Re: [Boost-users] [mpl][variant] Visitor with typelists

Subject: Re: [Boost-users] [mpl][variant] Visitor with typelists
From: Tilo Nitzsche (tricorus_at_[hidden])
Date: 2008-12-29 14:37:17

Thanks! That's certainly better than what I came up with:

class visitor : public boost::static_visitor<char*> {
public:
    template<typename T>
    char* operator()(const T& arg) const {
        typedef typename mpl::contains<v1, T>::type is_v1;
        typedef typename mpl::contains<v2, T>::type is_v2;
        return run(arg, is_v1(), is_v2());
    }
private:
    template<typename T>
    char* run(T arg, mpl::bool_<true>, mpl::bool_<false>) const {
        return arg.a;
    }
    template<typename T>
    char* run(T arg, mpl::bool_<false>, mpl::bool_<true>) const {
        return arg.b;
    }
};

-- Tilo

--- On Mon, 12/29/08, Steven Watanabe <watanabesj_at_[hidden]> wrote:

> From: Steven Watanabe <watanabesj_at_[hidden]>
> Subject: Re: [Boost-users] [mpl][variant] Visitor with typelists
> To: boost-users_at_[hidden]
> Date: Monday, December 29, 2008, 10:21 AM
> AMDG
> 
> Tilo Nitzsche wrote:
> > What is the best (shortest/most elegant) way to create
> a visitor for a Variant, where there are subsets of the
> variant types, for which the operation is the same?
> > 
> > typedef boost::mpl::vector<A, B, C> v1;
> > typedef boost::mpl::vector<D, E> v2;
> > typedef boost::mpl::joint_view<v1, v2> v;
> > typedef boost::make_variant_over< v >::type
> v_union;
> > 
> > struct visitor : public
> boost::static_visitor<int> {
> >     int operator()([T in v1] arg) { // do something
> for A, B, C
> >         return arg.a;
> >     }
> >     int operator()([T in v2] arg) { // do something
> for D, E
> >         return arg.b;
> >     }
> > };
> >   
> 
> Use enable_if
> 
> typedef boost::mpl::vector<A, B, C> v1;
> typedef boost::mpl::vector<D, E> v2;
> typedef boost::mpl::joint_view<v1, v2> v;
> typedef boost::make_variant_over< v >::type v_union;
> 
> struct visitor : public boost::static_visitor<int> {
>    template<class T>
>    typename boost::enable_if<boost::mpl::contains<v1,
> T>, int>::type
>    operator()(T arg) { // do something for A, B, C
>        return arg.a;
>    }
>    template<class T>
>    typename boost::enable_if<boost::mpl::contains<v2,
> T>, int>::type
>    operator()(T arg) { // do something for D, E
>        return arg.b;
>    }
> };
> 
> In Christ,
> Steven Watanabe
> 
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