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int main()
{
  try
  {
Create a server object to accept incoming client requests, and run the boost::asio::io_service object.
    boost::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;
}
class udp_server
{
public:
The constructor initialises a socket to listen on UDP port 13.
  udp_server(boost::asio::io_context& io_context)
    : socket_(io_context, udp::endpoint(udp::v4(), 13))
  {
    start_receive();
  }
private:
  void start_receive()
  {
        The function ip::udp::socket::async_receive_from()
        will cause the application to listen in the background for a new request.
        When such a request is received, the boost::asio::io_service object will
        invoke the handle_receive() function with two arguments: a value
        of type boost::system::error_code indicating whether the operation succeeded
        or failed, and a size_t value bytes_transferred
        specifying the number of bytes received.
      
    socket_.async_receive_from(
        boost::asio::buffer(recv_buffer_), remote_endpoint_,
        boost::bind(&udp_server::handle_receive, this,
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
  }
        The function handle_receive() will service the client request.
      
  void handle_receive(const boost::system::error_code& error,
      std::size_t /*bytes_transferred*/)
  {
        The error parameter contains the result of the asynchronous
        operation. Since we only provide the 1-byte recv_buffer_ to
        contain the client's request, the boost::asio::io_service object would return
        an error if the client sent anything larger. We can ignore such an error
        if it comes up.
      
    if (!error || error == boost::asio::error::message_size)
    {
Determine what we are going to send.
      boost::shared_ptr<std::string> message(
          new std::string(make_daytime_string()));
We now call ip::udp::socket::async_send_to() to serve the data to the client.
      socket_.async_send_to(boost::asio::buffer(*message), remote_endpoint_,
          boost::bind(&udp_server::handle_send, this, message,
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred));
When initiating the asynchronous operation, and if using boost::bind(), you must specify only the arguments that match the handler's parameter list. In this program, both of the argument placeholders (boost::asio::placeholders::error and boost::asio::placeholders::bytes_transferred) could potentially have been removed.
Start listening for the next client request.
start_receive();
        Any further actions for this client request are now the responsibility of
        handle_send().
      
} }
        The function handle_send() is invoked after the service request
        has been completed.
      
  void handle_send(boost::shared_ptr<std::string> /*message*/,
      const boost::system::error_code& /*error*/,
      std::size_t /*bytes_transferred*/)
  {
  }
  udp::socket socket_;
  udp::endpoint remote_endpoint_;
  boost::array<char, 1> recv_buffer_;
};
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