[Boost-users] [MPL] Filtering a sequence to leave only most-derived types

Subject: [Boost-users] [MPL] Filtering a sequence to leave only most-derived types
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2010-02-23 06:10:45

Hi,

I've got an MPL sequence containing a (rather small) number of classes. I
want to filter this sequence so that all classes that are a base of another
class in the sequence are filtered out. I thought I'd do something like
this (in pseudo-code):

is_strict_base_of(Base, Derived) =
  !is_same(Base, Derived) && is_base_of(Base, Derived)

Result =
  remove_if(
    Interfaces,
    find_if(
      Interfaces,
      is_strict_base_of(outer_value, inner_value)
    ) != end(Interfaces)
  )

where outer_value is the iteration value of the remove_if loop, and
inner_value is the iteration value of the count_if loop.
Yes, this is N², but since the length of the initial sequence is very
unlikely to exceed 8, I can live with that.

In MPL syntax:

template <typename Base, typename Derived>
struct is_strict_base_of :
    mpl::and_<
        mpl::not_<boost::is_same<Base, Derived> >,
        boost::is_base_of<Base, Derived>
    >
{};

typedef typename mpl::remove_if<
    Interfaces, // Remove from Interfaces
    mpl::not_<mpl::equal<
        mpl::find_if<
            Interfaces, // if there is any value in interfaces
            // that this is a base of
            is_strict_base_of<???>
        >,
        mpl::end<Interfaces>
    > >
>::type Bases;

How do I represent outer_value and inner_value in this expression? It
seems I can't simply put mpl::_1 and mpl::_2 there, since that gives me a
static assertion failure (argument is not available). I think I have to put
an apply somewhere in there, but I can't figure out where.

Sebastian