global reference

// ------- main.cpp --------#include <iostream>#include "extern.h"int main() {  std::cout << a;  func();  std::cout << a;}// ------- extern.h --------#ifndef EXTERN_H#define EXTERN_Hextern int a;void func();#endif// ------- extern.cpp -------#include "extern.h"int a = 0;void func() {  ++a;}

// ------- main.cpp --------#include <iostream>#include "screen.h"void main(void){    // Set the dimentions.    int dimention;    std::cout << "enter width: ";    std::cin >> dimention;    std::cout << endl;    set_width(dimention);    std::cout << "enter height: ";    std::cin >> dimention;    std::cout << endl;    set_height(dimention);    // Echo the dimentions.    std::cout << "Width: " << get_width();    std::cout << "Height: " << get_height();}// ------- screen.h --------#ifndef SCREEN_H#define SCREEN_Hint get_width(void);int get_height(void);void set_width(int width);void set_height(int height);#endif// ------- screen.cpp -------#include "screen.h"static int screen_width = 0;static int screen_height = 0;int get_width(void){    return screen_width;}int get_height(void){    return screen_width;}void set_width(int width){    screen_width = width;}void set_height(int height){    screen_height = height;}