Thread: Mutual header inclusion breaks variable definition

Hello there. I have not dabbled much in C++ in a long while but I am feeling like making a small project for fun. In this project I encountered this annoying problem on a secondary function of the application (i.e. not what it's core use is). Maybe I am just too tired and I am not seeing the horrible design or maybe I am missing something obvious :/

I have tried a few changes like moving declarations and definitions around, but it does not seem to go away.

Basically, bar.h uses a class defined in foo.h to instantiate from it and to create a macro using that instance. In turn, that macro will be used in both foo.cpp and main.cpp. I am getting the following error:

build/Debug/GNU-Linux-x86/bar.o:(.bss+0x0): multiple definition of `SOMEVAR'
build/Debug/GNU-Linux-x86/main.o:/home/alexandre/NetBeansProjects/tests/main.cpp:10: first defined here
collect2: ld returned 1 exit statuse

I have tried the simplest code possible (stripping *everything* that is not related to this issue) and get the exact same error so I know this is what is causing me trouble.

Here are the related files:

Code:

/* 
 * File:   foo.h
 * Author: alexandre
 *
 * Created on August 21, 2011, 12:49 AM
 */

#ifndef FOO_H
#define	FOO_H

class Foo {
public:
    void do_something(int t);
};

#endif	/* FOO_H */

Code:

/* 
 * File:   bar.h
 * Author: alexandre
 *
 * Created on August 21, 2011, 12:49 AM
 */

#ifndef BAR_H
#define	BAR_H

#define WHATEVER

#ifdef WHATEVER
    #include "foo.h"

    Foo SOMEVAR;
    #define SOMEMACRO(a) SOMEVAR.do_something(a)
#else // WHATEVER
    #define SOMEMACRO(a) //does nothing
#endif

#endif	/* BAR_H */

Code:

/*
 * File:   foo.cpp
 * Author: alexandre
 *
 * Created on August 21, 2011, 12:49 AM
 */

#include "foo.h"

void Foo::do_something() {
    // whatever
}

Code:

/*
 * File:   bar.cpp
 * Author: alexandre
 *
 * Created on August 21, 2011, 12:49 AM
 */

#include "bar.h"

Code:

/* 
 * File:   main.cpp
 * Author: alexandre
 *
 * Created on August 21, 2011, 12:48 AM
 */

#include "bar.h"

int main(int argc, char** argv) {
    SOMEMACRO("whatever");
    return 0;
}

This exact code generates the error. I am using the latest version of g++ on Netbeans IDE.

Finally, I have tried to add "guards" to try not to instantiate twice the same class. Apparently that does not do anything else different as I get the exact same problem.

Code:

#ifdef WHATEVER
    #ifndef WHATEVER_INIT
    #define WHATEVER_INIT
        #include "foo.h"

        Foo SOMEVAR;
        #define SOMEMACRO(a) SOMEVAR.do_something(a)
    #endif //WHATEVER_INIT
#else // WHATEVER
    #define SOMEMACRO(a) //does nothing
#endif

Sorry for the code dumping but I think the structure here matters a lot to the problem.

Thank you very much.

Thank you for the quick reply. To be honest I am trying to make a simple error logger (Bar in this case) aso that I know when something goes wrong and what goes wrong in my application. I wanted to make it so that if I call the macro it would just do nothing if DEBUG_MODE is not defined. However it seems that it would be difficult to do and maybe I should think of simply encapsulating my IO class (Foo, in this case).