Re: [boost] Interest in switch generator

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-02-03 18:42:28

Steven Watanabe wrote:
>> I'm interested. I'll need polymorphic return type handling though.
>> Something like:
>>
>> struct f
>> {
>>      template <typename Case> struct result;
>>
>>      template <> struct result<mpl::int_<0> >
>>          : mpl::identity<char> {};
>>
>>      template <> struct result<mpl::int_<2> >
>>          : mpl::identity<float> {};
>>
>>      template <> struct result<mpl::int_<5> >
>>          : mpl::identity<std::string> {};
>>
>>      char operator()(mpl::int_<0>) const;
>>      float operator()(mpl::int_<1>) const;
>>      std::string operator()(mpl::int_<5>) const;
>> };
>>
>> Regards,
> 
> So what goes here?
> 
> template<class Cases, class Int, class F>
> ??? switch_(Int i, F f);
> 
> ??? could be either any or variant.
> If any, why then, typedef boost::any result_type
> works just fine.  Variant is a little more
> difficult especially if operator() is a template.

Yeah, right. That's too hairy. I guess it's best to have that
(variant returns) as the responsibility of the function object:

struct f
{
      typedef variant<char, float, std::string> result_type;

      char operator()(mpl::int_<0>) const;
      float operator()(mpl::int_<1>) const;
      std::string operator()(mpl::int_<5>) const;
};

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net