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

I can omit the output when optional doesn't have a value, but I don't know
how

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!!
> >
> >
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