Date: Thu, 6 Mar 1997 12:52:49 GMT
Message-Id: <1.5.4.16.19970306125320.268fb65e@mailhost.sm.ic.ac.uk>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
To: an118 AT chebucto DOT ns DOT ca
From: Paul Dixon <p DOT dixon AT ic DOT ac DOT uk>
Subject: Re: HELP - need help resolving conundrum with classes
Cc: djgpp AT delorie DOT com

>How can I declare two different classes, where each class has a member 
>variable of the other class type. My problem is how to get the compiler to 
>recognize a variable type before it has been created.

It isnt possible (AFAIK) to do exactly what you want - the definitions for
'sizeof' become recursive - but you can create classes each with member
POINTERS to each other, as

// This block in the header file (thing.h)
// declare both classes 
class Owner;
class Cat;

// now define interface for each class
class Owner;
{
  Owner();
  ~Owner();

  int cats;
  Cat **cat;
public:
  void Own( Cat * );
}

class Cat
{
  Cat();
  ~Cat();

  int owners;
  Owner **owner;

public:
  void Owned( Owner * );
}       


// In the constructor for each, initialise empty lists

Cat::Cat()
{
 owners = 0;
 owner = NULL;
}

Owner::Owner()
{
 cats = 0;
 cat = NULL;
}

// and provide mechanisms for assigning the links

void Owner::Own( Cat *c )
{
 if (c)
 {
   // we ought to search the list and only add if not already present!
   if (!found)
   {
     cat = realloc( ++cats * sizeof( Cat * ) );
     cat[cats-1] = c;
     c->Owned( this );
   };
 };
}

etc etc

You do need to be careful about creating the links in a symmetrical fashion
Please note the code snippet above is based on code I use but rewritten
heavily to fit your example problem and is nowhere near complete - it is
only intended to demonstrate a technique....

Paul
---------------------------------------------------------------------
Paul Dixon                            
Software Engineer, Dept of Paediatrics  Telephone: (+44) 171 725 6258
St Mary's Hospital Medical School       Fax:       (+44) 171 725 6284
Norfolk Place, London W2 1PG, UK        Email:       p DOT dixon AT ic DOT ac DOT uk

A Constituent College of 
                 Imperial College of Science, Technology and Medicine
------------------------------------------------------------------------