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Thinking in C++
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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 reads these tags and creates the appropriate makefile so everything compiles properly (you’ll learn about makefiles at the end of this chapter).

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire