HP Integrity NonStop J-Series - Manual (Page 220)

Add Definitions for Virtual Functions
Class RWCollectable declares the following virtual functions:
virtual ~RWCollectable();
virtual Rwspace binaryStoreSize() const;
virtual int compareTo(const RWCollectable*) const;
virtual unsigned hash() const;
virtual RWClassID isA() const;
virtual RWBoolean isEqual(const RWCollectable*) const;
virtual RWCollectable* newSpecies() const;
virtual void restoreGuts(RWvistream&);
virtual void restoreGuts(RWFile&);
virtual void saveGuts(RWvostream&) const;
virtual void saveGuts(RWFile&) const;
In these functions 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 you as the class designer to provide definitions
tailored to the class at hand.
We've split our discussion of these virtual functions. We discuss the destructor below ( Object
Destruction ) , and the binaryStoreSize(), saveGuts(), and restoreGuts() functions below ( How to Add
Polymorphic Persistence ) , where we describe how to add persistence to a class. Virtual functions isA()
and newSpecies() are declared and defined by macros, so they were discussed above, in Virtual
Function isA() and Virtual Function newSpecies() . This section presents discussion on the
remaining functions: compareTo(), isEqual(), and hash(). A very brief example, showing how all
three functions deal with the same data, appears
below ( An example of compareTo()... ) .
Virtual Function compareTo()
The virtual function compareTo() is used to order objects relative to each other. This function is
required in collection classes that depend on such ordering, such as
RWBinaryTree or RWBTree .
Here is its declaration:
virtual int compareTo(const RWCollectable*) const;
The function int compareTo(const RWCollectable*) const should return a number greater than zero
if self is greater than the argument, a number less than zero if self is less than the argument, and
zero if self is equal to the argument.
The definition and meaning of whether one object is greater than, less than, or equal to another
object is left to the class designer. The default definition, found in class
RWCollectable , is to
compare the two addresses of the objects. This default definition should be considered a
placeholder; in practice, it is not very useful and could vary from run to run of a program.
Here is a possible definition of compareTo():