Consider that friendships are not corresponded if we do not explicitly specify so. In our example, CRectangle is
considered as a friend class by CSquare, but CRectangle does not consider CSquare to be a friend, so CRectangle
can access the protected and private members of CSquare but not the reverse way. Of course, we could have
declared also CSquare as friend of CRectangle if we wanted to.