How to declare a global variable that could be used in the entire program

global.h
extern int myVar;

global.cpp
#include "global.h"
int myVar = 0;  // initialize

class1.cpp
#include "global.h"
...

class2.cpp
#include "global.h"
...

class3.cpp
#include "global.h"
...

MyVar will be known and usable in every module as a global variable. You do not have to have global.cpp. You could initialize myVar in any of the class .cpp's but I think this is cleaner for larger programs.

While I would like to avoid global variables like the plague as our software cannot be multithreaded effectively due to the high reliance on global variables, I do have some suggestions:

Use a Singleton. It will allow you to keep the code and access clean. Part of the problem with a global variable is you don't know what code has modified it. You could set the value of global somewhere in your function relying on the hope that no one else will change it, but function your code calls, fooA, changes it and now your code is a) broken, and b) hard to debug.

If you have to use a global variable without touching the singleton pattern, look at fupsduck's response.

If you're not going to use the Singleton pattern as Lyndsey suggests, then at least use a global function (inside a namespace) to access the variable. This will give you more flexibily in how you manage that global entity.

// mymodule.h
namespace mynamespace // prevents polluting the global namespace
{
   extern int getGlobalVariable();
}

// mymodule.cpp
namespace mynamespace
{
   int myGlobalVariable = 42;

   int getGlobalVariable()
   {
      return myGlobalVariable;
   }
}