17. Inheritance vs. encapsulation

There are two ways to be a client of an existing class A:

• Inherit from type A

• Have a member of type A

Inheritance automatically provides interface of Base class.

Encapsulation allows restricting the interface, but requires republication of needed parts.

Usually, it's pretty obvious which is correct.

Class libraries often rely on inheritance (recall Printer example.)

This is known as Is-a vs. Has-a:

  class Employee (Base)
           | |
           | +--has-a--> int Salary;
           | `--has-a--> string Name;
           |
           |
           |
  class Manager (Derived) : (is-a) public Employee
             |
             `--has-a--> Employee subordinates[ 10 ];