Right. My rule of thumb would be "one implementation file per system", or what would be called a "module" in other languages. So that a moderately complex code

Right. My rule of thumb would be "one implementation file per system", or what would be called a "module" in other languages. So that a moderately complex code base ends up with a about a couple dozen source files to build. And each system should only have a single 'public interface header' to keep the number of cross-system include dependencies low.