Get numeric index from Boost multi-index iterator
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26-09-2019 - |
Question
I'm storing a bunch of the following
struct Article {
std::string title;
unsigned db_id; // id field in MediaWiki database dump
};
in a Boost.MultiIndex container, defined as
typedef boost::multi_index_container<
Article,
indexed_by<
random_access<>,
hashed_unique<tag<by_db_id>,
member<Article, unsigned, &Article::db_id> >,
hashed_unique<tag<by_title>,
member<Article, std::string, &Article::title> >
>
> ArticleSet;
Now I've got two iterators, one from index<by_title>
and one from index<by_id>
. What is the easiest way to transform these to indexes into the random access part of the container, without adding a data member to struct Article
?
Solution
Every index supports generation of an iterator by value using iterator_to. If you already have an iterator to the target value in one index, you could use this to convert to an iterator in another index.
iterator iterator_to(const value_type& x);
const_iterator iterator_to(const value_type& x)const;
For conversion to index you can likely follow the model in random_access_index.hpp
:
iterator erase(iterator first,iterator last)
{
BOOST_MULTI_INDEX_CHECK_VALID_ITERATOR(first);
BOOST_MULTI_INDEX_CHECK_VALID_ITERATOR(last);
BOOST_MULTI_INDEX_CHECK_IS_OWNER(first,*this);
BOOST_MULTI_INDEX_CHECK_IS_OWNER(last,*this);
BOOST_MULTI_INDEX_CHECK_VALID_RANGE(first,last);
BOOST_MULTI_INDEX_RND_INDEX_CHECK_INVARIANT;
difference_type n=last-first;
relocate(end(),first,last);
while(n--)pop_back();
return last;
}
OTHER TIPS
iterator_to
is a relatively new function in Boost (it's there since 1.35). It adds a little of the syntax sugar when using with default index. For older versions of Boost the function project
is the only choise. You can use project
as follows:
ArticleSet x;
// consider we've found something using `by_db_id` index
ArticleSet::index_const_iterator<by_db_id>::type it =
x.get<by_db_id>().find( SOME_ID );
// convert to default index ( `random_access<>` )
ArticleSet::const_iterator it1 = x.project<0>( it );
// iterator_to looks like:
ArticleSet::const_iterator it11 = x.iterator_to( *it );
// convert to index tagged with `by_title` tag
ArticleSet::index_const_iterator<by_title>::type it2 = x.project<by_title>( it );
// iterator_to doen't look better in this case:
ArticleSet::index_const_iterator<by_title>::type it2 = x.get<by_title>().iterator_to( *it );
// etc.