Global variables
Global variables are defined outside all
function bodies and are available to all parts of the program (even code in
other files). Global variables are unaffected by scopes and are always available
(i.e., the lifetime of a global variable lasts until the program ends). If the
existence of a global variable in one file is declared using the
extern keyword in another
file, the data is available for use by the second file. Here’s an example
of the use of global variables:
//: C03:Global.cpp
//{L} Global2
// Demonstration of global variables
#include <iostream>
using namespace std;
int globe;
void func();
int main() {
globe = 12;
cout << globe << endl;
func(); // Modifies globe
cout << globe << endl;
} ///:~
Here’s a file that accesses
globe as an extern:
//: C03:Global2.cpp {O}
// Accessing external global variables
extern int globe;
// (The linker resolves the reference)
void func() {
globe = 47;
} ///:~
Storage for the variable globe is
created by the definition in Global.cpp, and that same variable is
accessed by the code in Global2.cpp. Since the code in Global2.cpp
is compiled separately from the code in Global.cpp, the compiler must be
informed that the variable exists elsewhere by the declaration
extern int globe;
When you run the program, you’ll
see that the call to func( ) does indeed affect the single global
instance of globe.
In Global.cpp, you can see the
special comment tag (which is my
own design):
//{L} Global2
This says that to create the final
program, the object file with the name Global2 must be linked in (there
is no extension because the extension names of object files differ from one
system to the next). In Global2.cpp, the first line has another special
comment tag {O}, which says “Don’t try to create an
executable out of this file, it’s being compiled so that it can be linked
into some other executable.” The ExtractCode.cpp program in Volume
2 of this book (downloadable at www.BruceEckel.com) reads these tags and
creates the appropriate makefile so everything compiles properly
(you’ll learn about makefiles at the end of this
chapter).
