The code seems clean. I'm not sure this is a great idea in practice, though. Generally the only good reason for mapping stuff out of the filesystem is performance, and VM behavior with mmap() varies wildly across systems (and filesystem backends, and drivers if it's a hardware device, and hardware if it's a framebuffer, and...). Frankly on windows this is AFAIK a mostly-unheard-of technique. No one does mapping.

This just isn't really something that can be cleanly abstracted to do what you want it to do, even if you can make the code "look" the same.

But again, it looks like a nice, clean, modern C++ library. Just IMHO misapplied.