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

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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>> --
>> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>>
>>
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>
>
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