Derivation from an Abstract Base Class

An abstract base class is used to encapsulate common features of concrete classes. An abstract class cannot be instantiated. Nevertheless, this scheme is quite useful and efficient for organizing our knowledge of the vastly complex biological world. In the simplified taxonomic organization chart shown in Figure 6.2, we can see that a primate is a mammal that has certain additional characteristics, a gorilla is a hominid with certain additional characteristics, and so forth.

Figure 6.2. Animal taxonomy

A concrete class represents a particular kind of entity, something that really exists and can be instantiated. For example, walking through the woods you will never encounter a real, live animal that is completely described by the designation Carnivora or Felidae. You may, depending on where you walk, find a lion, a siamese cat, or a common housecat (Felis silvestris). But there is no instance of a Hominidae (i.e., of a base class) in the concrete world that is not also an instance of some particular species. If a biologist ever finds a concrete instance that does not fit into an existing species definition, then that biologist may define and name a new species and become famous.

To summarize, the more general categories (class, order, family, subfamily) are all abstract base classes that cannot be instantiated in the concrete world. They were invented by people to help with the classification and organization of the concrete classes (species).

Back to Programming

At first it might seem counterintuitive to define a class for an abstract idea that has no concrete representative. But classes are groupings of functions and data, and are useful tools to enable certain kinds of organization and reuse. Categorizing things makes the world simpler and more manageable for humans and computers.

As we study design patterns and develop frameworks and class libraries, we will sometimes design inheritance trees where only the leaf nodes can be instantiated, and the inner nodes are all abstract.

Once again, an abstract base class is a class that is impossible and/or inappropriate to instantiate. Features of a class that tell the compiler to enforce this rule are:

• There is at least one pure virtual function.
• There are no public constructors.

Figure 6.3 shows a class hierarchy with an abstract base class, Shape. Shape is abstract because it contains pure virtual functions.

Figure 6.3. Shapes UML diagram

A pure virtual function has the following declaration syntax:

virtual returnType functionName(parameterList)=0;

Example 6.13 shows the base class definition.

Example 6.13. src/derivation/shape1/shapes.h

getName(), area(), and getdimension() are all pure virtual functions. Because they are defined to be pure virtual, no function definition is required in the Shape class. Any concrete derived class must override and define all pure virtual base class functions for instantiation to be permitted. In other words, any derived class that does not override and define all pure virtual base class functions is, itself, an abstract class. Example 6.14 shows the derived class definitions.

Example 6.14. src/derivation/shape1/shapes.h

Rectangle, Circle, and Square are derived from Shape. Their implementations are shown in Example 6.15.

Example 6.15. src/derivation/shape1/shapes.cpp

We provide some client code to exercise these classes in Example 6.16.

Example 6.16. src/derivation/shape1/shape1.cpp

In the global function showNameAndArea() the base class pointer, pshp, is successively given the addresses of objects of the three subclasses. For each address assignment, pshp polymorphically invokes the correct getName() and area() functions. Example 6.17 shows the output of the program.

Example 6.17. src/derivation/shape1/shape.txt