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int main()
{
  try
  {
We need to create a server object to accept incoming client connections. The io_context object provides I/O services, such as sockets, that the server object will use.
    boost::asio::io_context io_context;
    tcp_server server(io_context);
Run the io_context object so that it will perform asynchronous operations on your behalf.
    io_context.run();
  }
  catch (std::exception& e)
  {
    std::cerr << e.what() << std::endl;
  }
  return 0;
}
class tcp_server
{
public:
The constructor initialises an acceptor to listen on TCP port 13.
  tcp_server(boost::asio::io_context& io_context)
    : io_context_(io_context),
      acceptor_(io_context, tcp::endpoint(tcp::v4(), 13))
  {
    start_accept();
  }
private:
        The function start_accept() creates a socket and initiates an
        asynchronous accept operation to wait for a new connection.
      
  void start_accept()
  {
    tcp_connection::pointer new_connection =
      tcp_connection::create(io_context_);
    acceptor_.async_accept(new_connection->socket(),
        boost::bind(&tcp_server::handle_accept, this, new_connection,
          boost::asio::placeholders::error));
  }
        The function handle_accept() is called when the asynchronous
        accept operation initiated by start_accept() finishes. It services
        the client request, and then calls start_accept() to initiate
        the next accept operation.
      
  void handle_accept(tcp_connection::pointer new_connection,
      const boost::system::error_code& error)
  {
    if (!error)
    {
      new_connection->start();
    }
    start_accept();
  }
        We will use shared_ptr and enable_shared_from_this
        because we want to keep the tcp_connection object alive as long
        as there is an operation that refers to it.
      
class tcp_connection
  : public boost::enable_shared_from_this<tcp_connection>
{
public:
  typedef boost::shared_ptr<tcp_connection> pointer;
  static pointer create(boost::asio::io_context& io_context)
  {
    return pointer(new tcp_connection(io_context));
  }
  tcp::socket& socket()
  {
    return socket_;
  }
        In the function start(), we call boost::asio::async_write()
        to serve the data to the client. Note that we are using boost::asio::async_write(),
        rather than ip::tcp::socket::async_write_some(),
        to ensure that the entire block of data is sent.
      
  void start()
  {
        The data to be sent is stored in the class member message_ as
        we need to keep the data valid until the asynchronous operation is complete.
      
message_ = make_daytime_string();
        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, since they are not being used in handle_write().
      
    boost::asio::async_write(socket_, boost::asio::buffer(message_),
        boost::bind(&tcp_connection::handle_write, shared_from_this(),
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
        Any further actions for this client connection are now the responsibility
        of handle_write().
      
  }
private:
  tcp_connection(boost::asio::io_context& io_context)
    : socket_(io_context)
  {
  }
  void handle_write(const boost::system::error_code& /*error*/,
      size_t /*bytes_transferred*/)
  {
  }
  tcp::socket socket_;
  std::string message_;
};
        You may have noticed that the error, and bytes_transferred
        parameters are not used in the body of the handle_write() function.
        If parameters are not needed, it is possible to remove them from the function
        so that it looks like:
      
  void handle_write()
  {
  }
The boost::asio::async_write() call used to initiate the call can then be changed to just:
  boost::asio::async_write(socket_, boost::asio::buffer(message_),
      boost::bind(&tcp_connection::handle_write, shared_from_this()));
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