make it possible to use sharg with std::optional

[feldroop]

As far as I know, this is not possible right now. It would be useful for when you want to have an non-required option and it is not sensible to provide a default (default then would be std::nullopt).

[smehringer]

Hi @feldroop,

If you have an option that has no sensible default, it might be enough to suppress the Default: 3 message in the help page. That is already possible, so would that suffice? Otherwise please elaborate on you use case :)

[feldroop]

How do I suppress the default message? I can't find it on my own.

[feldroop]

First of all, from a programming style standpoint I would like to have the ability to model cases like this with a std::optional. I can hide the invalid default from users, but why should I be forced to have it flying around in my code when patterns like the std::optional for exactly this use case exist?

Secondly, as far as I know (and I might have missed something), sharg does not tell me whether a certain flag was set by the user. It might happen that every possible value of my int option are valid user input, but I still want to handle the case where the option was not set differently.

The latter is not the case in my app at the moment. I could suppress the default print to the help menu (which I didn't know was possible before) and set the default to something like the maximum int. But I still feel like a std::optional would be a more convenient and elegant solution.

[eseiler]

Secondly, as far as I know (and I might have missed something), sharg does not tell me whether a certain flag was set by the user.

You can use parser.is_option_set('b') / parser.is_option_set("bar") to check if an option was given.

After #226, it will work with long or short IDs, for now you would have to check for both long and short (see PR).

[feldroop]

That's nice! I will use this. Then only my first point stands.

[smehringer]

You can make std::optional work with the parser by adding ostream/istream overloads:
See this HowTo as a reference.

namespace std
{

// Make external::bar satisfy sharg::ostreamable
template <typename optional_type>
    requires sharg::ostreamable<optional_type>
ostream & operator<<(ostream & output, std::optional<optional_type> const & opt)
{
    if (opt.has_value())
        output << opt.value(); // Adapt this for your type
    else
        output << "optional not set";
    return output;
}

// Make std::optional<optional_type> satisfy sharg::istreamable
template <typename optional_type>
    requires sharg::istreamable<optional_type>
istream & operator>>(istream & input, std::optional<optional_type> & opt)
{
    optional_type val;
    input >> val; // Adapt this for your type
    opt = val;
    return input;
}

} // namespace std

Add the above and then you can use std::optional for add_option and add_positional_option calls.

[feldroop]

Thanks! For me it still doesn't work, because I'd want to use an option of a number type with an arithmetic range validator. I'm sure it would also be possible to implement another concept for that, but at this point it doesn't seem worth the effort anymore.

I managed to suppress the default message, so I'm fine with the current state.