undefined reference when using extern

Question

I have the following setup (hopefully this is not too bare an example):

A.h

typedef std::map<unsigned int, float> MyClass;
extern MyClass inst;

A.cpp

MyClass inst;

B.h

#include <A.h>
void foo();

B.cpp

#include <B.h>
void foo {
    inst.myClassFunc();
}

Now, when I use inst in B.cpp I get undefined reference to inst.

Any idea on how to fix this?

Solution

I know this question is old, but it still might be helpful for someone.

The global variable (here: MyClass inst) should not be extern for the compilation unit which define it (here: A.cpp)

One way to achieve this:

• declare your global variable in a separate header (let's say global.h) and include this header in the *cpp using these.
• remove the extern keyword for the compilation unit which define them (e.g. with #ifdef) :

global.h looks like:

#ifdef A_H_
  #define EXTERN
#else
  #define EXTERN extern
#endif

EXTERN MyClass inst;

while A.h looks like:

#ifndef A_H_
#define A_H_

// your header content (without globals)

#endif /* A_H_ */

and A.cpp:

#include "A.h"
#include "global.h" // after A.h inclusion, we need A_H_ definition

Hope it helps!

Comments

[Mike Seymour]

Or in one step, g++ A.cpp B.cpp.