//this is intl.h
 
# ifndef _header_i
#define _header_i
 
#include "floatl.h"
#include <iostream>
using namespace std;
 
class intl
{
..
..
};
#endif

//this is float_l.h
#ifndef _header_f
#define _header_f
 
#include "intl.h"
#include <cstring>
using namespace std;
 
class float
{
..
..
};
//all methods defined here

//********* header b.h ***************
#ifndef _B_H_INCLUDED_
#define _B_H_INCLUDED_
struct A ;
// A has been declared; we can now declare (not yet define) functions using A
struct B
{
  inline int b_fun( const A& a ) ;
  int bm ;
  A* a ;
  friend inline int b_friend( B& b, const A& a ) ;
};
#endif // _B_H_INCLUDED_
//////////////////////////////////////////////////////////////////

//********* header a.h ***************
#ifndef _A_H_INCLUDED_
#define _A_H_INCLUDED_
struct B ;

struct A
{
  inline int a_fun( const B& b ) ;
  int am ;
  friend inline int a_friend( A& a, const B& b ) ;
};
#endif // _A_H_INCLUDED_
/////////////////////////////////////////////////////////////////////

//********* header anything_else.h ***************
#ifndef _ANYTHING_ELSE_H_INCLUDED_
#define _ANYTHING_ELSE_H_INCLUDED_

#include "a.h"
#include "b.h"
// A and B have been defined; we can now define functions using A and B

int A::a_fun( const B& b ) { return am += b.bm ; }
int B::b_fun( const A& a ) { return bm += a.am ; }
int a_friend( A& a, const B& b ) { return a.am += b.bm ; }
int b_friend( B& b, const A& a ) { return b.bm += a.am ; }
#endif // _ANYTHING_ELSE_H_INCLUDED_

struct A ; class A ; struct A ;
int foobar( A arg1, A& arg2 ) ; // ok, use in name
//int foobar( A arg1, A& arg2 ) { return arg2.a = arg1.a ; };
// uncomment the above and you get the error '.... undefined type ....'
class A
{
  public : int a ;
};
struct A ; class A ; struct A ;
int foobar( A arg1, A& arg2 ) { return arg2.a = arg1.a ; }; // ok
 
class B ;
 
struct B : public A
{
  private : int b ;
  public : operator int() const { return a + b ; }
};
 
int main()
{
  struct A a1 ; 
  class A a2 ;
  class B b1 ;
  struct B b2 ;
  return a1.a + a2.a + b1.a + b2 ;
}