How do I use a boost::program_options notifier with multiple options?

Question

I'm writing a physics simulation that reads in a whole bunch of system parameters with the boost::program_options library and I'd like to "automatically" set some parameters that arise as combinations of the user-input values. For example, if I have

[parameters]
    c0 = 299792458
    dt = 0.004

as the speed of light and timestep in my input file, I'd like to set a value cdt = c0*dt in the same structure I'm using to store c0 and dt after both options get read. Notifiers, as I understand, are a way to process an input option with a function, but I haven't seen a way to do the same thing with multiple options -- what's the best way to go about doing this?

Answer 1

I do not see any direct option how to achieve this using boost program_options. Notifiers are being called after only one option is parsed. However you can combine several workarounds to achieve acceptable solution - store values in separate structure, let program_options to fill all computed values (directly or using setter function) after parsing is complete.

My simplified suggestion:

#include <boost/program_options.hpp>
#include <iostream>

using namespace boost::program_options;

struct SimulationConfig {
  int c0;
  float dt;
  float cdt;

  void setCdt() {
    cdt = c0*dt;
  }
};

int main(int argc, const char *argv[])
{

  SimulationConfig config;
  try
  {
    options_description desc{"Options"};
    desc.add_options()
      ("help,h", "Help screen")
      ("c0", value<int>(&config.c0), "Speed of light")
      ("dt", value<float>(&config.dt), "Time interval");

    variables_map vm;
    store(parse_command_line(argc, argv, desc), vm);
    notify(vm);


    if (vm.count("help"))
      std::cout << desc << '\n';
    else if (vm.count("c0") && vm.count("dt")) {
      //config.cdt = vm["c0"].as<int>() * vm["dt"].as<float>();
      config.setCdt();
      std::cout << "Cdt is set to: " << config.cdt << std::endl;
    }

  }
  catch (const error &ex)
  {
    std::cerr << ex.what() << '\n';
  }
}