Asynchronous file reading in C++

A program written by my friend Tom to demonstrate multi treading concept of programming in C++. Thank you Tom for sharing your code to us.

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Program Listing

#include <iostream>
#include <iomanip>
#include <fstream>
#include <sstream>
#include <string>
#include <thread>
#include <future>
#include <chrono>

const std::string lorem = R"(
Lorem ipsum dolor sit amet, consetetur sadipscing
elitr, sed diam nonumy eirmod tempor invidunt ut
labore et dolore magna aliquyam erat, sed diam
voluptua. At vero eos et accusam et justo duo
dolores et ea rebum.
)";

void create_file(std::ostream& os, const std::string& s, int count)
{
   for (int i = 0; i < count; ++i)
     os << s << '\n';
}

std::string read_file(std::istream& is)
{
   std::stringstream ss;
   std::string line;
   int count = 0;

   while(std::getline(is, line))
   {
     ss << line << '\n';
     ++count;
     if (count % 5000 == 0)
     {
       std::cout << "\nThread " << std::this_thread::get_id() << " read
" <<
       count << " lines\n";
       std::this_thread::sleep_for(std::chrono::milliseconds(500));
     }
   }
   return ss.str();
}


int main()
{
   std::cout << "Main thread " << std::this_thread::get_id() << "
started\n\n";
   std::ofstream dest("lorem.txt");
   if (!dest)
   {
     perror("Error creating file. ");
     return EXIT_FAILURE;
   }
   create_file(dest, lorem, 10000);
   dest.close();

   std::ifstream src("lorem.txt");
   if (!src)
   {
     perror("Error opening file. ");
     return EXIT_FAILURE;
   }
   std::future<std::string> content = std::async(std::launch::async,
                                      [&] {return
read_file(std::ref(src));});
   for (int i = 0; i <= 100; ++i)
   {
     if (i % 10 == 0)
     {
       std::cout << "Thread: # " << std::this_thread::get_id()
                 << " i = " << i << '\n';
     }
     // sim long task
     std::this_thread::sleep_for(std::chrono::milliseconds(75));
   }
   std::string str = content.get();
   std::cout << "Length of file: " << str.size() << '\n';
}