Can we get a full working example of custom Validator/Converter?

[phlptp]

Discussed in #778

^{Originally posted by Ivan-SB September 20, 2022}
I'd like to write a date/time validator/converter but I'm not even able to write a trivial do nothing custom validator as a subclass and use it.

It would be nice if we could have an example class and how to use it.

I gave a look to this
https://cliutils.github.io/CLI11/book/chapters/advanced-topics.html
and this
https://cliutils.github.io/CLI11/book/chapters/validators.html

but I didn't come up with anything working.

thanks

[phlptp]

related to #780