Tools.h++ Manual

17-2 104011 Tandem Computers Incorporated
17
(RWBoolean is a typedef for an int , RWspace is a typedef for unsigned
long , and RWClassID is a typedef for an unsigned short .) Any class that
derives from class RWCollectable should be able to understand any of these
methods. Although default definitions are given for all of them in the base
class RWCollectable , it is best for the class designer to provide definitions
that are tailored to the class at hand.
17.2 Example
Suppose we are running a bus company. We want to write a class that
represents a bus, its set of potential customers, and its set of actual passengers.
In order to be a passenger, you must be a customer. Hence, the set of
customers is a superset of the set of passengers. Because 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, we want to make sure there are no
duplicates in either list.
We must be careful when it comes time to persist the bus. If we naïvely iterate
over the set of customers and then over the set of passengers, saving each one,
then if a person is both a customer and a passenger we will save him twice.
When the bus is restored, instead of the list of passengers referring to people
already in the customer list, it will have its own separate instances.
We need some way of recognizing when a person has already been saved to
the stream and, instead of saving him or her again, merely save a reference to
the previous instance.
This is the job of class RWCollectable . Objects inheriting from
RWCollectable have the ability to save not only their contents, but also their
relationships with other objects inheriting from RWCollectable . We call this
isomorphic persistence.
Here’s how we might declare our class Bus:
class Bus : public RWCollectable
{
RWDECLARE_COLLECTABLE(Bus)
public: