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

Probably I missed something from the answers above but another important concept in encapsulation is hiding of implementation. Reducing access to private data members (the implementation details of a class) allows much easier modification of the code later. If a friend directly accesses the private data, any changes to the implementation data fields (private data), break the code accessing that data. Using access methods mostly eliminates this. Fairly important I would think.