Re: [Boost-users] [Boost.Program_options] zero or one tokens with zero_tokens()?

Subject: Re: [Boost-users] [Boost.Program_options] zero or one tokens with zero_tokens()?
From: Bryan Green (bryan.d.green_at_[hidden])
Date: 2009-07-09 13:56:36

Marti Cuquet writes:
> Hello,
> 
> I am trying to use an option that can have either zero or one arguments.
> 
> I have seen an example in the documentation claiming that the following
> code would allow the user to provide either zero or one tokens to option
> 'verbose', but instead it throws an exception "extra parameter" when one
> token is given.
> 
> So the question is, does zero_tokens() allow the user to specify a value?
> 
> options_description desc;
> desc.add_options()
>     ("help", "produce help message")
>     ("compression", value<string>(), "compression level")
>     ("verbose", value<string>()->zero_tokens(), "verbosity level")
>     ("email", value<string>()->multitoken(), "email to send to")
>     ;

There is an undocumented "implicit_value" feature that may be what you are
looking for.  It allows zero or one token in the style of
getopt/getopt_long:

options_description desc;
desc.add_options()
    ("help", "produce help message")
    ("verbose,v", po::value<int>()->implicit_value(1),
                      "enable verbosity (optionally specify level)")

-bryan