如何在boost :: program_options中接受空值

#include<boost/program_options.hpp>

namespace po = boost::program_options;

po::options_description desc("Allowed options");
desc.add_options()
    ("help", "produce help message")
    ("input-file", po::value<std::string>(), "input file")
    ("output-file", po::value<std::string>(), "output file")
    ("optional-value", po::value<po::optional<std::string>>(), "optional value")
    ;

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 1;
}

if (vm.count("input-file")) {
    std::string input_file = vm["input-file"].as<std::string>();
    // do something with input_file
}

if (vm.count("output-file")) {
    std::string output_file = vm["output-file"].as<std::string>();
    // do something with output_file
}

if (vm.count("optional-value")) {
    po::optional<std::string> optional_value = vm["optional-value"].as<po::optional<std::string>>();
    if (optional_value) {
        std::string value = optional_value.get();
        // do something with value
    }
}