Re: [boost] Interest in switch generator

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-02-03 12:30:16

AMDG

Joel de Guzman <joel <at> boost-consulting.com> writes:

> 
> 
> 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.

template<class F>
struct get_result {
    template<class Case>
    struct apply {
        typedef typename F::template result<Case>::type type;
    };
};

template<class F, class Sequence>
struct variant_adapter : F {
    typedef typename mpl::transform<Sequence,
        mpl::protect<get_result<F> >,
        mpl::inserter<mpl::set0<>, mpl::insert<_1, _2> >
    >::type possible_results;
    typedef typename make_variant_over<possible_results>::type result_type;
    variant_adapter(const F& f) : F(f) {}
};

In Christ,
Steven Watanabe