What should go into an .h file?

Question

When dividing your code up into multiple files just what exactly should go into an .h file and what should go into a .cpp file?

Answer 1

What compiles into nothing (zero binary footprint) goes into header file.

Variables do not compile into nothing, but type declarations do (coz they only describe how variables behave).

functions do not, but inline functions do (or macros), because they produce code only where called.

templates are not code, they are only a recipe for creating code. so they also go in h files.