Re: [boost] Is type list defined in boost

From: Peng Yu (pengyu.ut_at_[hidden])
Date: 2007-07-17 11:17:31

On 7/17/07, David Abrahams <dave_at_[hidden]> wrote:
>
> on Tue Jul 17 2007, "Peng Yu" <pengyu.ut-AT-gmail.com> wrote:
>
> > On 7/17/07, David Abrahams <dave_at_[hidden]> wrote:
> >>
> >> on Tue Jul 17 2007, "Peng Yu" <pengyu.ut-AT-gmail.com> wrote:
> >>
> >> > I saw list in mpl. But I'm not familiar with it yet. Would you please
> >> > show me how to do the same thing in mpl as that in my original
> >> > email?
> >>
> >> typedef boost::mpl::list<Rect_, Square_, Triangle_, RightTriangle_> Shapes_;
> >>
> >
> > Hi,
> >
> > I actually meant to define a visitor class for those shapes like the
> > following. Is there anything similar to type_list::head and
> > type_list::tail in mpl::list?
>
> Well, there is, but in general you don't do it that way.  You'd want
> to write code that works just as well with
>
>   typedef boost::mpl::vector<Rect_, Square_, Triangle_, RightTriangle_> Shapes_;
>
> All those virtual functions are a waste of time; none of them override
> the others.  AFAICT all you're really trying to do is generate an
> overload set on the elements of Shapes_.
>
>   struct base_visitor
>   {
>       void visit();
>   };
>
>   template <class T, class Base>
>   struct visitor : Base
>   {
>       void visit(T&) { /*...*/ }
>       using Base::visit;
>   };
>
>   typedef mpl::fold<Shapes_, base_visitor, visitor<_2,_1> >::type v;

I know _2 and _1 are placeholders. But I'm not very clear how they are
used here. I tried to read the manual for "fold", but its explanation
refers to many other mpl structs.

Could you give me some easy explanation on how this works?

Thanks,
Peng