从Boost多索引迭代器获取数字索引 [英] Get numeric index from Boost multi-index iterator
问题描述
我存储了以下一组
struct文章{
std :: string title;
unsigned db_id; // MediaWiki数据库转储中的id字段
};在Boost.MultiIndex容器中的
,定义为
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& >
>
>文章集;
现在我有两个迭代器,一个来自 index< by_title>
和一个来自 index< by_id>
。在 struct Article
?
中添加数据成员时,将这些索引转换为容器的随机访问部分的最简单方法是什么? div class =h2_lin>解决方案
每个索引都支持使用iterator_to 。如果你已经在一个索引中有一个迭代器到目标值,你可以使用它来转换为另一个索引中的迭代器。
iterator iterator_to(const value_type& x);
const_iterator iterator_to(const value_type& x)const;
对于转换到索引,你可以按照 random_access_index.hpp
:
迭代器擦除(迭代器第一,迭代器最后)
{
BOOST_MULTI_INDEX_CHECK_VALID_ITERATOR (第一);
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;
}
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
?
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;
}
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