C++ Bimap Left unordered_map Right sorted mutable multimap

I need to implement the following datastructure for my project. I have a relation of

const MyClass* 

to

uint64_t

For every pointer I want to save a counter connected to it, which can be changed over time (in fact only incremented). This would be no problem, I could simply store it in a std::map. The problem is that I need fast access to the pointers which have the highest values.

That is why I came to the conclusion to use a boost::bimap. It is defined is follows for my project:

typedef boost::bimaps::bimap<
        boost::bimaps::unordered_set_of< const MyClass* >,
        boost::bimaps::multiset_of< uint64_t, std::greater<uint64_t> >
> MyBimap;
MyBimap bimap;

This would work fine, but am I right that I can not modify the uint64_t on pair which were inserted once? The documentation says that multiset_of is constant and therefore I cannot change a value of pair in the bimap.

What can I do? What would be the correct way to change the value of one key in this bimap? Or is there a simpler data structure possible for this problem?

Here's a simple hand-made solution.

Internally it keeps a map to store the counts indexed by object pointer, and a further multi-set of iterators, ordered by descending count of their pointees.

Whenever you modify a count, you must re-index. I have done this piecemeal, but you could do it as a batch update, depending on requirements.

Note that in c++17 there is a proposed splice operation for sets and maps, which would make the re-indexing extremely fast.

#include <map>
#include <set>
#include <vector>

struct MyClass { };


struct store
{

    std::uint64_t add_value(MyClass* p, std::uint64_t count = 0)
    {
        add_index(_map.emplace(p, count).first);
        return count;
    }

    std::uint64_t increment(MyClass* p)
    {
        auto it = _map.find(p);
        if (it == std::end(_map)) {
            // in this case, we'll create one - we could throw instead
            return add_value(p, 1);
        }
        else {
            remove_index(it);
            ++it->second;
            add_index(it);
            return it->second;
        }
    }

    std::uint64_t query(MyClass* p) const {
        auto it = _map.find(p);
        if (it == std::end(_map)) {
            // in this case, we'll create one - we could throw instead
            return 0;
        }
        else {
            return it->second;
        }
    }

    std::vector<std::pair<MyClass*, std::uint64_t>> top_n(std::size_t n)
    {
        std::vector<std::pair<MyClass*, std::uint64_t>> result;
        result.reserve(n);
        for (auto idx = _value_index.begin(), idx_end = _value_index.end() ;
             n && idx != idx_end ;
             ++idx, --n) {
            result.emplace_back((*idx)->first, (*idx)->second);
        }
        return result;
    }

private:
    using map_type = std::map<MyClass*, std::uint64_t>;
    struct by_count
    {
        bool operator()(map_type::const_iterator l, map_type::const_iterator r) const {
            // note: greater than orders by descending count
            return l->second > r->second;
        }
    };
    using value_index_type = std::multiset<map_type::iterator, by_count>;

    void add_index(map_type::iterator iter)
    {
        _value_index.emplace(iter->second, iter);
    }

    void remove_index(map_type::iterator iter)
    {
        for(auto range = _value_index.equal_range(iter);
            range.first != range.second;
            ++range.first)
        {
            if (*range.first == iter) {
                _value_index.erase(range.first);
                return;
            }
        }

    }

    map_type _map;
    value_index_type _value_index;
};