Inheritance

The mechanism that allows us to extend the definition of a class without making any physical changes to the existing class is called as inheritance. Or in other words by using inheritance child class can acquire the properties of parent class.

Inheritance lets you create new classes from existing class. Any new class that you create from an existing class is called as derived class and existing class is called as base class.

Above diagram represents the inheritance relationship. Mother and Father are represented as base class or super class or parent class and Daughter is represented as sub class or derived class or child class.

The inheritance relationship enables a derived class to inherit features from its base class. Furthermore, the derived class can add new features of its own. Therefore, rather than creating the new classes completely from the scratch, you can take the advantage of inheritance and reduce the software complexity.

Types of Inheritance

Single Inheritance:

It is the inheritance hierarchy wherein one derived class inherited from one base class.

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Multiple Inheritances: 

It is the inheritance hierarchy wherein one derived class inherited from multiple base class(es).

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Hierarchical Inheritance:

It is the inheritance hierarchy wherein multiple subclasses inherited from one base class.

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Multilevel Inheritance:

It is the inheritance hierarchy wherein subclass acts as a base class for other classes.

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Hybrid Inheritance: 

The inheritance hierarchy that reflects any legal combination of other four types of inheritance.

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Syntax for Inheritance

• In order to derive a class from another, we use a colon (:) in the declaration of the derived class using the following format :

    class derived_class : memberAccessSpecifier base_class
    {
                ...
    };

• Where derived_class is the name of the derived class and base_class is the name of the class on which it is based. The member Access Specifier may be public, protected or private
  

Following example further explains concept of inheritance:

Class Shape is a base class and we have derived Rectangle (Derived class) and Triangle (Derived class) classes from the base class.

Limitations of inheritance

• A derived class inherits every member of a base class except constructor and destructor. 
• Private members of base classes become members of derived class. But they are inaccessible by the members of derived class 
• Though object-oriented programming is frequently propagandised as an answer for complicated projects, inappropriate use of inheritance makes a program more complicated.
• Invoking member functions using objects create more compiler overheads.
• In class hierarchy various data elements remain unused; the memory allocated to them is not utilized.