[boost] Re: best way to make {reverse_}fold a sequence

From: David Abrahams (dave_at_[hidden])
Date: 2005-02-28 03:56:56

Larry Evans <cppljevans_at_[hidden]> writes:

> After creating:
>
>       typedef range_c<int,0,3> types;
>       typedef fold
>       < types
>       , empty_base
>       , inherit< arg<2>, arg<1> >
>       >::type tuple_type;
>
> I'd like to use for_each on tuple_type.  For example:
>
> struct print_integral_c
> {
>     template<int i, typename Right>
>     void operator()(inherit<integral_c<int,i>, Right>)const
>     {
>         std::cout<<i<<"\n";
>     }
> };
>
> for_each<tuple_type>(print_integral_c);
>
> However, tuple_type is not a sequence; hence, for_each won't work.
> I could define the necessary specializations for mpl::begin, etc.,
> to make it a sequence, but I was wondering if there's any way
> or any plans to somehow automagically make fold or reverse_fold
> into a sequence so I could use for_each on it.

No.  Since fold and reverse_fold are metafunctions (and therefore
class templates), they can't possibly be sequences (which are always
types).  fold and reverse_fold don't even have to _return_ sequences.

I can't understand why you're not using Fusion, or at least trying to
fit into the Fusion framework, but I'm going to stop flogging that
horse now.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com