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 tree example is a pretty good example : Having an object implemented in a few different class without having an inheritance relationship.

Maybe you could also need it to have a constructor protected and force people to use your "friend" factory.

... Ok, Well frankly you can live without it.