Как асинхронно обработать unordered_map

std::unordered_map<std::string, std::string>

unordered_map<string, string> m = Init();
vector<std::thread> threads;
 
for (size_t i = std::thread::hardware_concurrency(); i > 0; --i) {
    // for loop ???
    threads.emplace_back(do_smth, it); // Here it is iterator of element in m
  
}
 
while (!threads.empty()) {
     if (threads.back().joinable()) {
        threads.back().join();
     }
     threads.pop_back();
}

#include <iostream>
#include <string>
#include <unordered_map>
#include <algorithm>
#include <execution>
#include <thread>
 
int main()
{
    std::unordered_map<std::string, std::string> m
    {
        {"1","11111"},
        {"2","33333"},
        {"3","5555"},
        {"4","11111"},
        {"5","33333"},
        {"6","5555"},
        {"7","11111"},
        {"8","33333"},
        {"9","5555"},
        {"a","11111"},
        {"b","33333"},
        {"c","5555"},
        {"d","11111"},
        {"e","33333"},
        {"f","5555"},
    };
    
    std::for_each(std::execution::par,m.cbegin(),m.cend(),[](const auto& pair)
    {
        std::cout<<"thread id="<<std::this_thread::get_id()<<": "<<pair.first<<','<<pair.second<<'\n';
    });
}