recursive friend classes

Question

Is there any way around this:

class B;

class C { 
 public:
  C() { }
 private:
  int i;
  friend B::B();
};

class B { 
 public:
  B() { }
 private:
  int i;
  friend C::C();
};

Gives error:

prog.cpp:8: error: invalid use of incomplete type ‘struct B’
prog.cpp:1: error: forward declaration of ‘struct B’

Answer 1

You just can't do this. Remove the circular dependency.

Answer 2

According to IBM's documentation (which I realize is not normative):

A class Y must be defined before any member of Y can be declared a friend of another class.

So I think the answer is "no".

Of course, you can use

friend class B;

...instead of friend B::B(), but that grants friendship to all of B's members. And you probably already knew that.

Answer 3

Since you're very selective about friendship (access to specific member functions given to specific classes), the Attorney-Client Idiom may be what you need. I'm not sure how well this will work with constructors, though.

Answer 4

I realize that this is a really silly idea, but couldn't you—theoretically—accomplish this through inheritance, by making the parent class' constructors friends? The code compiles, at least, questionable though it may be.

class A {
 public:
  A() { }
 private:
  int i;
};

class D {
 public:
  D() { }
 private:
  int i;
};

class B : public A {
 public:
  B() { }
 private:
  friend D::D();
};

class C : public D {
 public:
  C() { }
 private:
  friend A::A();
};