Required and Optional Arguments Using Boost Library Program Options

Question

I'm using Boost Program Options Library to parse the command line arguments.

I have the following requirements:

1. Once "help" is provided, all the other options are optional;
2. Once "help" is not provided, all the other options are required.

How I can deal with this? Here is the my code handling this, and I found it's very redundant, and I think there must be an easy to do, right?

#include <boost/program_options.hpp>
#include <iostream>
#include <sstream>
namespace po = boost::program_options;

bool process_command_line(int argc, char** argv,
                          std::string& host,
                          std::string& port,
                          std::string& configDir)
{
    int iport;

    try
    {
        po::options_description desc("Program Usage", 1024, 512);
        desc.add_options()
          ("help",     "produce help message")
          ("host,h",   po::value<std::string>(&host),      "set the host server")
          ("port,p",   po::value<int>(&iport),             "set the server port")
          ("config,c", po::value<std::string>(&configDir), "set the config path")
        ;

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

        if (vm.count("help"))
        {
            std::cout << desc << "\n";
            return false;
        }

        // There must be an easy way to handle the relationship between the
        // option "help" and "host"-"port"-"config"
        if (vm.count("host"))
        {
            std::cout << "host:   " << vm["host"].as<std::string>() << "\n";
        }
        else
        {
            std::cout << "\"host\" is required!" << "\n";
            return false;
        }

        if (vm.count("port"))
        {
            std::cout << "port:   " << vm["port"].as<int>() << "\n";
        }
        else
        {
            std::cout << "\"port\" is required!" << "\n";
            return false;
        }

        if (vm.count("config"))
        {
            std::cout << "config: " << vm["config"].as<std::string>() << "\n";
        }
        else
        {
            std::cout << "\"config\" is required!" << "\n";
            return false;
        }
    }
    catch(std::exception& e)
    {
        std::cerr << "Error: " << e.what() << "\n";
        return false;
    }
    catch(...)
    {
        std::cerr << "Unknown error!" << "\n";
        return false;
    }

    std::stringstream ss;
    ss << iport;
    port = ss.str();

    return true;
}

int main(int argc, char** argv)
{
  std::string host;
  std::string port;
  std::string configDir;

  bool result = process_command_line(argc, argv, host, port, configDir);
  if (!result)
      return 1;

  // Do the main routine here
}

Answer 1

I've run into this issue myself. The key to a solution is that the function po::store populates the variables_map while po::notify raises any errors encountered, so vm can be used prior to any notifications being sent.

So, as per Tim, set each option to required, as desired, but run po::notify(vm) after you've dealt with the help option. This way it will exit without any exceptions thrown. Now, with the options set to required, a missing option will cause a required_option exception to be thrown and using its get_option_name method you can reduce your error code to a relatively simple catch block.

As an additional note, your option variables are set directly via the po::value< -type- >( &var_name ) mechanism, so you don't have to access them through vm["opt_name"].as< -type- >().

A code example is provided in Peters answer

Answer 2

Here is complete program as per rcollyer and Tim, whom the credits go to:

#include <boost/program_options.hpp>
#include <iostream>
#include <sstream>
namespace po = boost::program_options;

bool process_command_line(int argc, char** argv,
                          std::string& host,
                          std::string& port,
                          std::string& configDir)
{
    int iport;

    try
    {
        po::options_description desc("Program Usage", 1024, 512);
        desc.add_options()
          ("help",     "produce help message")
          ("host,h",   po::value<std::string>(&host)->required(),      "set the host server")
          ("port,p",   po::value<int>(&iport)->required(),             "set the server port")
          ("config,c", po::value<std::string>(&configDir)->required(), "set the config path")
        ;

        po::variables_map vm;
        po::store(po::parse_command_line(argc, argv, desc), vm);

        if (vm.count("help"))
        {
            std::cout << desc << "\n";
            return false;
        }

        // There must be an easy way to handle the relationship between the
        // option "help" and "host"-"port"-"config"
        // Yes, the magic is putting the po::notify after "help" option check
        po::notify(vm);
    }
    catch(std::exception& e)
    {
        std::cerr << "Error: " << e.what() << "\n";
        return false;
    }
    catch(...)
    {
        std::cerr << "Unknown error!" << "\n";
        return false;
    }

    std::stringstream ss;
    ss << iport;
    port = ss.str();

    return true;
}

int main(int argc, char** argv)
{
  std::string host;
  std::string port;
  std::string configDir;

  bool result = process_command_line(argc, argv, host, port, configDir);
  if (!result)
      return 1;

  // else
  std::cout << "host:\t"   << host      << "\n";
  std::cout << "port:\t"   << port      << "\n";
  std::cout << "config:\t" << configDir << "\n";

  // Do the main routine here
}

/* Sample output:

C:\Debug>boost.exe --help
Program Usage:
  --help                produce help message
  -h [ --host ] arg     set the host server
  -p [ --port ] arg     set the server port
  -c [ --config ] arg   set the config path


C:\Debug>boost.exe
Error: missing required option config

C:\Debug>boost.exe --host localhost
Error: missing required option config

C:\Debug>boost.exe --config .
Error: missing required option host

C:\Debug>boost.exe --config . --help
Program Usage:
  --help                produce help message
  -h [ --host ] arg     set the host server
  -p [ --port ] arg     set the server port
  -c [ --config ] arg   set the config path


C:\Debug>boost.exe --host 127.0.0.1 --port 31528 --config .
host:   127.0.0.1
port:   31528
config: .

C:\Debug>boost.exe -h 127.0.0.1 -p 31528 -c .
host:   127.0.0.1
port:   31528
config: .
*/

Answer 3

You can specify that an option is required easily enough [1], e.g.,:

..., value<string>()->required(), ...

but as far as I know there's no way to represent relationships between different options to the program_options library.

One possibility is to parse the command line multiple times with different option sets, then if you've already checked for "help" you can parse again with the three other options all set as required. I'm not sure I'd consider that an improvement over what you have, though.

Answer 4

    std::string conn_mngr_id;
    std::string conn_mngr_channel;
    int32_t priority;
    int32_t timeout;

    boost::program_options::options_description p_opts_desc("Program options");
    boost::program_options::variables_map p_opts_vm;

    try {

        p_opts_desc.add_options()
            ("help,h", "produce help message")
            ("id,i", boost::program_options::value<std::string>(&conn_mngr_id)->required(), "Id used to connect to ConnectionManager")
            ("channel,c", boost::program_options::value<std::string>(&conn_mngr_channel)->required(), "Channel to attach with ConnectionManager")
            ("priority,p", boost::program_options::value<int>(&priority)->default_value(1), "Channel to attach with ConnectionManager")
            ("timeout,t", boost::program_options::value<int>(&timeout)->default_value(15000), "Channel to attach with ConnectionManager")
        ;

        boost::program_options::store(boost::program_options::parse_command_line(argc, argv, p_opts_desc), p_opts_vm);

        boost::program_options::notify(p_opts_vm);

        if (p_opts_vm.count("help")) {
            std::cout << p_opts_desc << std::endl;
            return 1;
        }

    } catch (const boost::program_options::required_option & e) {
        if (p_opts_vm.count("help")) {
            std::cout << p_opts_desc << std::endl;
            return 1;
        } else {
            throw e;
        }
    }