Why can't i define my previously declared extern variable inside a function?

I’m quite new to programming in general and more specifically to c++. I’ve made a program using the following files:

my.h

extern int foo;
void print_foo();

my.cpp

#include <iostream>
#include "my.h"


void print_foo(){
    std::cout << "foo = " << foo <<std::endl;

}

use.cpp

#include "my.h"

int main(){
    int foo = 7;
    print_foo();
}

When i try to compile it I get the error message ‘undefined reference to `foo”, but when i define foo outside of my main() function like below, it works just fine. Why is that?

use.cpp

#include "my.h"

int foo;

int main(){
    foo = 7;
    print_foo();
}

>Solution :

When i try to compile it I get the error message ‘undefined reference to `foo”

Because when you define foo inside main, it is local to the main function. But the foo that you use inside print_foo is a global foo which you’ve not defined(globally).

Basically, extern int foo;(in your program) declares a global variable named foo and the foo used inside print_foo is the globally declared foo which you never define.