Adding Help

There isn't a special provision to add --help option in Argum, unlike some other libraries. Adding one is easy, though, just like any other kind of option.

#include <cstdlib>

int main(int argc, char * argv[]) {

    const char * progname = (argc ? argv[0] : "prog");

    Parser parser;
    parser.add(
        Option("--help", "-h"). 
        help("show this help message and exit"). 
        handler([&]() {  

            std::cout << parser.formatHelp(progname);
            std::exit(EXIT_SUCCESS);
    }));

    try {
        parser.parse(argc, argv);
    } catch (ParsingException & ex) {
        std::cerr << ex.message() << '\n';
        std::cerr << parser.formatUsage(progname) << '\n';
        return EXIT_FAILURE;
    }
}

Running this code produces

$ ./prog
$ ./prog --help
Usage: ./prog [--help]

options:
  --help, -h  show this help message and exit

$ ./prog  -h
Usage: ./prog [--help]

options:
  --help, -h  show this help message and exit

$ ./prog --help=foo
extraneous argument for option: --help
Usage: ./prog [--help]
$ ./prog --help foo bar
Usage: ./prog [--help]

options:
  --help, -h  show this help message and exit

Note the last invocation. The parser seem to have ignored the invalid arguments following --help. Why?

What is very important to understand is that argument handlers are invoked sequentially, as they are parsed - not after the whole command line was validated like most other parsers do. Thus when the handler for --help is invoked, the invalid arguments foo and bar haven't been handled yet. The --help handler we specified prints the help message and exits the program so no error is ever discovered.

Further Reading

Defining Options
Positional Arguments