Re: [Boost-users] [mpl] erase_key confusion

From: David Abrahams (dave_at_[hidden])
Date: 2007-03-23 10:47:28

on Fri Mar 23 2007, Scott Meyers <usenet-AT-aristeia.com> wrote:

> David Abrahams wrote:
>> Nothing; this is clearly a bug in MPL
>
> As a workaround, I thought I'd write an erase_first function as shown 
> below, but this seems to have the same problem.  Perhaps this is because 
>   I'm using erase in my implementation, and erase_key may, I suppose, be 
> implemented on top of erase.  

It is *extremely* likely that erase_key and erase produce the same
result.

> More likely, I'm doing something wrong. 

Not all that likely.

> More likely still, the way I'm doing it is gross.  

Well yeah.  The un-gross way is to wait for Aleksey to fix it.  He's
unfortunately in a work crunch at the moment, but I'd guess he could
address it within a couple of days.

> I'd appreciate 
> feedback on the implementation of my erase_first function, where I've 
> put comments showing the procedural equivalent of what I'm trying to do 
> in each statement.
>
> Also, does anybody have a workaround for the bug in erase_key?

It's highly unlikely that any opportunity for such a workaround exists
in the set code.

> Thanks,
>
> Scott
>
> #include <boost/mpl/set.hpp>
> #include <boost/mpl/equal.hpp>
> #include <boost/mpl/erase.hpp>
> #include <boost/mpl/if.hpp>
> #include <boost/mpl/end.hpp>
> #include <boost/mpl/find.hpp>
>
> namespace mpl = boost::mpl;
> using mpl::_1;
> using mpl::_2;
>
> struct A {};
> struct B {};
>
> typedef mpl::set<A, B> set1;
> typedef mpl::set<B, A> set2;
>
>
> // return Seq with the first occurrance of T removed
> template<typename Seq, typename T>
> struct erase_first {
>    // i = Seq.find(T)
>    typedef typename mpl::find<Seq, T>::type i;
>
>    // found = (i == Seq.end()) ? false : true
>    typedef typename mpl::if_<
>      typename boost::is_same<i,
>                              typename mpl::end<Seq>::type
>                             >::type,
>      mpl::false_,
>      mpl::true_
>      >::type found;

  mpl::if_<  pred, mpl::false_, mpl::true_>::type 

is usually better written

  mpl::not_<pred>
 
>    // type = found ? Seq : Seq.erase(i)
>    typedef typename mpl::if_<found,
>                              Seq,
>                              typename mpl::erase<Seq, i>::type
>                          >::type type;
> };

>From scratch (untested):

     template <class Seq, class E>
     struct erase_first
     {
         typedef typename mpl::find<Seq, T>::type i;
         typedef typename eval_if<
             boost::is_same<i, typename mpl::end<Seq> >
           , mpl::identity<Seq>
           , mpl::erase<Seq,i>
         >::type type;
     };

HTH,

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