HOWTO: Declare Classes That Refer to Each Other

ID: Q136005

The information in this article applies to:

The Microsoft C/C++ Compiler (CL.EXE)
- 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, 5.0, 6.0

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

MORE INFORMATION

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


Keywords          : kbcode kbVC100 kbVC151 kbVC152 kbVC200 kbVC210 kbVC220 kbVC400 kbVC410 kbVC500 kbVC600 
Version           : WIN3X:1.0,1.5,1.51,1.52;WINNT:1.0,2.0,2.1,2.2,4.0,4.1,5.0
Platform          : NT WINDOWS 
Issue type        : kbhowto

Last Reviewed: June 30, 1999