circular reference in parent/child classes

circular reference in parent/child classes

• David Seymour

  I didn't believe it until I tried it, but you are absolutely correct. Tell your boss that I said it was okay for you to take the rest of the day off. Here is the code which compiles just fine::
  
  ============== main.cpp ===============
  
  #include "stdafx.h"
  #include "bravo.h"
  #include "alpha.h"
  
  int _tmain(int argc, _TCHAR* argv[])
  {
  alpha aaa;
  return 0;
  }
  
  =============== alpha.h ================
  
  class alpha
  {
  public:
  int a;
  bravo *my_child; // variable
  alpha(); // constructor
  };
  
  =============== alpha.cpp ==============
  
  #include "stdafx.h"
  #include "bravo.h"
  #include "alpha.h"
  
  alpha::alpha()
  {
  // bravo.h needed because of call to "new"
  bravo *my_child = new bravo(this);
  a = 0;
  }
  
  =============== bravo.h =================
  
  class alpha;
  
  class bravo
  {
  public:
  int b;
  alpha *my_parent; // variable
  bravo(alpha *); // constructor
  };
  
  =============== bravo.cpp ===============
  
  #include "stdafx.h"
  #include "bravo.h"
  
  bravo::bravo(alpha* a_ptr)
  {
  my_parent = a_ptr;
  b = 0;
  }
  
  
  

• Moses Bunting

  Hi Harold. I think I have the solution (or at least "a solution").

  My first problem was that I was not using "incomplete class declarations". Amazing stuff, that.

  My second problem was that I was expecting too much from the "incomplete class declaration". Apparently, the "incomplete class declaration" will allow you to declare pointers to the incomplete class (the compiler knows exactly how many bytes are in a pointer). But, you can not declare an instance of the incomplete class, since there is not enough info in the "incomplete class declaration" for the compiler to know (as of yet) how many bytes the class will require.

  I'll include the final code that compiles.

  ============== main.cpp ===============

  // main.cpp : Defines the entry point for the console application.
  //

  #include "stdafx.h"
  #include "alpha.h"
  #include "bravo.h"

  int _tmain(int argc, _TCHAR* argv[])
  {

  alpha a;
  bravo b(&a);
  a.my_child = &b;
  return 0;
  }

  =============== alpha.h ================

  class bravo; // incomplete class declaration

  class alpha
  {
  public:
  int a;
  bravo *my_child;
  alpha();
  };

  =============== alpha.cpp ==============

  #include "stdafx.h"
  #include "alpha.h"

  alpha::alpha()
  {
  my_child = NULL;
  a = 0;
  }

  =============== bravo.h =================

  class alpha; // incomplete class declaration

  class bravo
  {
  public:
  int b;
  alpha *my_parent;
  bravo(alpha *);
  };

  =============== bravo.cpp ===============

  #include "stdafx.h"
  #include "bravo.h"

  bravo::bravo(alpha* a_ptr)
  {
  my_parent = a_ptr;
  b = 0;
  }

  
  
  

• TGhostH

  Try placing a default constructor (one with no parameters) in the bravo class.
  
  

• Mark TK

  You don't have to move a part of your code to the main() funtion to avoid this error -
  'bravo' : class has no constructors
  All you need to do is to include bravo.h in alpha.cpp. You need complete definition of brave class to create its instance (line 6 in alpha.cpp)
  

  
  

• LiquidLithium

  Hi Harold. I think you solved most of the problem with your suggestion. I was not familiar with the technique of "incomplete class declarations". Most of the errors went away. bravo.cpp compiled fine, but alpha.cpp got this error:

  Compiling...
  bravo.cpp
  alpha.cpp
  Projects\test\main\alpha.cpp(6) : error C2514: 'bravo' : class has no constructors
  Projects\test\main\alpha.h(1) : see declaration of 'bravo'

  I don't really understand what this is trying to tell me. The bravo class most certainly does have one (and only one) constructor. I'll include an updated version of the alpha and bravo files below. Thanks very much for your help.

  ============ main.cpp ===========
  
  #include "stdafx.h"
  int _tmain(int argc, _TCHAR* argv[])
  {
  return 0;
  }
  
  ============ alpha.h ============
  
  class bravo;

  class alpha
  {
  public:
  int a;
  bravo *my_child;
  alpha();
  };
  
  ============ alpha.cpp ===========
  
  #include "stdafx.h"
  #include "alpha.h"
  
  alpha::alpha()
  {
  bravo *my_child = new bravo(this);
  a = 0;
  }
  
  ============ bravo.h =============
  
  class alpha;

  class bravo
  {
  public:
  int b;
  alpha *my_parent;
  bravo(alpha *);
  };
  
  ============ bravo.cpp ===========
  
  #include "stdafx.h"
  #include "bravo.h"
  
  bravo::bravo(alpha* a_ptr)
  {
  my_parent = a_ptr;
  b = 0;
  }
  

  
  
  

• Susan2

  Hi,

  Just place the following at the top of every header file

  class bravo; (in the alpha header file)

  class alpha; (in the bravo header file)

  also remove the cross include calls for alpha.h and bravo.h

  It should compile!

  
  
  
• circular reference in parent/child classes