Question

Possible Duplicate:
Why does this not prevent multiple function declarations?

Global.h

#ifndef Global_h
#define Global_h

#include <iostream>

unsigned char exitStatus;

#endif

OutputHandler.h

#ifndef OutputHandler_h
#define OutputHandler_h

#include "Global.h"

class OutputHandler {
private:
    static bool instanceExists;
// more code  

#endif

Root.h

#ifndef Root_h
#define Root_h

// declarations

OutputHandler *output;

#endif

Root.cpp

#include "Root.h"
// gets instance of OutputHandler
// more code

I am getting errors regarding exitStatus, static bool instanceExists, and static class output being already defined by Root.obj in OutputHandler.obj. I assume the issue is with including the header file OutputHandler.h in both Root.h and OutputHandler.cpp. Anyone know how to fix this or how to better organize header files?

Answer 1

Because include guards only work at the translation unit level (you can, for this simple case, consider a single C file to be a translation unit).

That means a single C file, if it includes the header file twice, will not process it the second time due to the include guards.

However, if you include the header from two different C files, each of them will get a copy of the variables defined in that header.

Then, when you link them together, you get the duplicates.

The easiest way to get around this problem is to never define things in headers, only declare them.

So, in the header (eg, xyzzy.h), you have:

extern int xyzzy;  // declare but don't define.

and in all the C files that want to use that, put:

$include "xyzzy.h"

and, in one of those C files, also put:

int xyzzy;        // define it here, once.

You can think of declaration as a simple “I declare that this exists somewhere, just not here”, while definition is “I an creating this here and now”.