When should you use 'friend' in C++?

Question

I have been reading through the C++ FAQ and was curious about the friend declaration. I personally have never used it, however I am interested in exploring the language.

Answer 1

@roo: Encapsulation is not broken here because the class itself dictates who can access its private members. Encapsulation would only be broken if this could be caused from outside the class, e.g. if your operator << would proclaim “I'm a friend of class foo.”

friend replaces use of public, not use of private!

Actually, the C++ FAQ answers this already.