Re: [Boost-users] newbie's questions about multi_index

From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2007-04-02 11:20:40

Hi Guillaume

----- Mensaje original -----
De: Guillaume Lazzara <glazzara_at_[hidden]>
Fecha: Lunes, Abril 2, 2007 5:13 am
Asunto: [Boost-users] newbie's questions about multi_index
Para: boost-users_at_[hidden]

> Hi,
> 
> I'm currently using multi_index and I have few questions:
> 
> - I have a multi_index with composed keys, sometimes I need to get a
> range of items, iterate over them and modify their key. How can I do
> this properly?
> 
> sample code:
> 
> EquivSet::iterator tmp;
> std::pair<EquivSet::iterator, EquivSet::iterator> es =
> contains.equal_range(*it);
> for (EquivSet::iterator it_equiv = es.first; it_equiv != es.second;)
>  if (!X[it_equiv->id])
>  {
>    tmp = it_equiv++;
>    contains.modify_key(tmp, update_equiv(nbEns));
>  }
>  else
>    it_equiv++;
> 
> In this example, the second iterator in the pair, normaly used as end
> iterator, can be changed and, sometimes, the loop iterates beyond the
> latter.

Well, I'd say that the potential problem is not that es.second
is modified when inside the loop, but rather that elements are
repositioned between es.second-1 and es.second, so that the
loop revisits them again, right?

A solution is to modify the loop so as to not use es.second
but the immediately preceding position, just as this (beware,
uncompiled code follows):

  std::pair<EquivSet::iterator, EquivSet::iterator> es =
    contains.equal_range(*it);
  if(es.first!=es.second){
    EquivSet::iterator last=es.second;
    --last;
    EquivSet::iterator next=es.first;
    EquivSet::iterator it_equiv;
    do{
      it_equiv=next;
      ++next;
      contains.modify_key(it_equiv, update_equiv(nbEns));
    }while(it_equiv!=last);
  }

Does this work?

> - If I can choose between a std::set and a multi_index used as
> a set, which one should I use?

If you don't use any of the added functionality provided
by Boost.MultiIndex (suboject searching, modify(), etc.) using
std::set will yield faster compile times. Other than that,
a multi_index_container with a single ordered_unique index is
entirely equivalent to std::set.

> 
> Thank you very much for your answers!

Thank you for using Boost.MultiIndex, good luck with your
project,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo