Friendship rules

• A friend does get full access to private members of the class.

• Friendship is a one-way street:

  • Friendship is not symmetrical.

  • A class has no access to its friend.

• The friend of my friend is still a stranger to me -
  friendship is not transitive.

• Suggestions:

  • Prefer making functions friends rather than classes;

  • Declare friends at the top of class declarations;

  • Don't make friends lightly!