cpp get exact time and precision reach nanoseconds via std::chrono::high_resolution_clock

#include <chrono>
#include <ctime>
#include <iomapip>
#include  <iostream>
#include <sstream>


std::string get_time_now()
{
    std::chrono::time_point<std::chrono::high_resolution_clock> now = std::chrono::high_resolution_clock::now();
    time_t raw_time = std::chrono::high_resolution_clock::to_time_t(now);
    struct tm tm_info = *localtime(&raw_time);
    std::chrono::seconds seconds = std::chrono::duration_cast<std::chrono::seconds>(now.time_since_epoch());
    std::chrono::milliseconds mills = std::chrono::duration_cast<std::chrono::milliseconds>(now.time_since_epoch());
    std::chrono::microseconds micros = std::chrono::duration_cast<std::chrono::microseconds>(now.time_since_epoch());
    std::chrono::nanoseconds nanos = std::chrono::duration_cast<std::chrono::nanoseconds>(now.time_since_epoch());
    std::stringstream ss;
    ss << std::put_time(&tm_info, "%Y%m%d%H%M%S") << std::setfill('0') << std::setw(3) << std::to_string(mills.count() - seconds.count() * 1000)
       << std::setfill('0') << std::setw(3) << std::to_string(micros.count() - mills.count() * 1000) << std::setw(3) << std::setfill('0') << std::to_string(nanos.count() - micros.count() * 1000);
    return ss.str();
}

int main(int args,char **argv)
{
   int len=atoi(argv[1]);
　　for(int i=0;i<len;i++)
   {
　　   std::cout<<i+1<<","<<get_time_now()<<std::endl;
   }
}