text: //
  // server.cpp
  // ~~~~~~~~~~
  //
  // Copyright (c) 2003-2015 Christopher M. Kohlhoff (chris at kohlhoff dot com)
  //
  // Distributed under the Boost Software License, Version 1.0. (See accompanying
  // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  //
  
  #include <ctime>
  #include <iostream>
  #include <string>
  #include <boost/array.hpp>
  #include <boost/bind.hpp>
  #include <boost/shared_ptr.hpp>
  #include <asio.hpp>
  
  using asio::ip::udp;
  
  std::string make_daytime_string()
  {
    using namespace std; // For time_t, time and ctime;
    time_t now = time(0);
    return ctime(&now);
  }
  
  class udp_server
  {
  public:
    udp_server(asio::io_context& io_context)
      : socket_(io_context, udp::endpoint(udp::v4(), 13))
    {
      start_receive();
    }
  
  private:
    void start_receive()
    {
      socket_.async_receive_from(
          asio::buffer(recv_buffer_), remote_endpoint_,
          boost::bind(&udp_server::handle_receive, this,
            asio::placeholders::error,
            asio::placeholders::bytes_transferred));
    }
  
    void handle_receive(const asio::error_code& error,
        std::size_t /*bytes_transferred*/)
    {
      if (!error || error == asio::error::message_size)
      {
        boost::shared_ptr<std::string> message(
            new std::string(make_daytime_string()));
  
        socket_.async_send_to(asio::buffer(*message), remote_endpoint_,
            boost::bind(&udp_server::handle_send, this, message,
              asio::placeholders::error,
              asio::placeholders::bytes_transferred));
  
        start_receive();
      }
    }
  
    void handle_send(boost::shared_ptr<std::string> /*message*/,
        const asio::error_code& /*error*/,
        std::size_t /*bytes_transferred*/)
    {
    }
  
    udp::socket socket_;
    udp::endpoint remote_endpoint_;
    boost::array<char, 1> recv_buffer_;
  };
  
  int main()
  {
    try
    {
      asio::io_context io_context;
      udp_server server(io_context);
      io_context.run();
    }
    catch (std::exception& e)
    {
      std::cerr << e.what() << std::endl;
    }
  
    return 0;
  }
  