The exists check shouldn't be necessary; the return value from GetFileAttributesW should be INVALID_FILE_ATTRIBUTES for a non-existent file as well.

I would really like it if there were a way to simply open the hidden file rather than changing the attribute. There might be a set of mode options that change the CreateFile call to avoid the documented error state (I would think specifically anything that moves to "open only existing files" might do it. So more of a if (exists) open_existing_with_truncate() else create().

If that cannot be done, then at a minimum we should use a scope exit to reset the attributes.