Software Engineering and Computer Games

4.4. Composition and delegation

We say that ClassB is composed with ClassA if ClassB has a ClassA or ClassA* member; for short we can say ClassB has a ClassA. And, as before, ClassB inherits from ClassA if ClassB is derived from ClassA as a child class; for short we say ClassB is a ClassA.

As it turns out, you can always replace an inheritance relationship by a composition relationship as indicated in Figure 4.7. If ClassB has a ClassA member object *_pA, then (a) a ClassB object gets a set of ClassA data fields wrapped up inside *_pA and, (b) ClassB can implement the same methods as ClassA simply by passing these method calls off to *_pA. When you pass method calls to a composed object, this is called delegation.

Figure 4.7. Inheritance and ...