Re: [Boost-users] [MPL] 2 Newbie Questions

From: Scott Meyers (usenet_at_[hidden])
Date: 2007-03-17 01:38:10

David Abrahams wrote:
>   template <class OuterSequence, InnerSequence>
>   struct includes               
>     : boost::is_same<
>           typename mpl::find_if<
>               InnerSequence
>             , mpl::not_<
>                   mpl::contains<OuterSequence, _1>
>               >
>           >::type
>         , typename mpl::end<SubSequence>::type
>       >
>   {};

Okay, thanks very much.  This works for me, and I even pretty much 
understand it :-).

Unfortunately, I can't make the leap from this to what I really want, 
which is to provide a predicate to contains for determining whether two 
types are the same.

I think I need to do something like this, and I apologize in advance for 
what will probably strike you as appalling:

// returns whether Seq1 includes each of the elements in Seq2.
// Pred defines whether 2 types are the same.
template<typename Seq1,  // the putative superset
          typename Seq2,  // the putative subset
          typename Pred>  // whether T1 from Seq1 == T2 from Seq2
struct includes_if
: boost::is_same<
     typename find_if<
       Seq2,
       not_<
         is_same<
           find_if<Seq1,
                   Pred::apply<_1 from outer find_if,
                               _1 from inner find_if>
                  >
           typename end<Seq1>::type
        >
     >::type,
     typename end<Seq2>::type
   >
{};

I don't know how to refer to the current elements of the two calls to 
find_if, and I don't know whether it's a syntax issue or if I'm just 
approaching this incorrectly.

Help?

Thanks,

Scott