Composition Vs. Inheritance

• Inheritance hierarchies model IS-A relationships among base and derived classes. This is because a derived class object IS-A type of base class object. For example, a CHECKING ACCOUNT object (derived class object) IS-A type of ACCOUNT object (base class object).
• Composite classes, i.e., classes that contain other class objects as members, model HAS-A relationships. This is because the container class HAS-Another class as its member component. For example, an APPOINTMENT object (container class object) HAS-A DATE object as its member component.
• Both types of relationships may be used to design systems. There are no well defined criteria for choosing one over the other. The nature of the application being designed should dictate the choice. In some situations, however, the choice between Inheritance or Composition is trivial. For example, in the above mentioned example, to model CHECKING ACCOUNT as an object that IS-A type of ACCOUNT, makes more sense than to model CHECKING ACCOUNT as an object that HAS-An ACCOUNT.
• In addition, to these two types of relationships, there are two other relationships - USES-A and KNOWS-A. These are also referred to as Associations. Associations are implemented in C++ using pointer members or reference members.