Circular references in C++ in different files

Question

If i want a circular reference but in two different files in C++, how would I implement that?

For example

AUnit.h

#inclue <BUnit.h>
class AClass : public TObject
{

   __published
        BClass * B;
};

BUnit.h

#include <AUnit.h>
class BClass : public TObject
{
    __published
        AClass *A;     
};

I can't make it in only one file with forward declarations.

You probably refer to "cyclic reference" rather than "dual reference", and for that you need a forward declaration of one or the other. — David Rodríguez - dribeas, Jun 22 '11 at 17:35
what is a dual reference ? what is the meaning of keyword `__published`? — Naveen, Jun 22 '11 at 17:35
#pragma once (or #ifndef guards) - the topic elaborated on here - http://stackoverflow.com/questions/787533/is-pragma-once-a-safe-include-guard — holtavolt, Jun 22 '11 at 17:36
@holtavolt: That would not solve the problem, which is that the compiler needs to "see" `AClass` before the `BClass` declaration, and vice versa. — Oliver Charlesworth, Jun 22 '11 at 17:38
@Oli: I see - misread the question. Yes, forward declaration needed, as @dribeas mentions. — holtavolt, Jun 22 '11 at 17:39

score 6 · Answer 1 · answered Jun 22 '11 at 17:35

You can use forward declaration in this case too:

// AUnit.h
class BClass;
class AClass : public TObject
{

   __published
        BClass * B;
};

// BUnit.h
#include <AUnit.h>
class BClass : public TObject
{
    __published
        AClass *A;     
};

There is no difference to the scenario if they are both in one file, because #include does nothing but inserting the included file (it is really jut text-replacement). It is exactly the same. After preprocessing of BUnit.h, the above will look like this:

class BClass;

class AClass : public TObject
{

   __published
        BClass * B;
};

class BClass : public TObject
{
    __published
        AClass *A;     
};

There aren't any solutions in two different files? Another method is to declare one of them an object and to use dynamic_cast — user558126, Jun 22 '11 at 17:35
@user558126, you can have forward declarations in a two-file solution. — Carl Norum, Jun 22 '11 at 17:36

score 2 · Accepted Answer · answered Jun 22 '11 at 17:35

I assume you're talking about circular dependencies.

The answer is indeed to use a forward declaration, such as:

AUnit.h

#include <BUnit.h>
class AClass : public TObject
{
   BClass *B;
};

BUnit.h

class AClass;  // Forward declaration

class BClass : public TObject
{
   AClass *A;
};

You could even have a forward declaration in both header files, if you wanted.

Circular references in C++ in different files

2 Answers2

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