Persistent Object Identifiers
C++ and Java objects have an object-id (oid), typically represented by its virtual memory address. This oid corresponds at least conceptually to the pkey of an RDB tuple. This type of oid is not visible and not persistent, because it disappears when the program terminates.
One way to avoid loss of information and achieve persistence is to have the RDBMS take over or duplicate OS memory-mapping functions: moving large segments of virtual memory to/from mass storage in a fail-safe manner.
Another way to achieve persistence is to convert pkey/fkey relationships to/from object references during import/export data flows. (This is done by COOL/GEN.)