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

From: Scott Meyers (usenet_at_[hidden])
Date: 2007-03-23 00:22:55

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.  More likely, I'm doing something wrong. 
More likely still, the way I'm doing it is gross.  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?

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;

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

BOOST_MPL_ASSERT((
   mpl::equal<
     erase_first<set1, A>::type,
     erase_first<set2, A>::type
   >
));