boost-program-options: notifier for options with no value
One can use notifier for parsed options only if they have value_semantic. What is the best way for no-value options to be automatically handled by the given notifier?
The simple approach is to make a dummy value_semantic with implicit assignment, so a user can pass the option without a value. This leads to a possibility of explicitly provided values. One can add a run-time check if the value was provided and throw an error.
Update: BUT, this doesn't work in presence of positional options, because a positional option's value can follow no-value option raising an exception as s given value to it.
Answer
One guy OXPEHOMETP on a russian programmers forum gave me a pice of advice to use boost::program_options::bool_switch().
When defining an option with no value via value-permitted interface, one must pass not boost::program_options::typed_value() as semantics, but bool_switch(). This means that no value can be explicitly taken for this option from the command line.
One can find info about it at http://www.boost.org/doc/libs/release/doc/html/boost/program_options/bool_switch.html