Categories of Persistence Frameworks

Persistence may be explicit, meaning the programmer asks for an object to be saved or loaded. Alternatively, persistence may be implicit, in which case the system determines when and how to save and load objects. RDBMS systems are typically explicit, while ODBMS systems are typically implicit.

Although most persistence schemes built to date use explicit persistence, there are two great advantages to implicit persistence:

The application programmer does not have to worry about loading and saving his objects. This is done transparently by the persistence framework.
The application programmer does not have to worry about when his objects are in memory and when they are only in the persistent store. He therefore addresses them with normal C++ references all of the time, as if they were always memory resident. This is a very natural way for C++ programmers to work.