A passenger name can exist in the set pointed to by customers_ and in the set pointed to by passengers_; that is, both collections might contain the same string. When the Bus is restored, we want to make sure that the pointer relationship is maintained, and that our restoration does not create another copy of the string.

We do not have to do anything special to insure that the pointer relationship stays as it should be. Consider the call:

Because passenger_ is a subset of customer_, the function addPassenger puts the name on both the customer list and the passenger list. When we save aBus to aFile, both lists are saved in a single call: first the customer list, then the passenger list. The polymorphic persistence machinery saves the first reference to John, but for the second reference it merely stores a reference to the first copy. During the restore, both references will resolve to the same object, replicating the original morphology of the collection.