[Someone]

But one could use it scoped to the FileWatcher class or to each member function, or is my C++ too rusty?

[jhalstead]

I think it's too rusty, unless I'm misunderstanding https://abseil.io/tips/153. It says,

The vast majority of C++ users think that the using-directive is injecting names into the scope where it’s declared. ... In reality, the names are injected into the nearest common ancestor of the target namespace (::testing) and the usage namespace (::totw::example::anomymous). In our example, that’s the global namespace!

[favorited]

IIRC it can be used from namespace or function scope, but not class.