How to Declare a Global Variable in C++

1. Declare a Global Variable in a Single Source File in C++
2. Declare a Global Variable in Multiple Source Files in C++

This article will explain several methods of how to declare a global variable in C++.

Declare a Global Variable in a Single Source File in C++

We can declare a global variable with the statement that is placed outside of every function. In this example, we assume the int type variable and initialize it to an arbitrary 123 value. The global variable can be accessed from the scope of the main function as well as any inner construct (loop or if statement) inside it. Modifications to the global_var are also visible to each part of the main routine, as demonstrated in the following example.

#include <iostream>

using std::cout;
using std::endl;

int global_var = 123;

int main() {
  global_var += 1;
  cout << global_var << endl;

  for (int i = 0; i < 4; ++i) {
    global_var += i;
    cout << global_var << " ";
  }
  cout << endl;

  return EXIT_SUCCESS;
}

Output:

124
124 125 127 130

On the other hand, if additional functions are also defined in the same source file, they can directly access the global_var value and modify it in the function scope. The global variables can also be declared with a const specifier, which forces them to only be accessible through the current translation unit (source file with all included headers).

#include <iostream>

using std::cout;
using std::endl;

int global_var = 123;

void tmpFunc() {
  global_var += 1;
  cout << global_var << endl;
}

int main() {
  tmpFunc();

  return EXIT_SUCCESS;
}

Output:

124

Declare a Global Variable in Multiple Source Files in C++

Alternatively, there may be global variables declared in different source files, and needed to be accessed or modified. In this case, to access the global variable, it needs to be declared with an extern specifier, which tells the compiler (more precisely the linker) where to look for the glob_var1 definition.

#include <iostream>

using std::cout;
using std::endl;

int global_var = 123;
extern int glob_var1;  // Defined - int glob_var1 = 44; in other source file

void tmpFunc() {
  glob_var1 += 1;
  cout << glob_var1 << endl;
}

int main() {
  tmpFunc();

  return EXIT_SUCCESS;
}

There might be global variables declared with a static specifier in the different source files in some cases. These variables are accessible only within the file where they are defined and can’t be reached from other files. If you still try to declare them with extern in the current file, the compiler error will occur.

#include <iostream>

using std::cout;
using std::endl;

int global_var = 123;
extern int
    glob_var2;  // Defined - static int glob_var2 = 55; in other source file

void tmpFunc() {
  glob_var2 += 1;
  cout << glob_var2 << endl;
}

int main() {
  tmpFunc();

  return EXIT_SUCCESS;
}