Add support for exclusive/dependant arguments

[f-michaut]

Some arguments are exclusive.
Current workaround is to implement checks after the parsing like so :

[...]
program.parse_args(ac, av);

if (program.is_used("--foo") && program.is_used("--bar")) {
    throw "--foo and --bar are exclusive !";
}

Furthermore, some parameters often require others to be present in order to be meaningfull.
Once again, we currently need to check manualy that both are given.

---

It would be nice to be able to use constructs like this :

program.add_argument("--line").needs("--file");
program.add_argument("--file").exclusive("--ip");
program.add_argument("--ip"); // No need to specify again that they are exclusive
program.add_argument("--port").needs("--ip").default_value(443);

try {
  program.parse_args(ac, av);
} catch (const std::runtime_error& err) {
  std::cerr << "Wrong arguments: " << err.what() << std::endl;
}

Usage :

# Good
./program --file /tmp/file
./program --file /tmp/file --line 25
./program --ip 8.8.8.8
./program --ip 8.8.8.8 --port 80

# Bad
./program --ip 8.8.8.8 --line 25
./program --line 25
./program --port 80
./program --file /tmp/file --port 80
./program --file /tmp/file --ip 8.8.8.8

[p-ranav]

Added support for mutually exclusive arguments here: #301

[f-michaut]

Thank you for adding exclusive arguments ! That's very helpfull.

Do you intend to add dependant arguments as well ?

[p-ranav]

I think so, although I don't have a complete design yet.