friend function using c++

#include"iostream.h"

class sample
{
int a;
int b;
public:
void setvalue() {a=50; b=40; }

friend float mean(sample s);
};


float mean(sample s)
{
return float(s.a+s.b)/2.0;
}


int main()
{
sample X; //object X
X.setvalue();

cout<< "Mean value="<
return 0;
}