Some minor tweaks to what the author shows to make it even better. Give the macro a more descriptive name. For their example, call it GLOBAL_STRING instead of

Some minor tweaks to what the author shows to make it even better. Give the macro a more descriptive name. For their example, call it GLOBAL_STRING instead of X. I think this helps make things clearer. #undef the macro at the end of the header. That removes one line of boilerplate from every use of the header. Use #ifndef at the top of the header and emit a nice error if the macro isn't defined. This will make it easier to understand what's wrong if you forget the #define or misspell the macro.