Remembering how to remember friend class C++

• Friendships are not symmetric – If class A is a friend of class B, class B is not automatically a friend of class A.
  "I thought you were my friend, but the feelings were not mutual."
• Friendships are not transitive – If class A is a friend of class B, and class B is a friend of class C, class A is not automatically a friend of class C.
  "Just because he is your friend doesn't make him my friend."
• Friendships are not inherited – A friend of class Base is not automatically a friend of class Derived and vice versa; equally if Base is a friend of another class, Derived is not automatically a friend and vice versa.
  "You've changed. You are not the person I used to know. I don't like you anymore."
  
  More: https://en.wikipedia.org/wiki/Friend_class