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

As the reference for friend declaration says:

The friend declaration appears in a class body and grants a function or another class access to private and protected members of the class where the friend declaration appears.

So just as a reminder, there are technical errors in some of the answers which say that friend can only visit protected members.