Question

When I design classes and have to choose between inheritance and composition, I usually use the rule of thumb: if the relationship is ‘is-a’ – use inheritance, and if the relationship is ‘has-a’ – use composition.

Is it always right?

Thank you.

Answer 1

No – ‘is a’ does not always lead to inheritence. A well cited example is the relationship between a square and a rectangle. A square is a rectangle, but it will be bad to design code that inherits a Square class off a Rectangle class.

My suggestion is to enhance your ‘is a / has a’ heuristic with the Liskov Substitution Principle. To check whether an inheritence relationship complies with the Liskov Substitution Principle, ask whether clients of a base class can operate on the sub class without knowing that it is operating on a sub class. Of course, all the properties of the sub class must be preserved.

In the square / rectangle example, we must ask whether a client of rectangle can operate on a square without knowing that it is a square. All that the client must know is that it is operating on a rectangle. The following function demonstrates a client that assumes that setting the width of a rectangle leaves the height unchanged.

void g(Rectangle& r) {     r.SetWidth(5);     r.SetHeight(4);     assert(r.GetWidth() * r.GetHeight()) == 20); } 

This assumption is true for a rectangle, but not for a square. So the function cannot operate on a square and therefore the inheritence relationship violates the Liskov Substitution principle.

Other examples