How to access a variable in mumtiple cpp files contain main section in each file? - global

Question

I want to access a variable in multiple .cpp files. I looked into several resources. I could not solve it though. I am using cmake to build all the codes in this project. Following is an example that exactly matches with my problem. Basically, I want val to print 42 in both code1.cpp and code2.cpp. When, I build these three files it complains: undefined reference to 'he::val' collect2: error:ld returned 1 exit status for both the .cpp files.
header1.h
#ifndef HEADER1_H
#define HEADER1_H
#include <iostream>
namespace he {
extern int val;
}
#endif // HEADER1_H
code1.cpp
#include "header1.h"
#include <iostream>
using namespace he;
int func()
{
std::cout << val << std::endl;
}
int main()
{
val=20;
func();
return 0;
}
code2.cpp
#include <iostream>
#include "header1.h"
using namespace he;
int main()
{
std::cout << val << std::endl;
}

Answer 1

extern int val;
Is a variable declaration. You need, somewhere (in one of your cpp files), to define the variable:
int val;
Actually, since you have two main() functions, these are two separate programs. Then your variable definition needs to be in both, like this:
namespace he {
int val;
}
But your extern declaration in the header file makes your variable global, which is usually frowned upon. It all depends, of course on what your purpose is.
Also, since two main() functions mean two separate programs, there is no variable sharing.