Q136005: HOW TO: Declare Classes That Refer to Each Other

Article: Q136005
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,2.2,4.0,4.1,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbVC100 kbVC151 kbVC152 kbVC200 kbVC210 kbVC220 kbVC400 kbVC410 kbVC500 kbVC600
Last Modified: 13-DEC-2001

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The information in this article applies to:

- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52 
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 2.2, 4.0, 4.1 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SUMMARY
=======

In C++, you may need to have two classes that contain data members that refer to
the other class as in this example:

  class B
  {
   A * x;
  };

  class A
  {
   B * x;
  };

This article shows by example how to declare two classes that contain pointers to
the other class. The two classes (A and B) are derived from other classes (C and
D) to better demonstrate this issue.

If the two classes are not declared correctly, errors such as the following can
occur:

  test1.h(8) : error C2501: 'B' : missing decl-specifiers
  test1.h(8) : error C2143: syntax error : missing ';' before '*'
  test1.h(8) : error C2501: 'x' : missing decl-specifiers

Sample Code
-----------

  /* Compile options needed: none
  */ 

  //================================
  // test1.h
  //================================

  #ifndef _a_
  #define _a_

  class B;

  class A : public D
  {
   B * x;
  };

  #endif

  //================================
  // test2.h
  //================================

  #ifndef _b_
  #define _b_

  class A;

  class B : public C
  {
   A * y;
  };

  #endif

  //=================================
  // test.cpp
  //=================================

  class C
  {
   int q;
   };

  class D
  {
   int p;
  };

  #include "test1.h"
  #include "test2.h"

  void main(void)
  {
  }

Additional query words: 8.00 8.00c 9.00 9.10

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Version           : :1.0,1.5,1.51,1.52,2.0,2.1,2.2,4.0,4.1,5.0,6.0
Issue type        : kbhowto

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