问在C++中迭代地图的一部分

EN

#include <map>
#include <string>
#include <vector>
#include <algorithm>
#include <future>
#include <iostream>

typedef std::map<std::string, std::vector<double>> map_type;

void do_work(map_type::iterator b, map_type::iterator e)
{
    std::for_each(b, e, [] (map_type::value_type const& p) 
    {
        std::for_each(p.second.begin(), p.second.end(), [] (double d) 
        {
            /* Process an element of the vector... */
        });
    });
}

int main()
{
    map_type m;

    size_t s = m.size();
    int quarter = s / 4;
    auto i1 = m.begin();
    auto i2 = std::next(i1, quarter);
    auto i3 = std::next(i2, quarter);
    auto i4 = std::next(i3, quarter);
    auto i5 = m.end();

    std::vector<std::future<void>> futures;
    futures.push_back(std::async(do_work, i1, i2));
    futures.push_back(std::async(do_work, i2, i3));
    futures.push_back(std::async(do_work, i3, i4));
    futures.push_back(std::async(do_work, i4, i5));

    for (auto& f : futures) { f.wait(); }
}

typedef std::map<std::string,std::vector<double>> data;

void process( data::iterator beg, data::iterator end );
data dt;
{
   auto task1 = std::async( process, dt.begin(), dt.lower_bound( "n" ) );
   auto task2 = std::async( process, dt.lower_bound( "n" ), dt.end() );
}