I have a stream of pairs of elements of type T, and want to insert individual T's uniquely into a container and convert the pairs to a pair of indices into this container. Boost.MultiIndex seems to be cut out exactly for this job, but I not sure about how to obtain the indices or stability of them on addition: Take, namespace bmi = boost::multi_index; typedef bmi::multi_index_container< T, bmi::indexed_by < bmi::hashed_unique< bmi::identity<T> > , bmi::random_access<>

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MIC;

MIC elements; std::vector<std::pair<int,int>> pairs; for( const std::pair<T,T>& pt : stream ) { auto t1 = elements.insert(pt.first).first; // not all insertions are successful of course auto t2 = elements.insert(pt.second).first; pairs.insert( index_of(t1), index_of(t2) ); } // where index_of() is auto index_of = [&]( MIC::const_iterator i ) { return elements.project<1>(i) - elements.get<1>().begin(); } So my questions are (1) Is the implementation of index_of() correct? (or better, is there something like this already for random access indices) (2) Is the way they are used in above loop stable, where elements are inserted after an index is obtained? Thanks Nick