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<p>Solution 2 (maintaining a separate index for each serotype can be done with a little metaprogramming:</p> <pre><code>#include <boost/mpl/push_back.hpp> #include <boost/mpl/range_c.hpp> #include <boost/mpl/fold.hpp> #include <boost/mpl/for_each.hpp> #include <boost/mpl/vector.hpp> #include <boost/multi_index_container.hpp> #include <boost/multi_index/ordered_index.hpp> #include <boost/shared_ptr.hpp> using namespace boost::multi_index; struct host { static const int NUM_SEROTYPES=8; bool infections[NUM_SEROTYPES]; }; typedef boost::shared_ptr<host> host_ptr; template<typename Num> struct infection_extractor { typedef bool result_type; result_type& operator()(const host_ptr& p)const { return p->infections[Num::value]; } }; typedef boost::mpl::fold< boost::mpl::range_c<int,0,host::NUM_SEROTYPES>, boost::mpl::vector<>, boost::mpl::push_back< boost::mpl::_1, ordered_non_unique<infection_extractor<boost::mpl::_2> > > >::type index_list_type; typedef multi_index_container< host_ptr, index_list_type > multi_t; template<int Num> std::size_t infected_with(const multi_t& m) { return m.get<Num>().count(true); }; typedef std::size_t (*infected_with_type)(const multi_t&); struct infected_with_table_assigner { infected_with_table_assigner(infected_with_type* table):table(table) { boost::mpl::for_each< boost::mpl::range_c<int,0,host::NUM_SEROTYPES> >(*this); } template<typename Num> void operator()(Num)const{table[Num::value]=&infected_with<Num::value>;} infected_with_type* table; }; std::size_t infected_with(const multi_t& m,int n) { static infected_with_type table[host::NUM_SEROTYPES]; static infected_with_table_assigner assigner(table); return table[n](m); } #include <iostream> int main() { multi_t m; host h; for(int i=0;i<200;++i){ for(int j=0;j<host::NUM_SEROTYPES;++j)h.infections[j]=(i&(1<<j))?true:false; m.insert(host_ptr(new host(h))); } for(int n=0;n<host::NUM_SEROTYPES;++n) { std::cout<<"infected with serotype "<<n<<": " <<infected_with(m,n)<<std::endl; } } </code></pre> <p>But take into account that maintaining 8~10 indices has a memory and insertion time penalty. A simpler solution is to just maintain one <a href="http://www.boost.org/libs/multi_index/doc/tutorial/indices.html#rnd_indices" rel="nofollow noreferrer">random access index</a> and sort this appropriately (with a custom comparer suited to the particular serotype of interest in each moment) before querying.</p>
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1. PO(How) Can I approximate a "dynamic" index (key extractor) for Boost MultiIndex?
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1. USJoaquín M López Muñoz
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1. COThis is exactly the sort of thing that I haven't done before and wasn't sure could be done. Thanks also for pointing out the memory and insertion time penalties. I think I will avoid it for now, since we will probably start with many serotypes. The random access index, if I understand it correctly, is also slightly costly with insertions and deletions. Option four appears cheapest, assuming that most of the time I need only the number of people of a given age infected with a serotype.
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