1. The association of an object with a physical location in storage (as in C++) may change over time.

2. There are several degrees of permanence of identity:

• intraprocedure: Identity persists only during the execution of a single procedure, e.g., local variables within procedures.
• intraprogram: Identity persists only during the execution of a single program or query, e.g., global variables in programming languages, and main memory or virtual memory pointers.
• interprogram: Identity persists from one program execution to another, e.g., pointers to file system data on disk but may change if the way data is stored in the file system is changed.
• persistent: Identity persists not only among program executions but also among structural reorganizations of the data. This is the persistent form of identity required for object-oriented systems.

3. In persistent extension of C++, object identifiers are implemented as \persistent pointers" which can be viewed as a pointer to an object in the database.