Processing command-line arguments in C++

I just released arg as a standalone library under LGPL. It's a command-line parser for C++ programs. The aim is to keep the programming effort in adding command-line processing to C++ programs at a minimum. There are several considerations in this direction:

1. simple and intuitive syntax
2. requiring no redundancy
3. localized codes
4. extensibility
5. completeness

The simple example as given on the arg homepage,

#include <arg.hh>
#include <iostream>
int main(int argc, char ** argv)
{
        arg::Parser p;
        int n;
        p.add_opt('n').stow(n);
        p.parse(argc, argv);
        std::cout << n << '\n';
        return 0;
}

should be very close to the minimum as far as 1. goes.

Programming is for a programmer to describe what he wants the computer to do. Per point 2., he should not be asked to provide the same information multiple times. (Well, maybe except in situations where multiple confirmations are required: "Launch the missile. Please have the missile launched. Yes, I really want you to launch the missile! Launch the *&^%$ missile!!!"; Computer: "Aborted on Error: missile != *&^%$ missile".)

When working on an item, e.g., adding a new command-line option, the programmer won't be asked to go to multiple places in the codes if 3. is observed. While common and frequent usages should be supported and simplified in the library, new and novel applications will ask for 4. Finally, some rare, special, and/or tricky applications will demand 5. in the arsenal.