Boost Program Options is silently ignoring unknown value tokens. why?

Question

I'm using Boost Program Options to parse CLI.

The problem I'm facing is that if there is any token in CLI without '-' or '--' in front of it, the library silently ignores it instead of throwing exception.

Following is the sample program:

try
{
    options_description od;
    od.add_options()
        ("dummy,d", value<int>()->required(), "does nothing...");

    variables_map vm;
    wparsed_options po = parse_command_line(argc, argv, od);
    store(po, vm);
    notify(vm);

    cout << vm["dummy"].as<int>() << endl;
}
catch (const error& e)
{
    cout << e.what() << endl;
}

Following are some sample runs:

Debug>test
the option '--dummy' is required but missing

Debug>test -d
the required argument for option '--dummy' is missing

Debug>test -d 1
1

Debug>test -d 1 asas
1

Now, the first three runs are as expected. But, why is the third run not throwing any exception? 'asas' doesn't matches any option and -d doesn't accepts vector. What am I doing wrong? Or the library is designed this way?

Answer 1

1. a token without preceding dashes is called a positional argument
2. you should explicitly forbid positionals for the expected behaviour
3. to do this make empty list of positionals and feed it to the parser https://stackoverflow.com/a/3859400/670719