C++: Share variables with Multiple files

I'm trying to share global variables within multiple .cpp files. I had used extern keyword, but it doesn't meet what I want.

At the first, I have had four files as:

***** common.hpp *****

#ifndef COMMON_HPP
#define COMMON_HPP
#include <iostream>
void printVar();
void printVarfromA();
void setVar(char *str);
#endif

***** Base.cpp *****

#include "common.hpp"
extern char *var;
void printVar(){
    std::cout << var << std::endl;
}
void setVar(char *str){
    var = str;
}

***** A.cpp *****

#include "common.hpp"
char *var = (char*)"This is var from A";
void printVarfromA(){
    printVar();
}

***** main.cpp *****

#include "common.hpp"
int main(){
    printVarfromA();
    setVar((char*)"Var was Changed.");
    printVarfromA();
    return 0;
}

All thing goes fine, and the result was:

This is var from A
Var was Changed.

as I expected.

The problem arose when I have added a new file that shares the var variable, let's say B.cpp, where it contains a single line now:

***** B.cpp *****

char *var;

At this moment ld returned 1 exit status ERROR appeared.

I have tried several methods and searched a lot, but I couldn't get any solution.

My question. How to share variable within multiple files in C++?