Abstraction Relationship

* Three Relationships In General
- is-a
The public inheritance. Derived class is a special base class and could be used in any circumstances of base class. This is the key point of dynamic binding. On the contrary, base class is not derived class.
- has-a
The composite implementation with the handles to composite objects.
- use-a
Two classes interact at some time instant. No private inheritance implementation and use composite implementation with the handles to used objects.

* Has-a and Usa-a
It is difficult to distinguish these two relationships from the coding since it is generally using the handles to implement them. For has-a, the master class has the responsibility to create, maintain and delete its composite object. For use-a, the useful object might be shared with others. The setting of handles to comp might be these:
- Has-a
{ delete comp; comp = newcomp; }
{ delete comp; comp = newcomp->clone(); }
- Use-a
{ comp = newcomp; }