Re: [boost] [mpl] is there a or_seq like logical metafunction?

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-03-07 08:37:05

On Thu, Mar 6, 2008 at 8:12 PM, vicente.botet <vicente.botet_at_[hidden]> wrote:
>  From: "Steven Watanabe" <watanabesj_at_[hidden]>
>  > AMDG
>  >
>  > vicente.botet wrote:
>  >> Hello,
>  >>
>  >> I needs calculate the mlp::or_ of a sequence of nullary logical
>  >> metafunction.
>  >>
>  >> This seams quite simple. I have do the following:
>  >> <code>
>  >> template <typename Seq>
>  >> struct or_seq;
>  >>
>  >> template <typename Seq, long N>
>  >> struct or_seq_aux : mpl::or_<
>  >>    typename mpl::front<Seq>::type,
>  >>    or_seq<typename mpl::pop_front<Seq>::type>
>  >>   > {};
>  >>
>  >> template <typename Seq>
>  >> struct or_seq_aux<Seq, 1> : mpl::front<Seq>::type {};
>  >>
>  >> template <typename Seq>
>  >> struct or_seq : or_seq_aux<Seq, mpl::size<Seq>::value> {};
>  >> </code>
>  >>
>  >> Is there a boost or_seq like logical metafunction?
>  >> Is there a better way to do it?
>  >>
>  >> I would be extremely interested in any suggestions.
>  >>
>  > not_<is_same<typename find_if<Seq, _>::type, typename end<Seq>::type> >?
>  >
>  > In Christ,
>  > Steven Watanabe
>
>  I have tested your much more compact solution and it works as expected.
>  Here it is with the mpl namespace:
>
>  template <typename Seq>
>  struct or_seq : mpl::not_<
>         boost::is_same<typename mpl::find_if<Seq, mpl::_>::type,
>                                     typename mpl::end<Seq>::type>
>      > {};
>

It seems a perfect application for fold:

template<typename Seq>
struct or_seq
    : mpl::apply<
          mpl::fold<mpl::_1, mpl::false_,  mpl::quote2<mpl::or_> >
        , Seq
      > {};

template<typename Seq>
struct and_seq
    : mpl::apply<
          mpl::fold<mpl::_1, mpl::true_,  mpl::quote2<mpl::and_> >
        , Seq
      > {};

--
gpd