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"recursive" class reference (Solved)

Started by The C modest god Jul 14, 2005 at 5:01 AM 12 replies 1.5k views

Original Post

The C modest god

112

The C modest god

Jul 14, 2005 at 5:01 AM

Consider the following code:


class B

class A {
    public:
         void do(B &);
};

class B {
    public:
         void do(A &);
};

This code will compile. However, why it is impossible to put these two classes in two different .h files? [Edited by - The C modest god on July 14, 2005 12:56:55 PM]

Helter Skelter

Jul 14, 2005 at 5:13 AM

Quote:
Original post by The C modest god
This code will compile. However, why it is impossible to put these two classes in two different .h files?

It's not:

// file: A.hclass Bclass A {    public:         void do(B &);};// file: B.hclass A;class B {    public:         void do(A &);};

However before either class can access an object of the other type the full class declaration must be visible.

// file: B.cpp#include "A.h"#include "B.h"void B::do(A &a){    a.do(*this);}// File A.cpp#include "A.h"#include "B.h"void A::do(B &b){    printf("%p\n", &b);}

nonpop

Jul 14, 2005 at 5:17 AM

Quote:
Original post by The C modest god
This code will compile.

Actually it won't, because you're using a reserved word (do) as a function name and you forgot a semicolon ;)

Quote:
However, why it is impossible to put these two classes in two different .h files?

It isn't. I tried and it worked:

// classa.h:class B;class A {public:	void dostuff (B& b) {}};// classb.h:class A;	// You didn't have this, and it isn't		// required, if classa.h is included firstclass B{public:	void dostuff (A& a) {}};// main.cc:#include "classa.h"#include "classb.h"int main (){	A a;	B b;	a.dostuff (b);	b.dostuff (a);}

EDIT: Too late... :P

The C modest god

Author

112

The C modest god

Jul 14, 2005 at 8:48 AM

I have a problem with recursive headers

#ifndef ERRORHANDLE#define ERRORHANDLE#include "DataLists.h"using namespace std;class ErrorMessage {	public:		char * Message;		HRESULT Code;		ErrorMessage ();		ErrorMessage (const ErrorMessage & ErrMsg);		~ErrorMessage ();};template <class T> class LinkedList;class ErrorList {	public:		char * GetError (HRESULT Code);		void InsertError (HRESULT Code, char * Message);	private:		LinkedList<ErrorMessage> List;};

I get the following error:
error C2079: 'List' uses undefined class 'LinkedList<class ErrorMessage>'

SiCrane

Mod

473

SiCrane

Jul 14, 2005 at 8:53 AM

Does this article answer your question?

The C modest god

Author

112

The C modest god

Jul 14, 2005 at 9:39 AM

No it really doesnt help.
My problem is more severe.
As you can see in the code I posted I included the relevant header file and declared about the class I am using.
It doesnt help.
Is there a solution for such a problem?

Edit: When I turn list into a pointer to that class it works, but I dont want a pointer.

Nitage

125

Nitage

Jul 14, 2005 at 9:50 AM

You need to include the header which LinkedList is defined in.

Telastyn

268

Telastyn

Jul 14, 2005 at 9:56 AM

Quote:
Original post by The C modest god

Is there a solution for such a problem?

Yeah, put them in the same header. Seriously, if the two classes are that intertwined, you can never include one without the other anyways.

The C modest god

Author

112

The C modest god

Jul 14, 2005 at 9:57 AM

I did.
Its datalists.h

The C modest god

Author

112

The C modest god

Jul 14, 2005 at 10:06 AM

I tried to do the following:

// ErrorHandle.h#ifndef ERRORHANDLE#define ERRORHANDLE#include "DataLists.h"using namespace std;class ErrorMessage {	public:		char * Message;		HRESULT Code;		ErrorMessage ();		ErrorMessage (const ErrorMessage & ErrMsg);		~ErrorMessage ();};//template <class T> class LinkedList;class LinkedList_ErrorMessage; // <= Error!class ErrorList {	public:		char * GetError (HRESULT Code);		void InsertError (HRESULT Code, char * Message);	private:		LinkedList_ErrorMessage List;}// DataLists.h#ifndef DATALISTS#define	DATALISTS#include "ErrorHandle.h"template <class T>struct Linked {	Linked<T> * pNext;	T Data;};template <class T> class LinkedListPointer;template <class T>class LinkedList {	public:		LinkedListPointer<T> CreateLinkedListPointer();		BOOL IsEmpty();		T & Peek();		void Pop();		void Push (const T & Data);		LinkedList();		~LinkedList();	private:		void LocalFree();		Linked<T> * pList;};class ErrorMessage;typedef class LinkedList<ErrorMessage> LinkedList_ErrorMessage;

On "class LinkedList_ErrorMessage; " I get the following error:
error C2371: 'LinkedList_ErrorMessage' : redefinition; different basic types

Why am I getting such an error?
Why it thinks I redfine the class? is that a bug of the compiler?

The C modest god

Author

112

The C modest god

Jul 14, 2005 at 10:09 AM

Quote:
Original post by Telastyn
Quote:
Original post by The C modest god

Is there a solution for such a problem?

Yeah, put them in the same header. Seriously, if the two classes are that intertwined, you can never include one without the other anyways.

These are two generic class that needs to use each other. One is for handling errors, the other is a basic data type.
Logically they should be seperated, eventhough they need to use each other.

Besides, if something can be done in one file, but it cant be done in two seperate files. That means that the support for writting programs in seperate files suck big time.

Telastyn

268

Telastyn

Jul 14, 2005 at 10:17 AM

Quote:
Original post by The C modest god
Quote:
Original post by Telastyn
Quote:
Original post by The C modest god

Is there a solution for such a problem?

Yeah, put them in the same header. Seriously, if the two classes are that intertwined, you can never include one without the other anyways.

These are two generic class that needs to use each other. One is for handling errors, the other is a basic data type.
Logically they should be seperated, eventhough they need to use each other.

Besides, if something can be done in one file, but it cant be done in two seperate files. That means that the support for writting programs in seperate files suck big time.

Indeed, logically they should be seperated. But the dependancies/design you describe means they're completely inseperable.

Anyways, your problem is perhaps that you're trying to pre-declare LinkedList_ErrorMessage as a class, when it's not a class, it's a template <class T> class. I'm not sure of the correct syntax for something like that.

The C modest god

Author

112

The C modest god

Jul 14, 2005 at 1:00 PM

I have solved the problem.
The header file with the stream list was a template only. And it only used a class from the Error file in its code implementation of the methods. Not in the declerations of the classes in the stream list.
So I moved the include of the error after all the classes of the stream list but before the implementation of its methods. It worked.

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