Multiple including a head file with a variable definition

Question

I just build a simple C++ project. The codes are shown in the follows:

-------- head.h --------

#ifndef _HEAD_H_
#define _HEAD_H_

int my_var = 100;

#en         


        

Answer 1

Change the header the following way

#ifndef _HEAD_H_
#define _HEAD_H_

extern int my_var;

#endif

And for example add line in the module with main

#include "head.h"
int my_var = 100;
int main() { return 0; }

The problem is that as the header is included in two modules then each module contains a variable with external linkage with the same name as a variable in other module. And the linker does not know which variable to use.