Using the same variable across multiple files in C++

Question

In the process of changing some code, I have spilt some functions into multiple files. I have the files controls.cpp and display.cpp and I would like to be able to have access to the same set of variables in both files. I don't mind where they are initialized or declared, as long as the functions in both files can use them.

This was not an issue when the functions were in the same file, but now it seems almost impossible after an hour of googling and trying various things.

Answer 1

use those variables as extern i.e.

extern int i;

in another file declare same as normal global variable...

int i;//global

Answer 2

Define the variable in one file like:

type var_name;

And declare it global in the other file like:

extern type var_name;

Answer 3

All of the declarations you want visible in multiple compilation units (.cpp files), should go into a header file that you include in all places that need to use the variable, type, class, etc.

This is vastly better than extern, which essentially hides your intention to share the declaration.

Answer 4

Create two new files:

1. Something like Globals.h and declare all variables like: extern type name;
   • btw remember include guards.
2. Something like Globals.cpp and declare variables like: type name;

Then add #include "Globals.h" at the top of:

• Globals.cpp
• controls.cpp
• display.cpp

You may then want some functions to initialise them.