Suppose you run a bus company. To automate part of your ridership tracking system, you want to write classes that represent a bus, its set of customers, and its set of actual passengers. In order to be a passenger, a person must be a customer. Hence, the set of customers is a superset of the set of passengers. Also, a person can physically be on the bus only once, and there is no point in putting the same person on the customer list more than once. As the developer of this system, you must make sure there are no duplicates on either list.

These duplicates can be a problem. Suppose that the program needs to be able to save and restore information about the bus and its customers. When it comes time to polymorphically save the bus, if your program naively iterates over the set of customers, then over the set of passengers, saving each one, any person who is both a customer and a passenger is saved twice. When the program polymorphically restores the bus, the list of passengers will not simply refer to people already on the customer list. Instead, each passenger has a separate instantiation on both lists.

You need some way of recognizing when a person has already been polymorphically saved to the stream and, instead of saving him or her again, merely saving a reference to the previous instance.

We will be designing the class hierarchy around the RWCollectable class and will derive our own classes from it. RWCollectable contains the necessary serialization interface.