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

The guard only prevents the header from being included twice in the same compilation unit.

When you compile src1.o and src2.o, each one will contain a definition of my_var. When you link them to create a.out, the compiler cannot merge those definitions (even if they are identicals), and fails.

What you want to do is declare my_var as extern :

---- head.h ----

#ifndef _HEAD_H_
#define _HEAD_H_

extern int my_var;

#endif

---- my_var.c ----

#include "head.h"

int my_var = 100;

Then compile all source files :

g++ -c my_var.cpp -o my_var.o
g++ -c src1.cpp -o scr1.o
g++ -c src2.cpp -o src2.o
g++ -o a.out my_var.o src2.o src1.o

This way, my_var will be declared in every file, but will only be defined in my_var.o. There is an important difference here. Try to omit linking my_var.o, you'll see what the compiler has to say :)