reference count.
2. In addition to an object pointer,
   each smart pointer contains a
   previous and next pointer, thereby
   forming a doubly-linked list of
   smart pointers to a particular
   object. The reference count is
   implicit in the list. When a smart
   pointer is copied, it adds itself to
   the list. Upon destruction, each
   smart pointer removes itself from
   the list. If it’s the last one in
   the list it then frees the
   referenced object as well.

If you go here and scroll to the bottom, there is an excellent diagram which explains these methods much more clearly.