main.cpp

/*
 * Extern.cpp
 *
 *  Created on: Feb 22, 2012
 *      Author: behnam
 */
#include <iostream>
/*http://www.learncpp.com/cpp-tutorial/42-global-variables/
 * Variables declared outside of a block are called global variables. Global
variables have program scope,
 * which means they can be accessed everywhere in the program, and they are only
destroyed when the program ends.

int g_nX; // global variable

int main()
{
    int nY; // local variable nY

    // global vars can be seen everywhere in program
    // so we can change their values here
    g_nX = 5;
} // nY is destroyed here

 */

/*
In order to use a global variable that has been declared in another file, you
have to use a forward declaration or a header file, along with the extern
keyword.


global.cpp:
        // declaration of g_nValue
        int g_nValue = 5;

main.cpp:
        // extern tells the compiler this variable is declared elsewhere
        extern int g_nValue;

        int main()
        {
                g_nValue = 7;
                return 0;
        }


_________________________________________________________________________________________________________________

Here is an example of using a header file extern:

global.cpp:

        // declaration of g_nValue
        int g_nValue = 5;

global.h:
        #ifndef GLOBAL_H // header guards
        #define GLOBAL_H

        // extern tells the compiler this variable is declared elsewhere
        extern int g_nValue;

        #endif

main.cpp:
        #include "global.h"
        int main()
        {
                g_nValue = 7;
                return 0;
        }


 */

using namespace std;

int Externmain()
// in main()
{
  return 0;
}