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

Return to the tutorial index

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