Question

file a.cc

int a = 0;

file b.cc

#include "a.cc"

file main.cc

#include "b.cc"
extern int a;

int main() {


}

g++ -c a.cc
g++ -c b.cc
g++ main.cc a.o b.o
error: multiple definitions of a

What am I doing wrong here?

Answer 1

You include a .cc (or .cpp) files, which is wrong. Do not do that. You need a header, and in that put the extern int a;:

// a.h
// include guards omitted
extern int a;

// a.cc
#include "a.h"

int a;

// b.cc
#include "a.h"

// main.cc
#include "a.h"

int main(){
  // use a
}