Friend function in separate files

[juanix]

Hello,

I need to use friend functions into two classes in separate files. I compiled and run the following sample program without problems but when I try to separate it into different files I get several compile errors related to the classes declaration.

------- friend1.cc -----------
class ONE;

class TWO
{
public:
void print(ONE& x);
};

class ONE
{
private:
int a, b;
friend void TWO::print(ONE& x);
public:
ONE() : a(1), b(2) { }
};

void TWO::print(ONE& x)
{
cout << "a is " << x.a << endl;
cout << "b is " << x.b << endl;
}

int main()
{
ONE xobj;
TWO yobj;
yobj.print(xobj);
}

In separated files:
------- one.h ---------
//class TWO;
//#include "two.h"

class ONE
{
private:
int a, b;
friend void TWO::print(ONE& x);
public:
ONE() : a(1), b(2) { }
};
------- one.cc --------
#include "one.h"

void TWO::print(ONE& x)
{}
------- two.h ---------
class ONE;
class TWO
{
public:
void print(ONE& x);
};
------- two.cc --------
#include "two.h"
void TWO::print(ONE& x)
{
cout << "a is " << x.a << endl;
cout << "b is " << x.b << endl;
}
------- friend2.cc ----
#include "one.h"
#include "two.h"

int main()
{
ONE xobj;
TWO yobj;
yobj.print(xobj);
}

These are the compilation errors:
$ g++ one.cc two.cc friend2.cc -o friend
In file included from one.cc:1:
one.h:10: error: âTWOâ has not been declared
one.cc:3: error: âTWOâ has not been declared
two.cc: In member function âvoid TWO::print(ONE&)â:
two.cc:4: error: invalid use of incomplete type âstruct ONEâ
two.h:4: error: forward declaration of âstruct ONEâ
two.cc:5: error: invalid use of incomplete type âstruct ONEâ
two.h:4: error: forward declaration of âstruct ONEâ
In file included from friend2.cc:2:
one.h:10: error: âTWOâ has not been declared

Could you get me ideas in order to get the best solution?

[maikel]

/**
 * Path: ./one.hpp 
 */
#ifndef _ONE_HPP
#define _ONE_HPP 1

#include "two.hpp"

class One {
private:
    int a, b;    
    friend void Two::print(One&);
public:
    One() : a(1), b(2) {}
};

#endif

/**
 * Path: ./two.hpp
 */

#ifndef _TWO_HPP
#define _TWO_HPP 1

class One;
class Two {
public:
    void print(One&);
};

#endif

/**
 * Path: ./two.cpp
 */
#include "one.hpp"
#include "two.hpp"
#include <iostream>
using std::cout;
using std::endl;

void Two::print(One& x)
{
    cout << "a is " << x.a << endl;
    cout << "b is " << x.b << endl;
}

/**
 * Path: ./friend.cpp
 */
#include "one.hpp"
#include "two.hpp"

int main()
{
    One obj1;
    Two obj2;
    obj2.print(obj1);
}

[jsmith]

Just out of curiosity... why does class Two know how to print class One, but class One doesn't know how
to print itself?