Example of friend class and friend function

The following example shows a simple friend class and friend function example:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//This program shows a basic example of friend class and function
#include<iostream>

using namespace std;

class A {
public:
  int square(int num)
  {return num*num;}
protected:
  int reduced_square(int num)
  {return (num-1)*(num-1);}
private:
  int cube(int num)
  {return num*num*num;}
  friend class B;
};

//Class B friend of class A
class B {
public:
  int b_square(int num)
  {
    A a;
    return a.square(num);
  }
  int b_reduced_square(int num)
  {
    A a;
    return a.reduced_square(num);
  }
  int b_cube(int num)
  {
    A a;
    return a.cube(num);
  }
private:
  int b_priv;
  int b_default_cube()
  { 
    return b_cube(b_priv); 
  }
  friend int C(int c);
};

//function C friend of Class B
int C(int c)
{
  B b;
  b.b_priv = c;
  //A a;
  //return a.cube(b.b_priv); - Not possible as C is not friend of A
  return b.b_default_cube();
}


int main()
{
  cout<<"\nExample of Friend Class and Function"<<endl;
  A a;
  cout<<"\nA: Square of 3 = "<<a.square(3)<<endl;
  //The following is not possible
  //cout<<"Reduced Square of 3 = "<<a.reduced_square(3)<<endl;
  //cout<<"Cube of 3 = "<<a.cube(3)<<endl;

  B b;
  cout<<"\nB: Square of 4 = "<<b.b_square(4)<<endl;
  cout<<"B: Reduced Square of 4 = "<<b.b_reduced_square(4)<<endl;
  cout<<"B: Cube of 4 = "<<b.b_cube(4)<<endl;

  cout<<"\nC: Cube of 5 = "<<C(5)<<endl;

  return 0;
}

The output is as follows: