Inheritance

Provides a way to create a new class from an existing class.
New class is a specialized version of the existing class.

• Base Class(or Parent): inherited by child class.
• Derived Class(or child): inherits from base class.

class Student{ //base class
 //body
}; 

class UnderGrad : public Student{  //derived class
  //body
};

Inheritance establishes an “is a” relationship between classes.

An object of the derived class “is a” object of the base class. For example:

• An UnderGrad is Student.

A derived object has all​ characteristics of the base class.

An object of child class has:

• All members defined in the child class.
• All ​members declared in the parent class.

An object of child class can use:

• All public members defined in the child class.
• All public members defined in the parent class.

Protected Members

• protected member access specification: similar to private, but accessible by objects of derived class.

Class Access Specifiers

• public: the object of the derived class can be treated as an object of the base class.
• protected: more restrictive than public, but allows derived classes to know details of parents.
• private: prevents objects of the derived class to be treated as objects of base class.

#include <iostream>
using namespace std;

// Base class
class Shape {
   public:
      Shape(){length = 0; width = 0;} //default constructor
      void setlength(int l, int w) {length = l; width = w;}
  protected:
      int length, width;
};

// Derived class
class Square: public Shape {
   public:
      Square() : Shape() {} //declaring and initializing derived class constructor 
      int get_Area(){ return (length * width); }
};

int main(void) {
   Square sq; //making object of child class Square
   sq.setlength(5, 5); //setting length equal to 5
   // Print the area of the object.
   cout << "Total area of square is: " << sq.get_Area() << endl;

   return 0;
}

As you can see in the example above,

• The shape class is the parent class whereas the Square class is the child class derived from it.
• In our child class Square, we use members from the parent class such as
  • the protected length and width variable which gets initialized to zero in the default constructor.
  • Length and width also gets used in child class function get_Area to compute the area of the square.
• In main the setlength function which is a public member function of the parent class is accessible to the child class object sq
  • The dot operator is used to access setlength in the main.

Try the following example in the editor below.

With reference to above example, create a derived class Rectangle inherited from class Shape, your task is to create a public function in it named get_Perimeter which return the perimeter of rectange.