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

> 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)")
>
>