LinuxQuestions.org - C++ Compilation Error

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- - C++ Compilation Error (https://www.linuxquestions.org/questions/programming-9/c-compilation-error-575338/)

Well, I've managed to strip it to just a few files. I'm posting the code below.

File: reliableudp.h

Code:

#ifndef RELIABLEUDP_H_

#define RELIABLEUDP_H_





// Forward declarations to pacify the compiler :)

namespace ReliableUDP

{

        class BaseSocket;

        class Socket;

        class ServerSocket;

}





#include "BaseSocket.h"

#include "Socket.h"

#include "ServerSocket.h"





#endif /*RELIABLEUDP_H_*/

File: BaseSocket.h

Code:

#ifndef BASESOCKET_H_

#define BASESOCKET_H_



#include "reliableudp.h"

#include <string>



using std::string;



namespace ReliableUDP

{

        class BaseSocket

        {

        };

}

                

#endif /*BASESOCKET_H_*/

File: ServerSocket.h

Code:

#ifndef SERVERSOCKET_H_

#define SERVERSOCKET_H_



#include "reliableudp.h"

#include <string>





namespace ReliableUDP

{



        class ServerSocket : public BaseSocket

        {

        };



}



#endif /*SERVERSOCKET_H_*/

File: Socket.h

Code:

#ifndef SOCKET_H_

#define SOCKET_H_



#include "reliableudp.h"

#include <string>



namespace ReliableUDP

{



        class Socket : public BaseSocket

        {

        };



}



#endif /*PEER_H_*/

The above is a complete program. I'm compiling it as a shared library. As you can see, it's just a skeleton program. But I still get the error.

I forgot to mention, I'm using the cmake utility. The CMakeLists.txt file is written as follows:

Code:

SET(RUDP_SRC

BaseSocket.cpp

ServerSocket.cpp

Socket.cpp

)



ADD_LIBRARY(rudp SHARED ${RUDP_SRC})

The corresponding .cpp files are empty. They do not contain anything.

guards are the combination #ifndef with #define and #endif which wraps around the definition in the header files. Hence BaseClass.h would be:

Code:

#ifndef BASE_CLASS

#define BASE_CLASS



#include <iostream>



using namespace std;



class BaseClass

{

  protected:

      int number;

  public:

      BaseClass(int num = 1){number = num;}

      int num () {return number;}

};



#endif

The identifier (for example BASE_CLASS) needs to be unique for every header file that you have.

Well, I tried changing the guard, just to be sure it wasn't conflicting with any other. The problem persists.

whatever i think is that you have saved your code in .c file if its so then change it to cpp. its just a guess from my side.

Nopes. All the code is in .cpp files.

On the little that you have given, you have had quite a bit of help. It's probably up to you now to help yourself and post some code...

But I already have posted some code. Scroll up and look for yourself. It fulfills all of your requirements. It's complete and concise. What more is required?

An update. On a fluke, I commented the line #include "reliableudp.h" in BaseSocket.h. The error disappeared. Unfortunately, I can't do this in the main project because it introduces a lot of dependency issues. Also, the same thing is done in all of the other files, but it doesn't give an error there. I do not understand what the problem is?

Quote:

Originally Posted by kamransoomro84 (Post 2862882)

But I already have posted some code. Scroll up and look for yourself.

I'm sorry I completely missed it... that'll teach me to jump to the next unread!

I'm not certain why you are using the forward declarations. I'm sure that it your problem. if you do need them then put them in the file that requires it, that is if ServerSocket also needs to know about Socket then put it in ServerSocket.h

Hi,

I've had a proper look at it now and managed to duplicate your problem. In C++ you can't have a forward declaration of a base class, that is the following will fail.

Code:

#ifndef SOCKET_H_

#define SOCKET_H_



class BaseSocket;



namespace ReliableUDP

{



        class Socket : public BaseSocket

        {

        };



}



#endif /*SOCKET_H_ */

Instead what you must have is:

Code:

#ifndef SOCKET_H_

#define SOCKET_H_



#include "BaseSocket.h"



namespace ReliableUDP

{



        class Socket : public BaseSocket

        {

        };



}



#endif /*SOCKET_H_ */

Where the file BaseSocket.h defines the class BaseSocket.

To recreate the problem I had the line #include "BaseSocket.h" in the file BaseSocket.cpp

Now what is happening with your code is as follows:

BaseSocket.cpp includes BaseSocket.h
BaseSocket.h includes reliableudp.h
reliableudp.h has forward declarations and then includes BaseSocket.h
The guards of BaseSocket.h stop the class BaseSocket from defined at this moment, and control is returned to reliableudp.h
reliableudp.h now includes Socket.h
Socket.h includes reliableudp.h
The guards of reliableudp.h stop further processing and control is returned to Socket.h
Socket.h attempts to define class Socket but it doesn't have a definition of BaseSocket hence the error.

What to do?

Replacing the line #include "BaseSocket.h" with #include "reliableudp.h" in the file BaseSocket.cpp will possibly fix it
Removing the file reliableudp.h and including the files that are needed when they are needed will fix it.

Personally I'd go with the second but you may find that the first approach works for you.

hello i'm no expert by i think they are refering to your guard

Code:

#ifndef BASE_CLASS_HPP

#define BASE_CLASS_HPP

#endif

i found out the hard way about guards, if you're using more than one header file you will need to include them, if you already have then i'm sorry for wasting your time :)
hth
paul