FAQ 5.04 What are the fundamental concepts of object-oriented technology?

Objects, classes, and inheritance.

By definition, object-oriented programming languages must provide objects, classes, and inheritance. In addition, polymorphism and dynamic binding are also required for all but the most trivial applications. Together, these five features provide the tools for fulfilling the goals of OO: abstraction, encapsulation, comprehensibility, changeability, and reusability.

An object is a software entity that combines state and behavior. A class is a programming construct that defines the common state and behavior of a group of similar objects; that is, a class has a name, and it describes the state (member data) and services (member functions) provided by objects that are instances of that class. The runtime system creates each object based on a class definition.

As an analogy, consider a home. A home is like an object since it has state (whether the lights are on, ambient temperature, and so on) and it provides services (a button that opens the garage door, a thermostat that controls the temperature, an alarm system that detects intruders, and so on). To carry the home metaphor a bit further, a blueprint for a home is like a class since a blueprint defines the characteristics of a group of similar homes.

Simple data types as int and float can be thought of as classes; variables of these types can be thought of as objects of the associated class.

Classes can be related by inheritance. In C++, inheritance facilitates polymorphism and dynamic binding. Polymorphism allows objects of one class to be used as if they were objects of another, related class. Dynamic binding ensures that the code that is executed is always consistent with the type of the object. Rather than selecting the proper code fragment by some ad hoc technique such as complex decision logic, the proper code fragment is automatically selected in a manner that is both extensible and always correct. Together, these allow old code to call new code; new classes can be added to a working system without affecting existing code.