[Moved][Solved] Compiler header guard problem

Jake007

Hi!

I'm having problem compiling my project. It seems that header guards aren't working as they should ( or I messed up something).
I've a header file globals.h, for global variables. I'm including it in several other files.

When compiling, I get errors, that variables were already declared. The number of errors is the same as the number of times I included globals.h.

Has someone ever experienced similar, or knows how to solve this problem?

Regards,
Jake

lgeyer

Well, how do your guards look like?
@
#ifndef GLOBALS_H
#define GLOBALS_H
...
#endif // GLOBALS_H
@

<code>#pragma once</code> is a non-standard include guard and may or may not work depending on your compiler.

Jake007

My header guards are exactly like yours:

@
#ifndef GLOBALS_H
#define GLOBALS_H
...
#endif // GLOBAL_H
@

My global variables are const qreal pi, QList, one enum definition and one struct definition.

#pragma once did not work.

lgeyer

Can you provide a small, compilable (our in your case non-compilable) example to reproduce your problem?

Jake007

I uploaded the example on 2shared:
http://www.2shared.com/file/ZVJnyK7f/Example.html?

lgeyer

Is there a reason you include <code>Globals.h</code> within <code>Globals.h</code>?
@
#ifndef GLOBALS_H
#define GLOBALS_H

#include <QList>
#include <QString>

#include "Globals.h" // <---

struct SVar
{
QString name;
QString value;
};

QList<SVar *> varList;

#endif // GLOBALS_H
@

Jake007

No.
Look like I accidentally added it in there to.
But I'm getting the same result even if I remove it.

And also, if I know correctly, header guards should prevent problems with including file in itself and compiling file again if included in several different places.

EDIT:
I'm also open for other ideas how could I implement few variables visible everywhere. I've tried static class, but I have a problem calling QList variable out. Passing pointers unfortunately isn't an option.
But it also bugs me, why header guards aren't working.

goetz

I moved this to the C++ Gurus forum, as it's nothing Qt related.

You don't run into a header guard issue, those work as expected. And you don't get a compiler error that a variable is already declared, but the linker errors that a symbol is defined twice.

This is caused by this line in your header file:

@
QList<SVar *> varList;
@

As the header is finally included in two .cpp files (via their "own" header files), the compiler generates two objects of the same name. As you did not put it into a namespace or the like they both reside as a global variable - and voilà: a name clash.

Fortunately it's easy to circumvent this:

Change Globals.h to

@
extern QList<SVar *> varList;
@

and add to one and only one source file (e.g. main.cpp) this line:

@
QList<SVar *> varList;
@

The change to the header file makes the variable just a declaration, so that the compiler just knows that there is a global variable of the name varList somewhere. And as you acutally need a physical incarnation of the variable, you will have to add it to some source file. Which source file doesn't matter, but main.cpp is a good candidate for such things.