How to initialize same variable in multiple files in C++

I’m having a problem. I have two header files (a.h, b.h) and three cpp files (main.cpp, a.cpp, b.cpp) and I’m trying to initialize the same ten variables in each one, and I’m lost. How do I go about doing that?

put the
extern type var_name
into a header and include this in all the other files instead of putting the declaration into them manually.