#include <iostream>

class B;

class A {
friend class B;
private:
  int x;
};

class B {
  static void print(const A& a) {
    // x is private, but B is a friend of A, so it's fine
    std::cout << a.x << std::endl;
  }
};

int main() {
  B b;
  A a;
  b.print(a);
  return 0;
}

#include <iostream>

class A {
private:
  int x;
};

class B : public A {
  void print() {
    // error A::x is private!
    std::cout << A::x << std::endl;
  }
};

int main() {
  B b;
  b.print();
  return 0;
}

#include <iostream>

class A {
protected:
  int x;
};

class B : public A {
  void print() {
    // A::x is protected, but B inherits from A, so it's fine
    std::cout << A::x << std::endl;
  }
};

int main() {
  B b;
  b.print();
  A a;
  // error a::x is protected and can only be accessed from A or from any class which inherits from A
  // std::cout << a.x << std::endl;
  return 0;
}

class A
{
    int a1;
};

class B : public A
{
    int b1 ;
};

class B : public A
{
    int b1 ;
    A *a;

    void foo()
    {
       std::cout << a->a1; --> possible because B is a friend of A
    }
}