How to Properly Manage Global Variables Across Multiple Source Files?

Managing Global Variables Across Multiple Files

Introduction

Access to common variables between source files is an essential aspect of program design. This article examines the optimal approaches to declare and access global variables within multiple files.

Best Practice

To declare a global variable accessible across multiple files, the following steps should be followed:

1. Header File Declaration: Declare the global variable as extern in a header file that both source files include.

   //common.h
   extern int global;

2. Source File Definition: Define the global variable in only one of the source files. The file that defines the variable must not include the extern keyword.

   //source1.cpp
   #include "common.h"
   int global = 42;

Alternative Approaches

While the recommended approach is to use an extern declaration and a single definition, there are alternative methods that are less desirable:

• Static Declaration: Declaring the global variable as static in the header file would make it internal to each source file, preventing shared access.
• Direct Definition in Both Files: Defining the global variable in both source files would create multiple instances of the variable, which could lead to conflicts.

Example Implementation

Consider the following example:

//source1.cpp
#include "common.h"
int function();
int main() {
  global = 42;
  function();
  return 0;
}

//source2.cpp
#include "common.h"
int function() {
  if (global == 42)
    return 42;
  return 0;
}

By following the best practice of using an extern declaration and a single definition, both source files can share the global variable global effectively.

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