https://stackoverflow.com/questions/59034114/custom-omp-reduction-on-stdmaps

criticalSection.cpp

#include <iostream>
#include <map>
#include <utility>
#include <vector>

// template <class maptype>
void mapAdd(std::map<size_t, double> &inout, std::map<size_t, double> &in) {
  for (auto initer = in.begin(), outiter = inout.begin(); initer != in.end(); ++initer, ++outiter) {
    outiter->second += initer->second;
  }
}

template <class vectype, class typevectype>
std::map<size_t, double> foo(const vectype &typeDoubleVec,
                             const typevectype &typevec) {
  std::map<size_t, double> mymap;

  // ensure that mymap has all elements of myvec.
  for (auto &elem : typevec) {
    mymap[elem];
  }

#pragma omp parallel default(none) shared(typeDoubleVec, mymap)
  {
    // we create a local copy of mymap for each thread!
    auto localmap = mymap;
#pragma omp for
    // iterate over the vector and add them to the local map
    for (size_t i = 0; i < typeDoubleVec.size(); ++i) {
      localmap.at(typeDoubleVec[i].first) += typeDoubleVec[i].second;
    }
#pragma omp critical
    // sum up the local map to the global map using a critical section
    mapAdd(mymap, localmap);
  }
  return mymap;
}

int main() {
  std::vector<std::pair<size_t, double>> myvec{{1, 2.}, {3, 4.}, {1, 3.}, {3, 5.}, {2, 1.}};
  std::vector<size_t> types{1, 3, 2};

  auto sumMap = foo(myvec, types);

  for (auto &[type, sum] : sumMap) {
    std::cout << "type:" << type << "sum:" << sum << std::endl;
  }
}