Friend Class in C++

//use of friend class
#include<iostream>
using namespace std;

class Test
{
int num1;
int num2;
public:
friend class Demo;//creating friend class
void setData()
{
num1=100;
num2=200;
}
void getData()
{
cout<<endl<<"num1="<<num1;
cout<<endl<<"num2="<<num2;
}
};//end of demo class
class Demo
{
Test t1;
int res;
public:
void input()
{
cout<<"\n using private data members of Test class";
t1.num1=50;
t1.num2=40;
}
void sum()
{
//using private data members of Test class
res=t1.num1+t1.num2;
cout<<"\n Result is "<<res;
}
};

int main()
{
Test t;
Demo d; //friend class
t.setData();
t.getData();
d.input();
d.sum();
return(0);
}//end of main