thread promise get_future(),get(), promise set_value()

#include <chrono>
#include <ctime>
#include <future>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>
#include <thread>
#include <unistd.h>
#include <uuid/uuid.h>

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::stringstream ss;
    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());
    ss << std::put_time(&tm_info, "%Y%m%d%H%M%S")
       << std::setw(3) << std::setfill('0') << std::to_string(mills.count() - seconds.count() * 1000)
       << std::setw(3) << std::setfill('0') << 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();
}

char *uuid_value = (char *)malloc(40);
char *get_uuid()
{
    uuid_t new_uuid;
    uuid_generate(new_uuid);
    uuid_unparse(new_uuid, uuid_value);
    return uuid_value;
}

void fill_promise(std::promise<std::string> *promise_obj,const int&len)
{
    std::stringstream ss;
    for(int i=0;i<len;i++)
    {
        ss<<i+1<<","<<get_uuid()<<std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
    }
    std::cout<<get_time_now()<<",in "<<__FUNCTION__<<std::endl;
    promise_obj->set_value(ss.str());
}

void thread_promise_get(const int &len)
{
    std::promise<std::string> promise_obj;
    std::thread t1(fill_promise,&promise_obj,std::cref(len));
    std::cout<<promise_obj.get_future().get()<<std::endl;
    t1.join();
}

int main(int args, char **argv)
{
    thread_promise_get(atoi(argv[1]));
}

g++ -g -std=c++2a -I. *.cpp -o h1 -luuid