try {
    po::options_description desc("Allowed options");

    desc.add_options()
        ("help,h","output help message")
        ("width,w",po::value<int>()->required()," width")
    ;

    po::positional_options_description p;
    p.add("width",1);

    po::variables_map vm;
    po::store(po::command_line_parser(argc,argv).
              options(desc).positional(p).run(),vm);

    if (vm.count("help")) {
        std::cout << "USAGE: " << av[0] << &p <<  std::endl;
        return 0;
    }

    po::notify(vm);

    if (vm.count("width")) {
        std::cout << "width: " << vm["width"].as<int>() << "\n";
    }
} catch (std::exception& e) {
    std::cout << e.what() << std::endl;
    return 1;
} catch (...) {
    std::cout << "Exception of unknown type!" << std::endl;
}

我想在没有传递参数的情况下显示帮助,但我没有找到一种方法来获取变量的总数,而不依赖于argc.