TITLE: Choosing inheritance (is-a) versus containment (uses)


[ Tip adapted from "Inheritance Versus Containment", Rainer H. Liffers,
  ACM SigPlan Notices, V. 28, No. 9, Sept 93, p. 36

  Designers using C++ sometimes have confusion about whether the relationship
  between two classes should be is-a or has-a. This problem is exacerbated
  because of the ability to use non-virtual multiple inheritance to mimic 
  containment [ugh].

     class Robot : public Arm, public Arm, public Leg, public Leg {...}

  Below is a simple guideline you can use to help determine which 
  relationship you should use.

  -adc ]

Let X be a class for which it is not clear whether it should be derived
from class Y or it should contain a member of class Y. Assume further
that there are at least two different instances of both X and Y. Then,
for any instance of X, if we exchange the current Y component against
a different instance of Y, and we regard the new instance of X as being
identical to the instance of X before the exchange, then the correct
relationship between X and Y is the has-a relationship, and otherwise
it is the is-a relationship.

Example 1: If, for a specific instance of class Human of which class
Heart is a component, we exchange the current instance of class Heart
against a different instance, then we would still regard the new
instance of Human as being identical to the instance before the exchange.
Therefore, a Human "has a" Heart, and it is not the case that a Human
"is a" Heart.

Example 2: If, for a specific instance of class Human of which class
Primate is a component, we exchange the current instance of class
Primate against a different instance, then the Human instance would
lose its identity. Therefore, a Human "is a" Primate, and it is not the
case that a Human "has a" Primate.

[ Of course, no guideline is razor-sharp. If we exchange a Brain instead
  of a Heart, we might be a little fuzzy in answering the question of
  identity. This particular problem is due more to a "resemblence
  relationship" than an "identity relationship".  -adc ]