Inheritance

The word inheritance means to pass on the features of one generation to the next. Its like a child inherits some or all characteristics of his parents and develops new characteristics of his own.

Is case of OOPs, inheritance means to derive a new class from the existing class without modifying the existing one. The new class is called derived class (or subclass or child class or descendent class) and the existing or old class is called base class (or super class or parent class or ancestor class). The derived class can inherit some or all features (data and functions) of the base class and can add its own features to extend the functionality of the base class. But in this whole procedure the base class does not get affected. The process of inheritance is also known as ‘derivation’.

The concept of inheritance is used at the time when programmer has to use such objects in his program that have certain features in common like shown below. Consider the following example-

Suppose we have three objects of an organization- worker, manager and clerk. We will have to make three classes one for each object. These are shown below-

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Here these three classes have certain data functions in common. The common data is - emp_id, ename and salary. The common functions are - get_data ( ) and put_data ( ). If we make these three classes as the way they are, then we will have to write, debug and compile common data and functions again and again and it will result in wastage of resources and time. Moreover if we have to add a new feature address that is common for all classes then we will have to add it to each of these classes. It means if we have to do some common change that is required for all classes, then we will have to make that change to all classes even though it is common. It is very time consuming and tedious job.

The solution for this is to make a class and put all the common data and functions in that class. So we will make class employee and will put the common features like emp_id, ename, salary, get_data ( ) and put_data ( ) in the class. This class will act as base class. And now we will derive other classes worker, manager and clerk from it and will add uncommon features to these classes i.e. the features that are not common to all these three classes like hr_wages, units_assigned, units_made, calc_salary ( ) belong only to worker class; reports, report_details ( ) to manager class and sales, purchase, audit ( ) to clerk class. It is shown below-



Now if we have to add any new feature, like address, that is common to all three classes, that will be put into employee class instead of adding separately to worker, manager and clerk classes. Only distinct features are required to be added separately to these classes.

Inheritance works only in one direction. The derived class inherits the features of the base class but base class does not know anything about the derived class.

The derived class can inherit some or all the properties of the base class and can also pass on the inherited properties to the class which will be inherited from it. It depends on the accessibility level of the data and functions of the base class and the way the base class is inherited. The accessibility levels and the way of derivation can be- Private, Protected and Public.



From the above figure it is clear that only the Protected and Public members are inherited and not the Private members. And inherited members will be available for further inheritance only if they are inherited in public way or protected way otherwise not.

The advantages of inheritance are-

1. With it the already written code can be reused. Thus the programmer does not need to do the same thing again and again and he can concentrate on new things.
2. The quality and reliability of the program is increased because the thing, which we reuse, has been used and tested many times.
3. We can easily modify the program.

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