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How to access public members of the class containing the object

Posted on 2008-06-13
10
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Last Modified: 2013-11-12
Hi experts,

Is it possible in C++ to access public members of the class containing the object?

For example, in this way it is impossible:

class AClass
{
public:
      BClass m_BClass; // impossible!
      m_BClass->m_AClass = this;
      
      int test;
}

class BClass
{
public:
      AClass* m_ AClass;
      void Test();
}

void BClass::Test()
{
      int test = m_ AClass->test;
}

I am asking that because in Java it is possible, since there classes are grouped in packages...
0
Comment
Question by:pamir
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10 Comments
 
LVL 40

Accepted Solution

by:
evilrix earned 125 total points
ID: 21776981
You need to (a) reorder a little and (b) forward declare.
http://www.devx.com/tips/Tip/12583
class AClass;
 
class BClass
{
public:
	AClass* m_AClass;
	void Test();
};
 
class AClass
{
public:
	BClass m_BClass; // impossible!
 
	AClass()
	{
		m_BClass.m_AClass = this;
	}
 
	int test;
};
 
void BClass::Test()
{
	int test = m_AClass->test;
}

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0
 
LVL 40

Expert Comment

by:evilrix
ID: 21776987
^^^Oops, I forgot to remove the "// impossible!" from the code I reordered -- ignore it, it is possible :)
0
 
LVL 11

Expert Comment

by:DeepuAbrahamK
ID: 21777345
Class is blue print of an object,You need to write this in a public fuction
<return type> <some function name>()
{
m_BClass.m_AClass = this;
}

or in a constuctor.
0
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Author Comment

by:pamir
ID: 21778086
Thanks evilrix,
I see that line 1 in your code is doing the trick.

Howerver, in my real situation, I am using header files and I am receiving an error:
BClass.cpp(14) : error C2027: use of undefined type 'AClass'

So I am wondering what is wrong here?

The code is:

AClass.h:
=======================================================================
#pragma once
#include ".\bclass.h"

class AClass
{
public:
      AClass(void);
      ~AClass(void);

      BClass m_BClass;
      int test;
};


AClass.cpp:
=======================================================================
#include "StdAfx.h"
#include ".\aclass.h"

AClass::AClass(void)
{
      m_BClass.m_AClass = this;
}

AClass::~AClass(void)
{
}


BClass.h:
=======================================================================
#pragma once

class AClass;

class BClass
{
public:
      BClass(void);
      ~BClass(void);

      AClass* m_AClass;
      int test;

      void Test();
};


BClass.cpp:
=======================================================================
#include "StdAfx.h"
#include ".\bclass.h"

BClass::BClass(void)
{
}

BClass::~BClass(void)
{
}

void BClass::Test()
{
      test = m_AClass->test;
}
0
 
LVL 40

Expert Comment

by:evilrix
ID: 21778120
You need to include aclass.h in bclass.cpp

The forward declaration only allows you to define a pointer or reference, to use it the compiler must see the full class definition.

BTW (off topic): Don't prefix  .\ to file names when including headers, it's not portable. Since the headers are in the same folder as the .cpp files just use "aclass.h". If you do need to use a path you should use / and not \ as the / char is portable on both Windows and Linux where as \ is a Windows only seperator.
0
 

Author Comment

by:pamir
ID: 21778122
P.S. the error is in the line
 test = m_AClass->test;
0
 
LVL 40

Expert Comment

by:evilrix
ID: 21778133
^^^ oh, and "#pragma once" isn't portable either, prefer to use include guards.
http://en.wikipedia.org/wiki/Include_guard
0
 
LVL 40

Expert Comment

by:evilrix
ID: 21778146
>> You need to include aclass.h in bclass.cpp
See below.

AClass.h:
=======================================================================
#pragma once
#include ".\bclass.h"
 
class AClass
{
public:
      AClass(void);
      ~AClass(void);
 
      BClass m_BClass;
      int test;
};
 
 
AClass.cpp:
=======================================================================
#include "StdAfx.h"
#include ".\aclass.h"
 
AClass::AClass(void)
{
      m_BClass.m_AClass = this;
}
 
AClass::~AClass(void)
{
}
 
 
BClass.h:
=======================================================================
#pragma once
 
class AClass;
 
class BClass
{
public:
      BClass(void);
      ~BClass(void);
 
      AClass* m_AClass;
      int test;
 
      void Test();
};
 
 
BClass.cpp:
=======================================================================
#include "StdAfx.h"
#include ".\bclass.h"
#include "abclass.h" // <--- LOOK !!!
 
BClass::BClass(void)
{
}
 
BClass::~BClass(void)
{
}
 
void BClass::Test()
{
      test = m_AClass->test;
}

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0
 

Author Comment

by:pamir
ID: 21778246
Yes, it works, the forward declarations are very useful, thanks a lot!

OK, I have read the include guards article, and also the article for #pragma once http://en.wikipedia.org/wiki/Pragma_once - but it seems that #pragma once does the work with less code?...
0
 
LVL 40

Expert Comment

by:evilrix
ID: 21778266
>> #pragma once does the work with less code?...
It does but it is NOT part of the C/C++ standard, it is a Microsoft only extension. As long as you know this and are happy to accept your code will not be portable then use it as your wish but if you need to write code to compile on other platforms then you cannot use it.

It was just an advisory :)
0

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