761: Mainline

Nevertheless, the member functions of the header class absolutely need access to the private member variables and to the private constructor of the link class. Accordingly, C++ provides a mechanism whereby the link class can grant complete access privileges to the member functions of the header class. To use this privilege-granting mechanism, you add a friend declaration to the definition of the link class:

class link {
  friend class header;  
  private: 
    link *next_link_pointer; 
    railroad_car *element_pointer; 
    link (railroad_car *e, link *l) { 
      element_pointer = e; 
      next_link_pointer = l; 
    } 
};