Friend declaration in C++ - difference between public and private

No, there's no difference - you just tell that class B is a friend of class A and now can access its private and protected members, that's all.

Since the syntax friend class B doesn't declare a member of the class A, so it doesn't matter where you write it, class B is a friend of class A.

Also, if you write friend class B in protected section of A, then it does NOT mean that B can access only protected and public members of A.

Always remember that once B becomes a friend of A, it can access any member of A, no matter in which section you write friend class B.

c++ has the notion of 'hidden friends': http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1601r0.pdf

Which only applies to friend functions that are defined inline. This make it so the functions can only be found via argument-dependent lookups, removing them from enclosing namespace.