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Subject: Re: [Boost-users] [multi-index] Container of polymorphic pointers
From: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2008-10-30 03:10:39

Daniel Dilts <diltsman <at>> writes:

> I'm am using boost::multi_index to hold a set of polymorphic entities.
> Due to efficiency concerns, I have to be able sort the container based
> on actual type.  I was planning on sorting based on the return result of
> typeid.  When I try this I get errors in <functional>.  I am using
> VC++ 2005 Standard Edition SP1.  What am I doing wrong, or, what is a
> better way of accomplishing what I am trying to do?
>     struct extractType
>     {
>         typedef const type_info& result_type; 

Here is the first problem: you've got to define

    typedef type_info result_type

and let operator() return value be const result_type& (instead
of the current return_type).

Past the first hurdle you'll find that type_info is not comparable
with <, so you'll also need to provide a custom comparer for this type
based on type_info::before. The following summarizes all the changes
needed (note: do not forget to include <typeinfo>; also, you're using
type_info instead of std::type_info, which seems to work in VS but is not
standard as far as I know):

namespace Entities
    struct type{};
    struct extractType
        typedef std::type_info result_type;

        const result_type& operator()(const Entity * r)const
            return typeid(*r);
    struct lessType
      bool operator()(const std::type_info& x, const std::type_info& y)const
            return x.before(y);
    typedef boost::multi_index::multi_index_container<
        Entity *,
                boost::multi_index::tag<type>, extractType, lessType
> EntityContainer;

Hope this helps. Thank you for using Boost.MultiIndex.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

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