LNK2005: " already defined error

Question

I am trying to use a global variable from separated .cpp files. I have got an init.h file as:

//init.h
#ifndef init
#define init
int a = 3;
#endif

I have got an init.cpp file as: //init.cpp #include init.h

Then finally my main.cpp file is:

//main.cpp
#include "init.h"
int main(void)
{
    while(1)
    {
    }
}

After this, I get the error:

1>init.obj : error LNK2005: "int a" (?a@@3HA) already defined in main.obj
1> ..deneme.exe : fatal error LNK1169: one or more multiply defined symbols found

Why my #infdef control does not solve this problem?. I also tried using #pragma once but I got same error. What is wrong with my code?

Answer 1

You need to mark your variable as extern and define it only once in an implementation file.

As the code is now, you're breaking the one definition rule. The include guards don't help in this case, since all translation units that include that header re-define the variable.

What you actually need:

//init.h
#ifndef init
#define init
extern int a;
#endif

and the definition:

//init.cpp
#include "init.h"
int a = 3;

Also, think twice before using globals. What is it that you're actually trying to achieve?