Example

/* File one.cpp */#include "two.h"#include <iostream>int a = 0;int main() {test();// Prints 22std::cout << a << std::endl;return 0;}/* File two.h */void test(void);/* File two.cpp */// Says the a we use in this file is the same one as in one.cppextern int a;void test(void) {a = 22;}