Re: [boost] Re: Re: Formal Review: Command Line & Config library

From: Pavol Droba (droba_at_[hidden])
Date: 2003-05-23 10:21:14

 > 
> > I like the idea proposed by Tanton. po::parameter should have a validation
> > predicate as an argument.
> > A reasonable set of arguments can be provided by library, and the user can
> > always provide a specific ones.
> 
> The story is a bit more complex. There's a 'validator' which can be set to
> do anything.
> 
>    desc.add_options()
>        ("foo", "bar").validator(your_nifty_functor)
>        ;
> 
> However, say you extensively use value ranges. You'd have to write this
> ".validator()" part everywhere, and "your_nifty_functor" will actually be
> creating of some functor, which is passed two values. This all is verbose.
> 
> You can, however, write a single function, say int_range, which will take
> parameter name, and two ints, and set up the validation function. That
> would be more convenient. 
> 
> The only problem with current 'validator' is that it should validate
> *string* and return the value. So, the example given by Tanton:
> 
>    _1 >= 1 && _1 <= 9)
> 
> would not really work --- the validator function is given a string. It's
> probably possible to use 'notify' callback to do such kind of validation,
> though. Or add another validator --- which is given converted value.
> 

I see. However I think this is a little bit clumsy to use. I think it may be better
to strip validation from the conversion.

What I had in mind is an extension to po::parameter funtion to something like

        template< typename T >
        *unspecified* po::parameter<T>( 
                        string parameter_name, 
                        T& variable, 
                        const boost::function1<bool, T)& validator );

so the framework would convert string parameter to a specified type and then run the validator
functor on the result to validate it.

> >> >       * Format specification and checking for string. Regex
> >> >       specification of something like scanf would be nice.
> >> >         This can be useful for example to check if the parameter is a
> >> >         filename.
> >> 
> >> Could you clarify a bit how scanf-like specification can work? You mean
> >> it can be used to specify syntax?
> >> 
> > 
> > I mean something like "%02.4f" or something like that... it is just an
> > idea. If there would be a validation predicate as an argument, regex
> > library can be used to define regex predicate which would cover most of
> > the cases.
> 
> Ah... ok. Indeed --- the validator can do that already, and in case of 
> string validation, the current code will work.
> 
That's nice. Can you post a piece of code as an example?

Pavol.