Bryan Green wrote:

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)")

Thanks, that is exactly what I was looking for. Maybe the documentation should be updated as it says "For the third option, the user may either provide a single token for the value, or no token at all", refering to the ("verbose", value<string>()->zero_tokens(), "verbosity level") Marti