A simple and complete C++ program to demonstrate friend function of another class.

#include <iostream>
class B;
class A
{
public:
void showB(B& );
};
class B
{
private:
int b;
public:
B() { b = 0; }
friend void A::showB(B& x); // Friend function
};
void A::showB(B &x)
{ // Since show() is friend of B, it can
// access private members of B
std::cout << “B::b = ” << x.b;
}
int main()
{ A a;
B x;
a.showB(x);return 0;
}

Output:
B::b = 0