15: Polymorphism &
Virtual Functions

Polymorphism (implemented in C++ with virtual functions) is the third essential feature of an object-oriented programming language, after data abstraction and inheritance.

It provides another dimension of separation of interface from implementation, to decouple what from how. Polymorphism allows improved code organization and readability as well as the creation of extensible programs that can be “grown” not only during the original creation of the project, but also when new features are desired.

Encapsulation creates new data types by combining characteristics and behaviors. Access control separates the interface from the implementation by making the details private. This kind of mechanical organization makes ready sense to someone with a procedural programming background. But virtual functions deal with decoupling in terms of types. In Chapter 14, you saw how inheritance allows the treatment of an object as its own type or its base type. This ability is critical because it allows many types (derived from the same base type) to be treated as if they were one type, and a single piece of code to work on all those different types equally. The virtual function allows one type to express its distinction from another, similar type, as long as they’re both derived from the same base type. This distinction is expressed through differences in behavior of the functions that you can call through the base class.

In this chapter, you’ll learn about virtual functions, starting from the basics with simple examples that strip away everything but the “virtualness” of the program.