That isn't an 'implementation file' in the relevant sense. If the file is intended to be used with #include, it's a header file, and should be given the .h or

That isn't an 'implementation file' in the relevant sense. If the file is intended to be used with #include, it's a header file, and should be given the .h or .hpp extension. If the file corresponds to an object file in the build, it should be given the .cpp extension (or .cxx). If you want to break apart your template metaprogramming into several .hpp files, you can do so.