### Daytime.3 - An asynchronous TCP daytime server

##### The main() function

    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.

        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;
    }

##### The tcp_server class

    class tcp_server
    {
    public:

The constructor initialises an acceptor to listen on TCP port 13.

      tcp_server(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(),
            std::bind(&tcp_server::handle_accept, this, new_connection,
              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 std::error_code& error)
      {
        if (!error)
        {
          new_connection->start();
        }

        start_accept();
      }

##### The tcp_connection class

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 std::enable_shared_from_this<tcp_connection>
    {
    public:
      typedef std::shared_ptr<tcp_connection> pointer;

      static pointer create(asio::io_context& io_context)
      {
        return pointer(new tcp_connection(io_context));
      }

      tcp::socket& socket()
      {
        return socket_;
      }

In the function `start()`, we call asio::async_write() to serve the data to the client. Note that we are using 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 `std::bind`, you must specify only the arguments that match the handler's parameter list. In this program, both of the argument placeholders (asio::placeholders::error and asio::placeholders::bytes_transferred) could potentially have been removed, since they are not being used in `handle_write()`.

        asio::async_write(socket_, asio::buffer(message_),
            std::bind(&tcp_connection::handle_write, shared_from_this(),
              asio::placeholders::error,
              asio::placeholders::bytes_transferred));

Any further actions for this client connection are now the responsibility of `handle_write()`.

      }

    private:
      tcp_connection(asio::io_context& io_context)
        : socket_(io_context)
      {
      }

      void handle_write(const std::error_code& /*error*/,
          size_t /*bytes_transferred*/)
      {
      }

      tcp::socket socket_;
      std::string message_;
    };

##### Removing unused handler parameters

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 asio::async_write() call used to initiate the call can then be changed to just:

    asio::async_write(socket_, asio::buffer(message_),
        std::bind(&tcp_connection::handle_write, shared_from_this()));

See the full source listing

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