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

You are defining my_var once per compilation unit. Remember that include guards operate on a per compilation unit basis.

To remedy, you should declare my_var as extern in the header:

#ifndef _HEAD_H_
#define _HEAD_H_

extern int my_var;

#endif

and define it in one of the source files using

int my_var = 100;

Then the linker sees only one definition and all will be fine.