dependent classes friend functions, are they possible?

Question

I was reading an old C++ book to do some retro coding, C++ for C Programmers by Sharam Hekmatpour, I remember reading it when I was way younger and I liked it.

In the friend functions chapter he shows having two classes with friend functions:

class RealSet;

class IntSet {
public:
    void ToRealSet(RealSet*);

    friend void RealSet::ToIntSet(IntSet*);
};

class RealSet {
public:
    void ToIntSet(IntSet*);

    friend void IntSet::ToRealSet(RealSet*);
};

I know this won't compile because while we had defined RealSet as a forward declaration, we don't have a definition to RealSet::ToIntSet and that will make the compiler fail.

Yes, I know I could set the whole class as a friend class and that will solve the problem:

class RealSet;

class IntSet {
public:
    ToRealSet(RealSet*);

    friend class RealSet;
};

class RealSet {
public:
    ToIntSet(IntSet*);

    friend class IntSet;
};

My question is simple: Was this ever possible or is it a typical book mistake?

there are some problems in the code not directly related to the question. missing `;` after class definition, `ToRealSet` has to return something.... — 463035818_is_not_an_ai, Apr 27 '20 at 08:56
`friend char* X::foo(int); // members of other classes can be friends too` see: https://en.cppreference.com/w/cpp/language/friend — Richard Critten, Apr 27 '20 at 09:04
Yes @RichardCritten that is very clear but that is **not** the question. — cprieto, Apr 27 '20 at 09:08
Indeed @freakish I think that is the case, I cannot find any reference in the old Borland or Watcom Manuals so I assume it is an error in Hekmatpour's book. — cprieto, Apr 27 '20 at 09:10
incomplete class. Cannot use anything that isn't defined and those methods weren't defined yet. Only way to do it is through a friend global function — Swift - Friday Pie, Apr 27 '20 at 09:10
@cprieto som old Pre-ISO compilers had severe gray zones, e.g. I have book with code for Borland C++, a vector graphics library of a kind where none of the classes would compile in C++98. — Swift - Friday Pie, Apr 27 '20 at 09:12
the crux of the problem is not really friends, but the mutual dependency and that needs to be solved on a higher level than the friends declaration. — 463035818_is_not_an_ai, Apr 27 '20 at 09:12
for a real use-scenario there are solutions, but without more context it is not obvious how to resolve the mutual dependency. As written the code is broken and there is no easy fix — 463035818_is_not_an_ai, Apr 27 '20 at 09:13

dependent classes friend functions, are they possible?

0 Answers0