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

The short answer would be: use friend when it actually improves encapsulation. Improving readability and usability (operators << and >> are the canonical example) is also a good reason.

As for examples of improving encapsulation, classes specifically designed to work with the internals of other classes (test classes come to mind) are good candidates.