Question

I haven’t yet worked out a specific case. But I am about to embark on writing some code that I feel will end up needing this; and so I wanted to know if:

1. Two classes can friend each other; so that they can freely access the private
   and protected members of the other (I believe the answer is yes, and ofcourse I
   can simply try it out!). Any detailed references or other question links with answers
   are also very welcome. I am aware of forward declarations and include guard compiler
   pre-directives and their use. My questions are rather more related to the semantics
   of the C++ language in terms of what it can offer with regard to this possibility
   of mutual friendship and how to use it properly.
2. Is this generally recommended? Do people employ this kind of design on a regular basis?
   Under what circumstances would this be a recommended design (preferably with some
   examples).

Answer 1

You can have mutual friendship:

class A {
    friend class B;
};

class B {
    friend class A;
};

Whether or not this makes sense depends entirely on the problem you are trying to solve. It definitely could make sense in certain circumstances.

The only example from my current project that utilizes mutual friendship is a container implementation: the container class is a friend of its iterator class and vice versa.