C++ with Extern “C” causing Duplicate Symbols Error

Question

I am trying to use a extern "C" function inside my header file for a c++ class.

When I compile I keep getting the error

duplicate symbol _currentInstance in:
main.o
GLHandler.o

I thought I had the right guards but can't seem to figure out why this is happening. Any help would be much appreciated.

Here is the header file.

#ifndef GLHANDLER_H
#define GLHANDLER_H

#include "LoadedObject.h"


#ifdef __cplusplus
extern "C" {
void displayCallback();
}
#endif



class GLHandler {

private:
    LoadedObject *object;

public:
    GLHandler(LoadedObject *);
    void initializeVBO(LoadedObject *);
    void renderObject(struct model *);
    void displayFunction(void);
    model *createModel(void);
    void setupDisplayCallback();


};

GLHandler *currentInstance;

#ifdef __cplusplus
}

#endif

#endif

EDIT: Quickly pointed out by David, the extern GLHandler *currentInstance fixed the error.

Answer 1

This problem has nothing to do with the extern "C" declaration - you're defining a global variable in the header, so it gets defined in each compilation unit:

GLHandler *currentInstance;

In the header, you should instead use:

extern GLHandler *currentInstance;

then in exactly one .cpp file have:

GLHandler *currentInstance;

As a side note, as it stands right now, the header is valid only for C++, since it has a class definition. The #ifdef __cplusplus directives are pointless clutter (though harmless).