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

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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