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

Post more detail, like your grammar, and perhaps someone can help you.

On Fri, Apr 21, 2017 at 11:21 AM, Emiliano Canedo
<[email protected]> wrote:
> 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!!
>> >> >
>> >> >
>> >>
>> >> > >------------------------------------------------------------------------
>> >> >
>> >>
>> >> >
>> >> > > >------------------------------------------------------------------------------
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>> >> >_______________________________________________
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>> >> >[email protected]
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>> >>
>> >> --
>> >> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>> >>
>> >>
>> >>
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>
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