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@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Joaquin M Lopez Munoz Sent: Tuesday, October 13, 2009 8:16 PM To: boost-users@lists.boost.org 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
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