C++ Global variable declaration

Question

What I want to do is just to define a variable in a header file and use it on two different cpp files without redefinition that variable each time I include that header
Here is how I tried :

Variables.h

#ifndef VARIABLES_H // header guards #define VARIABLES_H  static bool bShouldRegister;  #endif 

(I also tried extern but nothing changed)

And in a cpp file I give it a value ::bShouldRegister = true or bShouldRegister = true;

In my another cpp file I check it's value by creating a thread and check its value in a loop (and yes my thread function works well)

 while (true)  {      if (::bShouldRegister) // Or if (bShouldRegister)         {             MessageBox(NULL,"Value Changed","Done",MB_OK|MB_ICONINFORMATION);         }   Sleep(100);  } 

And yes, that MessageBox never appears (bShouldRegister never gets true :/)

Answer 1

You must use extern, otherwise you will have separated bShouldRegister variables in each translation unit with probably different values.

Put this in a header file (.h):

extern bool bShouldRegister; 

Put this in one of implementation files (.cpp):

bool bShouldRegister; 

Answer 2

If you can use C++17, consider using an inline variable:

// in a header file inline bool bShouldRegister = true; 

See How do inline variables work? for more information.