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.

The problem is that you defined a variable in a header. So, if the header is included in multiple translation units (source file + headers), the variable will be defined in each translation unit that it is included, breaking the one definition rule. Header inclusion guards cannot prevent this as they only guard against multiple inclusion within a single translation unit.

Why do you need the macro in the first place?

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).

Finally I decided to create a third class which will be taking in charge matters related to error logging (which may get a little more complex should I work seriously on this project). I had the same problem but remembered the "extern" keyword which solved my problem of multiple definitions.

Code:

//
// system.h
//

#ifndef SYSTEM_H
#define	SYSTEM_H

#define DEBUG_MODE

#ifdef DEBUG_MODE
    #include "iomanagement.h"

    class C_DBG_LOGGER {
    private:
        IOManagement io;
    public:
        C_DBG_LOGGER();

        void Write(const char*);
    };

    extern C_DBG_LOGGER DBG_LOGGER;

    #define DIE(a) DBG_LOGGER.Write(a)
#else
    #define DIE(a) // left empty
#endif // DEBUG_MODE

#endif	// SYSTEM_H

This is very basic and has no features at all but does the job for the moment.

Ah, I was hesitant to suggest extern because global variables should be avoided, but in this case your use of it is legitimate.

Incidentally, instead of DEBUG_MODE, you might consider if checking for the standard macro NDEBUG would be more appropriate.