Introduction

As we saw earlier in our introduction to OOP, inheritance is a mechanism for deriving a new class from an older base class. That is, the base class, sometimes called the super class, is supplemented or selectively altered to create the new derived class. Inheritance provides a powerful code reuse mechanism since a hierarchy of related classes can be created that share the same code. A class can be derived from an existing base class using the module construct illustrated in Figure 6.1.

We note that the inheritance is invoked by the USE statement. Sometimes an inherited entity (attribute or member) needs to be slightly amended for the purposes of the new classes. Thus, at times one may want to bring into the new class selectively only certain entities from the base class. The modifier ONLY in a USE statement allows one to select the desired entities from the base class as illustrated in Figure 6.2. It is also common to develop name conflicts when combining entities from one or more related classes. Thus, a rename modifier, =>, is also provided for a USE statement to allow the programmer to pick a new local name for an entity inherited from the base class. The form for that modifier is given in Figure 6.3.

It is logical to extend any or all of the aforementioned inheritance mechanisms to produce multiple inheritance. Multiple Inheritance allows a derived class to be created by using inheritance from more than a single base class.