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

When implementing tree algorithms for class, the framework code the prof gave us had the tree class as a friend of the node class.

It doesn't really do any good, other than let you access a member variable without using a setting function.