Re: [Boost-users] [boost-users][multi-index] iterating though index keys

From: JOAQUIN M. LOPEZ MUÑOZ (joaquin_at_[hidden])
Date: 2008-06-12 15:58:59

_______________________________________
De: boost-users-bounces_at_[hidden] [boost-users-bounces_at_[hidden]] En nombre de Igor R [boost.lists_at_[hidden]]
Enviado el: jueves, 12 de junio de 2008 19:58
Para: boost-users_at_[hidden]
Asunto: Re: [Boost-users] [boost-users][multi-index] iterating though   indexkeys

>  Hi Joaquín,
>
>  >  for(Entries::iterator
>  > it_key=entries.begin(),end=entries.end();it_key!=end;){
>  >   // get all elements with key it_key->someForeignKey
>  >   std::pair<Entries::iterator,Entries::iterator> values=
>  >     entries.equal_range(it_key->someForeignKey);
>  >   while(values.first!=values.second){
>  >     // process element
>  >     ++values.first;
>  >   }
>  >   // next key is just that of  the element right after the current value
>  > group
>  >   it_key=values.second;
>  >  }
>
>  Thank you! It seems to be exactly what I need.
>  However, I'm not sure I got the principle right: how come that "next
>  key is just that of  the element right after the current value"? Is
>  this the general behavior of equal_range - despite of the kind of the
>  index (ordered/hashed, unique/non-unique)?

Yep, and the behavior of STL containers like std::multiset, for that
matter. This is easily seen if you consider that indices of a
multi_index_container (and STL containers) guarantee that elements
with equivalent key are grouped together. So, if you hop from
a group of same-key elements to the next you end up visiting
all the keys present in the container.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo