Loosely speaking, a persistent object is one that lives on after the program which created it has stopped. Persistent objects can even outlive various versions of the creating program, can outlive the disk system, the operating system, or even the hardware on which the OS was running when they were created. The challenge with persistent objects is to effectively store their method code out on secondary storage along with their data bits (and the data bits and method code of all subobjects, and of all their subobjects, etc). This is non-trivial when you have to do it yourself. In C++, you have to do it yourself. C++/OO databases can help hide the mechanism for all this.
