Re: [Boost-users] [mpl] Implementing eraseVal

From: David Abrahams (dave_at_[hidden])
Date: 2007-03-29 21:52:39

on Thu Mar 29 2007, Scott Meyers <usenet-AT-aristeia.com> wrote:

> David Abrahams wrote:
>> Yeah, why struggle?  This problem looks tailor-made for filter_view.
>
> Skipping over the struggling comment (it's not like I'm deliberately choosing 
> it), 

Sorry.

> what makes filter_view a better choice?  With both implementations, doesn't 
> eraseVal::type yield a new sequence?

I didn't look too closely at what

  template<typename Seq, typename T>
  struct eraseVal
     : mpl::eval_if<
         boost::is_same<typename mpl::find<Seq, T>::type,
                        typename mpl::end<Seq>::type>,
         typename mpl::identity<Seq>::type,
         eraseVal<typename mpl::erase<Seq,
                    typename mpl::find<Seq,T>::type,
                    typename mpl::next<typename mpl::find<Seq,T>::type>::type
                    >::type,
                  T>
       >
  {};

was doing, but it seemed like a much bigger struggle than

  template<typename Seq, typename T>
  struct eraseVal
    : filter_view<Seq, mpl::not_<boost::is_same<_, T> > >
  {};

> If filter_view<Seq, Pred> is always better than copy_if<Seq, Pred>,
> why do we need both?

Oh, I didn't see copy_if anywhere.  I'm not sure filter_view is always
better, but the question of why we need both has definitely crossed my
mind, and I don't have a good answer.

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