Hi,
I want to suggest that we refrain from using leading underscores in the future because of clause 17.4.3.1.2/1 which states that ...
Certain sets of names and function signatures are always reserved to the implementation:
* Each name that contains a double underscore (_ _) or begins with an underscore followed by an upper-case letter (2.11) is reserved to the implementation for any use.
* Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace.
In particular the first bullet point can lead to issues and did so on Cygwin which did not like the variable name _X. My suggestion to completely stop using leading underscores is just a precaution in order to prevent any future issues.
Regards,
Hauke