Friend Function in C++

//use of friend function
#include<iostream>
using namespace std;
class Test
{
int num1;
int num2;
public:
friend void display();//creating friend function
void setData()
{
num1=100;
num2=20;

}
void getData()
{
cout<<endl<<"num1="<<num1;
cout<<endl<<"num2="<<num2;
}
};
void display()//this is non member function
{ //accessing private data members of Test class
Test t;
t.num1=1000;
t.num2=2000;
cout<<endl<<"The sum is "<<t.num1+t.num2;
}
int main()
{
display();
return(0);
}//end of main

//use of friend function of class
#include<iostream>
using namespace std;
class Demo
{
public:
void fun1();
void fun2();

};

class Test
{
int num1;
int num2;
public:
friend void display();//creating friend function
friend void Demo::fun1();
void setData()
{
num1=100;
num2=20;

}
void getData()
{
cout<<endl<<"num1="<<num1;
cout<<endl<<"num2="<<num2;
}
};//end of demo class
void Demo::fun1()
{
Test t;
t.num1=100;
t.num2=90;
cout<<endl<<"The sum is "<<t.num1+t.num2;
}//end of function
void Demo::fun2()
{

}
void display()//this is non member function
{ //accessing private data members of Test class
Test t;
t.num1=1000;
t.num2=2000;
cout<<endl<<"The sum is "<<t.num1+t.num2;
}
int main()
{
display();
Demo d;
d.fun1();

return(0);
}//end of main