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

your problem boils down to understanding C++ storage classes; static and extern more precisely.

extern says: this variable is defined in a different file, this 
             is only a declaration
static says: this variable's lifetime is the program lifetime and
             restrict it's visibility to the current file. 



What happens in your case:
   - each file includes the variable declaration and definition `int my_var = 100;`,
     so both object scr1.o and scr2.o will contain the variable definition. When
     linking them, the linker cannot solve the ambiguity and throws an error.
   - adding the extern qualifier and defining the variable in one of the files 
     solves this problem: in one source it'll be an external unresolved at compile
     time, symbol and the other will contain the actual symbol.

As for the second part of your question; why adding static seems to help the linker. Look at what I just said about static storage class: basically each object will have their own copy of the variable - not the same one!