Question

I would like to know why you can not declare a global with the same name in 2 different .cpp files. My understanding is considering scope, that it should only be visible to that particular .cpp file and no where else but it is obviously complaining. The reason I’m doing this is for commonality in code and that’s it. any ideas?

Edit for Clarity

a.cpp

int g_x;

b.cpp

int g_x;

Answer 1

To make a global variable (or function) only visible to the file it’s declared in,

1. declare it static, or
2. (the way that is preferred in C++) put the variable in a nameless namespace. It would seem like that would make it inaccessible from outside the namespace, but what it actually does is make it only visible to the file in which it is.

To access a global variable (that is not static or in an anonymous namespace) that is declared in some other file, use extern.

The reason why is the same reason why you can’t have a function with the same name in two different files. It confuses the linker because global variables by default have external linkage. static or being in an anonymous namespace gives them internal linkage, which makes them like a “local global variable”.