boost-program-options: notifier for options with no value

Question

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?

Answer 1

Use the zero_tokens modifier. It seems you also need to use implicit_value, but anything provided after the option name won't be consumed by the option parser. Instead, when the option is noticed on the command line, the implicit value will be assigned to the option, triggering the option's notifier (so make sure to provide a notifier function). Apparently, it's also important for the option's value type to be string. I'm not clear on why.

void got_foo(std::string const&);

desc.add_options()
  ("foo", 
   po::value<std::string>()
     ->implicit_value("")
     ->zero_tokens()
     ->notifier(&got_foo),
   "foo description")
;

Answer 2

This is full example on providing a flag as an option to complement current answers:

#include <iostream>

#include <boost/program_options.hpp>

using namespace std;

namespace po = boost::program_options;


int main(int ac, char* av[])
{
    po::options_description desc("Allowed options");

    desc.add_options()
        ("help", "produce help message")
        ("overwrite,o", po::bool_switch()->default_value(false),
                        "enable file overwrite");

    po::variables_map vm;
    po::store(po::parse_command_line(ac, av, desc), vm);

    boolalpha(cout);  // display true and false for bool
    cout << "overwrite is: " << vm["overwrite"].as<bool>() << endl;

    return 0;
}

Mine qmake pro file (I use Qt 5.4):

TEMPLATE = app
CONFIG += console
CONFIG += c++14
CONFIG -= app_bundle
CONFIG -= qt

SOURCES += main.cpp


include(deployment.pri)
qtcAddDeployment()

INCLUDEPATH += /opt/boost_1_57_0

unix:!macx: LIBS += -L/opt/boost_1_57_0/stage/lib -lboost_program_options

Running the program without any options results in:

./untitled4
overwrite is: false

However running with '-o' option/flag gives:

./untitled4 -o
overwrite is: true

Answer 3

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