Hi, I would like to be able to support an empty optional argument for long options. This is something that is supported by gnu getopt_long and argp. Consider for instance a simplified version of the first simple example program: #include <iostream> #include int main(int ac, char* av[]) { namespace po = boost::program_options; using namespace std; po::options_description desc("Allowed options"); desc.add_options() ("help", "produce help message") ("compression", po::value<string>()->implicit_value(""), "set compression level") ; po::variables_map vm; po::store(po::parse_command_line(ac, av, desc), vm); po::notify(vm); if (vm.count("help")) cout << desc << "\n"; else if (vm.count("compression")) cout << "Compression level was set to " << vm["compression"].as<string>() << ".\n"; else cout << "Compression level was not set.\n"; } when I run it, in "typical cases", I have: $ ./a.out Compression level was not set. $ ./a.out --compression=foo Compression level was set to foo. $ ./a.out --compression foo Compression level was set to foo. $ ./a.out --compression Compression level was set to . all of this is fine. However, how can I explicitly pass the empty string? $ ./a.out --compression= libc++abi.dylib: terminating with uncaught exception of type boost::exception_detail::clone_implboost::program_options::invalid_command_line_syntax >: the argument for option '--compression' should follow immediately after the equal sign zsh: abort ./a.out --compression= Of course I could simply to my user that there is a special token, say '-', that means "empty", but it is still more complex than simply accepting the empty string to mean the empty string. Thanks! (There are some stack overflow questions, but they do not address this particular point, --foo='').