Single Inheritance – Public Inheritance in C++

Inheritance in which the derived class is derived from only one base class is called single inheritance.

Let us consider a simple example to illustrate the single inheritance. Program given below shows a base class B and a derived class D. The class B contains one private data member, one public data member and three public member functions. The class D contains one private data member and two public member functions.

#include<iostream.h>

class B
   {
      private:
         int a;      //private; not inheritable
      public:
         int b;      //public; ready for inheritance
         void get_ab();
         int get_a();
         void show_a();
   };

class D : public B      //public derivation
   {
      private:
         int c;
      public:
         void mul();
         void display();
   };

/*...Defining of functions...*/

void B :: get_ab()
   {
      a = 5;
      b = 10;
   }

int B :: get_a()
   {
      return a;
   }

void B :: show_a()
   {
      cout << "a = " << a <<endl;
   }

void D :: mul()
   {
      c = b * get_a();
   }

void D :: display()
   {
      cout << "a = " << get_a() << endl;
      cout << "b = " << b << endl;
      cout << "c = " << c << endl;
   }

/*...Main Program...*/

void main()
   {
      D x;
   
      x.get_ab();
      x.mul();
      x.show_a();
      x.display()

      x.b = 20;
      x.mul();
      x.display();
   }

Given below is the output of the above program:

a = 5
a = 5
b = 10
c = 50

a = 5
b = 20
c = 100

In the above program, following things require to be noted:

1. The class D is a public derivation of the base class B. Therefore, class D inherits all the public members of class B an retains their visibility. Thus, a public member of the base class B is also a public member of the derived class D
2. The private members of the base class B cannot be inherited by the derived class D
3. The class D, in effect, will have more members than what it contains at the time of declaration
4. The program illustrates that the objects of class D have access to all the public members of class B

Let us have a look at the function show_a() and mul():

void B :: show_a()
   {
      cout << "a = " << a <<endl;
   }

void D :: mul()
   {
      c = b * get_a();
   }

Although the data member a is a private member of base class B and cannot be inherited, objects of derived class D are able to access it through an inherited member function of class B.