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Programming for Containment/Delegation

As noted a bit earlier in this chapter, code reuse comes in two flavors. We have just explored the

classical “is-a” relationship. Before we examine the third pillar of OOP (polymorphism), let’s exam-

ine the “has-a” relationship (also known as the 

containment/delegation model or aggregation).

Assume you have created a new class that models an employee benefits package:

// This type will function as a contained class.

class BenefitPackage

{

// Assume we have other members that represent

// 401K plans, dental/health benefits, and so on.

public double ComputePayDeduction()

{

return 125.0;

}

}

Obviously, it would be rather odd to establish an “is-a” relationship between the

BenefitPackage class and the employee types. (Employee “is-a” BenefitPackage? I don’t think so.)

However, it should be clear that some sort of relationship between the two could be established.

In short, you would like to express the idea that each employee “has-a” BenefitPackage. To do so,

you can update the Employee class definition as follows: