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POHow to move an element without removing and re-inserting it into a boost::multi_index_container?
 

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  1. POHow to move an element without removing and re-inserting it into a boost::multi_index_container?
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    <p>I'm using a <a href="http://www.boost.org/doc/libs/1_46_1/libs/multi_index/doc/index.html" rel="nofollow">boost::multi_index_container</a> to provide a random access and a hash-based access to a collection of elements. I wanted to change the random access index of an element, without changing the hash-based index.</p> <p>Here is a piece of code :</p> <pre><code># include &lt;string&gt; # include &lt;boost/multi_index_container.hpp&gt; # include &lt;boost/multi_index/random_access_index.hpp&gt; # include &lt;boost/multi_index/hashed_index.hpp&gt; # include &lt;boost/multi_index/member.hpp&gt; using namespace std ; using namespace boost ; using namespace boost::multi_index ; // class representing my elements class Element { public : Element(const string &amp; new_key) : key(new_key) {} string key ; // the hash-based index in the multi_index_container // ... many stuff skipped private : // ... many stuff skipped } ; typedef multi_index_container&lt; Element, indexed_by&lt; random_access&lt; &gt;, hashed_unique&lt; member&lt;Element, string, &amp;Element::key&gt; &gt; &gt; &gt; ElementContainer ; typedef ElementContainer::nth_index&lt;0&gt;::type::iterator ElementRandomIter ; typedef ElementContainer::nth_index&lt;1&gt;::type::iterator ElementHashedIter ; int main(int, char*[]) { ElementContainer ec ; // insert some elements ec.push_back(Element("Alice")) ; // random-access index = 0 ec.push_back(Element("Bob")) ; // random-access index = 1 ec.push_back(Element("Carl")) ; // random-access index = 2 ec.push_back(Element("Denis")) ; // random-access index = 3 // Here I want to move "Denis" to position 1 // The (bad looking) solution I found involves removing and inserting the element ElementRandomIter it = ec.get&lt;0&gt;().begin() + 3 ; Element e = *(it) ; // store a copy ec.get&lt;0&gt;().erase(it) ; // remove the element it = ec.get&lt;0&gt;().begin() + 1 ; ec.get&lt;0&gt;().insert(it, e) ; // insert the copy // Elements are now in the following order // random-access index 0 : Alice // random-access index 1 : Denis // random-access index 2 : Bob // random-access index 3 : Carl return 0 ; } </code></pre> <p>I know that even if I only used random-access iterators in this example to manipulate elements, hashing occurs behind the scenes at least twice in the <code>multi_index_container</code>, in addition to an object copy, which can be expensive.</p> <p>Is there a method to change the random access index of an element inside a <code>boost::multi_index</code> without requiring an expensive remove-and-insert-while-keeping-a-copy ugliness ?</p> <p>I've searched in the <code>multi_index_container</code> documentation, perhaps I missed something. Thanks for any advice !</p> <p>Note : Sorry for possible English mistakes :)</p>
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