// friends.cpp
#include <iostream>
using namespace std;

class A1;  // incomplete type definition needed because B1 refers to A1
           // and A1 refers to B1

class B1
{
  public:
    B1(int pub, int pri){b1pub=pub; b1pri=pri;}
    void B1_out(A1 aa);
    int b1pub;
  private:
    int b1pri;
};

class A1
{
  friend B1;  // says B1 can access this classes private part
  public:
    A1(int pub, int pri){a1pub=pub; a1pri=pri;}
    void A1_out(B1 bb);
    int a1pub;
  private:
    int a1pri;
};

void A1::A1_out(B1 bb)
{
  cout << "A1_out " << a1pub << a1pri << bb.b1pub /* << bb.b1pri */ << '\n';
                           // B1 public OK  -----          ----- no private
}

void B1::B1_out(A1 aa)
{
  cout << "B1_out " << b1pub << b1pri << aa.a1pub << aa.a1pri << '\n';
                    // both public and private OK because of 'friend'
}

int main(void)
{
  A1 a(1,2);
  B1 b(3,4);

  cout << "friends.cpp \n";

  a.A1_out(b);
  b.B1_out(a);

  return 0;
}

// results: friends.cpp 
//          A1_out 123
//          B1_out 3412