Boost Program Options

Basic Usage

Boost program options provides a simple and safe way to parse and handle command line arguments.

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

int main(int argc, char** argv) {
  namespace po = boost::program_options;

  po::variables_map vm;
  po::options_description desc("Allowed Options");

  // declare arguments
  desc.add_options()
    ("name", po::value<std::string>()->required(), "Type your name to be greeted!");

  // parse arguments and save them in the variable map (vm)
  po::store(po::parse_command_line(argc, argv, desc), vm);

  std::cout << "Hello " << vm["name"].as<std::string>() << std::endl;

  return 0;
}

Compile and run with:

 $ g++ main.cpp -lboost_program_options && ./a.out --name Batman
Hello Batman

You can output a boost::program_options::options_description object to print the expected argument format:

std::cout << desc << std::endl;

would produce:

Allowed Options:
  --name arg                Type your name to be greeted!

Error Handling

boost::program_options::notify can be used to report any errors in the paramters passing

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

int main(int argc, char** argv) {
  namespace po = boost::program_options;

  po::variables_map vm;
  po::options_description desc("Allowed Options");

  // declare options
  desc.add_options()
    ("name", po::value<std::string>()->required(), "Type your name to be greeted!");

  // parse arguments
  po::store(po::parse_command_line(argc, argv, desc), vm);

  // check arguments
  try {
    po::notify(vm);
  } catch (std::exception& e) {
    std::cout << "Error: " << e.what() << std::endl;
    std::cout << desc << std::endl;
    return 1;
  }

  // program logic
  std::cout << "Hello " << vm["name"].as<std::string>() << std::endl;

  return 0;
}

Passing illegal arguments produces helpful errors messages

 $ ./a.out
Error: the option '--name' is required but missing
Allowed Options:
  --name arg            Type your name to be greeted!

Default Values

A default valued command line argument can be specified easily:

// declare options
desc.add_options()
  ("name", po::value<std::string>()->required(), "Type your name to be greeted!")
  ("rank", po::value<std::string>()->default_value("Dark Knight"), "Your rank");

Its value is also added to the variable map:

  std::cout << "Hello " << vm["name"].as<std::string>() << " " << vm["rank"].as<std::string>() << std::endl;

The default value is shown in the description…

$ ./a.out
Error: the option '--name' is required but missing
Allowed Options:
  --name arg                Type your name to be greeted!
  --rank arg (=Dark Knight) Your rank

… and used if not specified…

$ ./a.out --name Batman
Hello Batman Dark Knight

… but can be overwritten at command line:

 $ ./a.out --name Batman --rank FlyingSquirrel
Hello Batman FlyingSquirrel

Switches

A switch is a command line argument which takes no value. It can be specified with:

desc.add_options()
  ("hidden", po::bool_switch()->default_value(false), "Hide your name");

And used with:

if (vm["hidden"].as<bool>())
   std::cout << "Hello *****" << std::endl;

from the command line:

 $ ./a.out --name Batman --hidden
Hello *****

and in the description it shows as:

Allowed Options:
  --name arg                Type your name to be greeted!
  --rank arg (=Dark Knight) Your rank
  --hidden                  Hide your name