After all, since only one file will include the header, that code is unnecessary. Here’s the file: ...

Basically once you've put a using namespace std; in your header, and someone has included your header, there's no way to undo the using. This can reduce your options when a name conflict occurs.

C++ splits source into header and implementation files. It requires a big monitor to see .hpp and .cpp header file extensions and file name extensions at the same time.