RE: [boost] mpl transform

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-11-26 05:10:10

David Abrahams wrote:
> Well I hope that was all instructive, but it was probably way more
> complicated than neccessary. I think it would actually be far superior
> to build a solution around an mpl iterator adaptor like this one:
> 
>    template <class IteratorSeq> struct zip_iterator
>    {
>         typedef typename mpl::transform<
>               IteratorSeq,mpl::apply0<mpl::_1>
>          >::type type;
> 
>         typedef zip_iterator<
>             typename mpl::transform<
>                IteratorSeq,mpl::next<mpl::_1>
>             >::type
>         > next;
>    };
> 
> and an operator adaptor like this one (don't know how to generalize
> this one to N arguments yet):
> 
>   template <class Op, class Seq>
>   struct binop_adaptor
>       : mpl::apply2<
>            Op
>            , typename mpl::begin<Seq>::type
>            , typename mpl::next<mpl::begin<Seq>::type>::type
>         >
>   {
>   };
> 
> How about that?

To finish the sketch:

    template<
          typename Sequences
        >
    struct zip_view
    {
        typedef typename transform< Sequences, begin<_1> >::type
first_ones_;
        typedef typename transform< Sequences, end<_1> >::type last_ones_;
        
        typedef nested_begin_end_tag tag;
        typedef zip_iterator<first_ones_> begin;
        typedef zip_iterator<last_ones_> end;
    };

    // corrected 'binop_adaptor'
    template< typename BinaryOp >
    struct apply_two_args_seq
    {
        template< typename Args > struct apply
            : apply2<
                  BinaryOp
                , typename at_c<Args,0>::type
                , typename at_c<Args,1>::type
                >
        {
        };
    };

    // user code
    typedef range_c<int,0,10> s1;
    typedef range_c<int,10,20> s2;

    typedef transform_view<
          zip_view< list<s1,s2> >
        , apply_two_args_seq< plus<> >
        > result;

    // here!
    BOOST_STATIC_ASSERT((equal< 
          result
        , filter_view< range_c<int,10,30>, math::is_even<_> >
        , equal_to<_,_>
        >::type::value));

The 'zip_view' is in the CVS, as well as the rest of the sample -
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/boost/boost/libs/m
pl/test/zip_view.cpp. 'apply_two_args_seq' needs generalization, so it
didn't go into a header. 

Note that the sample doesn't work on all the supported compilers yet.

Aleksey