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Inheritance in C++

The ability to use the object-oriented programming is an important feature of C++. classes in C++ introduced the idea of the class; if you have not read it and do not know the basic details of classes, you should read it before continuing this tutorial. 

Inheritance is an important feature of classes; in fact, it is integral to the idea of object oriented programming. Inheritance allows you to create a hierarchy of classes, with various classes of more specific natures inheriting the general aspects of more generalized classes. In this way, it is possible to structure a program starting with abstract ideas that are then implemented by specific classes. For example, you might have a class Animal from which class dog and cat inherent the traits that are general to all animals; at the same time, each of those classes will have attributes specific to the animal dog or cat.

Inheritance offers many useful features to programmers. The ability, for example, of a variable of a more general class to function as any of the more specific classes which inherit from it, called polymorphism, is handy. For now, we will concentrate on the basic syntax of inheritance. 

A quick example of inheritance:
class Animal
  void eat();
  void sleep();
  void drink();

  int legs;
  int arms;
  int age;
//The class Animal contains information and functions
//related to all animals (at least, all animals this lesson uses)
class Cat : public Animal
  int fur_color;
  void purr();
  void fish();
  void markTerritory();
//each of the above operations is unique
//to your friendly furry friends
//(or enemies, as the case may be)
A discussion of the keywords public, private, and protected is useful when discussing inheritance. The three keywords are used to control access to functions and variables stored within a class.


The most open level of data hiding is public. Anything that is public is available to all derived classes of a base class, and the public variables and data for each object of both the base and derived class is accessible by code outside the class. Functions marked public are generally those the class uses to give information to and take information from the outside world; they are typically the interface with the class. The rest of the class should be hidden from the user using private or protected data (This hidden nature and the highly focused nature of classes is known collectively as encapsulation). The syntax for public is:
Everything following is public until the end of the class or another data hiding keyword is used. 

In general, a well-designed class will have no public fields--everything should go through the class's functions. Functions that retrieve variables are known as 'getters' and those that change values are known as 'setters'. Since the public part of the class is intended for use by others, it is often sensible to put the public section at the top of the class.


Variables and functions marked protected are inherited by derived classes; however, these derived classes hide the data from code outside of any instance of the object. Keep in mind, even if you have another object of the same type as your first object, the second object cannot access a protected variable in the first object. Instead, the second object will have its own variable with the same name - but not necessarily the same data. Protected is a useful level of access control for important aspects to a class that must be passed on without allowing it to be accessed. The syntax is the same as that of public. specifically,


Private is the highest level of data-hiding. Not only are the functions and variables marked private not accessible by code outside the specific object in which that data appears, but private variables and functions are not inherited (in the sense that the derived class cannot directly access these variables or functions). The level of data protection afforded by protected is generally more flexible than that of the private level. On the other hand, if you do not wish derived classes to access a method, declaring it private is sensible.
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