[foofie]

> The existence of .h files

C or C++ don't specify or require .h files. The languages don't even specify any file extension.

What C and C++ specify is the concepts of declaration and definition, and require only one definition across all translation units. Thus it's customary to simply have a single source for declarations to ensure they all stay consistent in spite of their usages, and those declarations are used once by wrapping them in include guards.