Re: [Boost-users] multi_index_comtainer questions

Subject: Re: [Boost-users] multi_index_comtainer questions
From: Vladimir Voronkov (voronkovv_at_[hidden])
Date: 2009-10-16 10:59:20

I understand the idea with exposing interval for interator. This is not always useful. For example I implement some class with this container as a private member. I would expose useful interface from this class which hides all work with these flags as well as data representation. And at the same time I would like to avoid copying data into another container just to expose the necessary data. Imagine the following function.

UsersSelected& MyClass::GetSelectedUsers() const
{
  return m_container.get<2>(--- here we would put some condition ---);
}

Client of this code might not be aware of what container "UsersSelected" is, and just use it as regular set. This interface seems me more elegant then returning range of iterators because client should know what lies behind these iterators.

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] [mailto:boost-users-
> bounces_at_[hidden]] On Behalf Of Andrew Holden
> Sent: Friday, October 16, 2009 6:29 PM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] multi_index_comtainer questions
> 
> Vladimir Voronkov wrote on Thursday, October 15, 2009 10:20 AM:
> > Hi again,
> >
> > Thanks for the answers, they helped my. Now I have come with some
> another probably stupid questions.
> >
> > I've changed the initial example by adding some attribute to the
> element. Let's say int.
> >
> > typedef std::string UserInfo;
> > typedef std::less<std::string> UserInfoLess;
> >
> > typedef boost::multi_index_container<
> >	std::pair<UserInfo, int>,
> >	boost::multi_index::indexed_by<
> >		boost::multi_index::sequenced<>,
> >		boost::multi_index::ordered_unique<
> >			boost::multi_index::member<std::pair<UserInfo, int>,
> UserInfo, &std::pair<UserInfo, int>::first>,
> >			UserInfoLess>,
> >		boost::multi_index::ordered_non_unique<
> >			boost::multi_index::member<std::pair<UserInfo, int>,
> int, &std::pair<UserInfo, int>::second> >
> 	> >
> > UsersContainer;
> >
> >typedef UsersContainer::nth_index<2>::type UsersSelected;
> >
> > This flag has not many possible values and most probably there will be
> many pairs in the container with the > same value of this flag. Now I want
> to enumerate all values in the container with some specific value of this
> flag. There are many ways to do that.
> >
> > 1. Use lower_bound for getting iterator for the UsersSelected index.
> > 2. implement own iterator that use some int value as a parameter.
> >
> > But the ideal solution would be to expose some kind of "view" of the
> container that behaves like list, so we > could use it's iterator from
> begin() to end() taking only necessary elements.
> 
> I have found this to be a very common use case for multi-index containers.
> I normally use ordered indices and composite keys to do this.
> Specifically, I would recommend the following index:
> 
> boost::multi_index::ordered_unique  //ordered_non_unique will also work
> <
> 	boost::multi_index::composite_key
> 	<
> 		std::pair<UserInfo, int>,
> 		boost::multi_index::member<std::pair<UserInfo, int>, int,
> &std::pair<UserInfo, int>::second>,
> 		boost::multi_index::member<std::pair<UserInfo, int>, UserInfo,
> &std::pair<UserInfo, int>::first>
> 	>
> >
> 
> typedef UsersContainer::nth_index<2>::type::iterator UsersByFlagIterator;
> 
> 
> You can now get a sorted list of all users with a certain flag using:
> 
> std::pair<UsersByFlagIterator, UsersByFlagIterator> range =
> my_container::get<2>().equal_range(boost::make_tuple(desired_flag));
> 
> 
> This index can also find a specific flag, name combination like this:
> 
> UsersByFlagIterator entry =
> my_container::get<2>().find(boost::make_tuple(desired_flag,
> desired_name));
> 
> 
> > -----Original Message-----
> > From: boost-users-bounces_at_[hidden] [mailto:boost-users-
> > bounces_at_[hidden]] On Behalf Of Joaquin M Lopez Munoz
> > Sent: Tuesday, October 13, 2009 8:16 PM
> > To: boost-users_at_[hidden]
> > Subject: Re: [Boost-users] multi_index_comtainer questions
> >
> > Andrew Holden <aholden <at> charteroaksystems.com> writes:
> >
> > >
> > > On Tuesday, October 13, 2009 9:53 AM, Vladimir Voronkov wrote:
> > > > Do iterators keep the property of set’s
> > > > iterators that they remain valid after insertion of an element?
> > > [...]
> > >
> > > As far as I know, all iterators to all indices of a multi-index
> > > remain valid unless they point to a deleted element.
> >
> > This is correct.
> >
> > Joaquín M López Muñoz
> > Telefónica, Investigación y Desarrollo
> >
> >
> >
> > _______________________________________________
> > Boost-users mailing list
> > Boost-users_at_[hidden]
> > http://lists.boost.org/mailman/listinfo.cgi/boost-users
> 
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