Re: spirit::karma specialization for std::optional (C++17)

Yes, but I need to handle this with the library. When a optional don't have
a value, karma must skip this attribute. The same behavior like with
boost::optional (Karma handles boost::optional).

For example, I can't do something like this:
static type call(std::experimental::optional<Attribute> const& attr,
Context& ctx)
{
    if (attr)
       return attr.value();
    else
       return nullopt;
}

The return type always must be Attribute.

On Fri, Apr 21, 2017 at 12:13 PM, Michael Powell <[email protected]>
wrote:

> On Fri, Apr 21, 2017 at 11:06 AM, Emiliano Canedo
> <[email protected]> wrote:
> > I can omit the output when optional doesn't have a value, but I don't
> know
> > how
>
> if (opt.has_value()) {
>    /* output value */
> }
>
> http://en.cppreference.com/w/cpp/utility/optional
>
> > On Fri, Apr 21, 2017 at 11:52 AM, Michael <[email protected]> wrote:
> >>
> >>
> >>
> >> On April 21, 2017 10:20:42 AM EDT, Emiliano Canedo
> >> <[email protected]> wrote:
> >> >Hello,
> >> >
> >> >I'm working in a spirit::karma specialization for std::optional
> >> >(C++17). I
> >> >have this code for now:
> >> >
> >> >//-----------------------------------
> >> >
> >> >namespace boost
> >> >{
> >> >namespace spirit
> >> >{
> >> >namespace traits
> >> >{
> >> >
> >> >// This handles std::optional attributes
> >> >template <typename Attribute, typename Exposed>
> >> >struct extract_from_attribute<std::experimental::optional<Attribute>,
> >> >Exposed>
> >> >{
> >> >    typedef Attribute const& type;
> >> >    template <typename Context>
> >> >   static type call(std::experimental::optional<Attribute> const&
> attr,
> >> >Context& ctx)
> >> >    {
> >> >        return attr.value();
> >> >    }
> >> >};
> >> >
> >> >template <typename Attribute, typename Exposed>
> >> >struct extract_from_attribute<std::experimental::optional<Attribute
> >> >const>,
> >> >Exposed>
> >> >{
> >> >    typedef Attribute const& type;
> >> >    template <typename Context>
> >> >   static type call(std::experimental::optional<Attribute const>
> const&
> >> >attr, Context& ctx)
> >> >    {
> >> >        return attr.value();
> >> >    }
> >> >};
> >> >
> >> >}
> >> >}
> >> >}
> >> >
> >> >//-----------------------------------
> >> >
> >> >This works good to extract the value, but, I need to handle the calls
> >> >if
> >> >the optional have, or not, a value. I tried to understand how karma
> >> >handle
> >> >this for boost::optional, but I'm stuck with that. Any idea?
> >>
> >> Are you trying to invoke the output regardless of optional state? Or can
> >> you omit the output when optional does not have a value?
> >>
> >> >Thanks!!
> >> >
> >> >
> >> >-----------------------------------------------------------
> -------------
> >> >
> >>
> >> > >-----------------------------------------------------------
> -------------------
> >> >Check out the vibrant tech community on one of the world's most
> >> >engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> >> >
> >> >-----------------------------------------------------------
> -------------
> >> >
> >> >_______________________________________________
> >> >Spirit-general mailing list
> >> >[email protected]
> >> >https://lists.sourceforge.net/lists/listinfo/spirit-general
> >>
> >> --
> >> Sent from my Android device with K-9 Mail. Please excuse my brevity.
> >>
> >>
> >> ------------------------------------------------------------
> ------------------
> >> Check out the vibrant tech community on one of the world's most
> >> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> >> _______________________________________________
> >> Spirit-general mailing list
> >> [email protected]
> >> https://lists.sourceforge.net/lists/listinfo/spirit-general
> >
> >
> >
> > ------------------------------------------------------------
> ------------------
> > Check out the vibrant tech community on one of the world's most
> > engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> > _______________________________________________
> > Spirit-general mailing list
> > [email protected]
> > https://lists.sourceforge.net/lists/listinfo/spirit-general
> >
>
> ------------------------------------------------------------
> ------------------
> Check out the vibrant tech community on one of the world's most
> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> _______________________________________________
> Spirit-general mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/spirit-general
>

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot

_______________________________________________
Spirit-general mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/spirit-general