A function can be a member of one class and a friend of another : friend function « Class « C++ Tutorial

1. C++ Tutorial
2. Class
3. friend function

#include <iostream> 
using namespace std; 
 
class Cylinder; // a forward declaration 
 
enum colors { red, green, yellow }; 
 
class Cube { 
 colors color; 
public: 
  Cube(colors c) { color= c; } 
  bool sameColor(Cylinder y); 
}; 
 
class Cylinder { 
 colors color; 
public: 
  Cylinder(colors c) { color = c; } 
  friend bool Cube::sameColor(Cylinder y); 
}; 
 
bool Cube::sameColor(Cylinder y) { 
  if(color == y.color) 
    return true; 
  else 
    return false; 
} 
 
int main() 
{ 
  Cube cube1(red); 
  Cube cube2(green); 
  Cylinder cyl(green); 
 
  if(cube1.sameColor(cyl)) 
    cout << "cube1 and cyl are the same color.\n"; 
  else 
    cout << "cube1 and cyl are different colors.\n"; 
 
  if(cube2.sameColor(cyl)) 
    cout << "cube2 and cyl are the same color.\n"; 
  else 
    cout << "cube2 and cyl are different colors.\n"; 
 
  return 0; 
}

cube1 and cyl are different colors.
cube2 and cyl are the same color.