Example of Friend function cannot be invoked with the object - C++

The friend function cannot be invoked with the object as the friend function is not in the scope of that class.

a friend function cannot be invoked using the object as it is not in the scope of that class

The program shows an error because the Friend function cannot be invoked with the object.

#include <iostream>
using namespace std;
class one {
private:
int n1=1;
public:
friend int add(one);
};
int add(one object1)
{
return (object1.n1 + 1);
}
int main()
{
one object1;
object1.add();
cout<<"Sum: "<< add(object1);
return 0;
}

#include <iostream>

using namespace std;

class one {

private:

int n1=1;

public:

friend int add(one);

};

int add(one object1)

{

return (object1.n1 + 1);

}

int main()

{

one object1;

object1.add();

cout<<"Sum: "<< add(object1);

return 0;

}

Correct program

#include <iostream>
using namespace std;
class one {
private:
int n1=1;
public:
friend int add(one);
};
int add(one object1)
{
return (object1.n1 + 1);
}
int main()
{
one object1;
cout<<"Sum: "<< add(object1);
return 0;
}