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<title>Daytime.3 - An asynchronous TCP daytime server</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tutdaytime3"></a><a class="link" href="tutdaytime3.html" title="Daytime.3 - An asynchronous TCP daytime server">Daytime.3 - An asynchronous
      TCP daytime server</a>
</h3></div></div></div>
<h5>
<a name="asio.tutorial.tutdaytime3.h0"></a>
        <span><a name="asio.tutorial.tutdaytime3.the_main___function"></a></span><a class="link" href="tutdaytime3.html#asio.tutorial.tutdaytime3.the_main___function">The
        main() function</a>
      </h5>
<pre class="programlisting">int main()
{
  try
  {
</pre>
<p>
        We need to create a server object to accept incoming client connections.
        The <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> object provides
        I/O services, such as sockets, that the server object will use.
      </p>
<pre class="programlisting">    asio::io_context io_context;
    tcp_server server(io_context);
</pre>
<p>
        Run the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> object
        so that it will perform asynchronous operations on your behalf.
      </p>
<pre class="programlisting">    io_context.run();
  }
  catch (std::exception&amp; e)
  {
    std::cerr &lt;&lt; e.what() &lt;&lt; std::endl;
  }

  return 0;
}
</pre>
<h5>
<a name="asio.tutorial.tutdaytime3.h1"></a>
        <span><a name="asio.tutorial.tutdaytime3.the_tcp_server_class"></a></span><a class="link" href="tutdaytime3.html#asio.tutorial.tutdaytime3.the_tcp_server_class">The
        tcp_server class</a>
      </h5>
<pre class="programlisting">class tcp_server
{
public:
</pre>
<p>
        The constructor initialises an acceptor to listen on TCP port 13.
      </p>
<pre class="programlisting">  tcp_server(asio::io_context&amp; io_context)
    : io_context_(io_context),
      acceptor_(io_context, tcp::endpoint(tcp::v4(), 13))
  {
    start_accept();
  }

private:
</pre>
<p>
        The function <code class="computeroutput">start_accept()</code> creates a socket and initiates an
        asynchronous accept operation to wait for a new connection.
      </p>
<pre class="programlisting">  void start_accept()
  {
    tcp_connection::pointer new_connection =
      tcp_connection::create(io_context_);

    acceptor_.async_accept(new_connection-&gt;socket(),
        boost::bind(&amp;tcp_server::handle_accept, this, new_connection,
          asio::placeholders::error));
  }
</pre>
<p>
        The function <code class="computeroutput">handle_accept()</code> is called when the asynchronous
        accept operation initiated by <code class="computeroutput">start_accept()</code> finishes. It services
        the client request, and then calls <code class="computeroutput">start_accept()</code> to initiate
        the next accept operation.
      </p>
<pre class="programlisting">  void handle_accept(tcp_connection::pointer new_connection,
      const asio::error_code&amp; error)
  {
    if (!error)
    {
      new_connection-&gt;start();
    }

    start_accept();
  }
</pre>
<h5>
<a name="asio.tutorial.tutdaytime3.h2"></a>
        <span><a name="asio.tutorial.tutdaytime3.the_tcp_connection_class"></a></span><a class="link" href="tutdaytime3.html#asio.tutorial.tutdaytime3.the_tcp_connection_class">The
        tcp_connection class</a>
      </h5>
<p>
        We will use <code class="computeroutput">shared_ptr</code> and <code class="computeroutput">enable_shared_from_this</code>
        because we want to keep the <code class="computeroutput">tcp_connection</code> object alive as long
        as there is an operation that refers to it.
      </p>
<pre class="programlisting">class tcp_connection
  : public boost::enable_shared_from_this&lt;tcp_connection&gt;
{
public:
  typedef boost::shared_ptr&lt;tcp_connection&gt; pointer;

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

  tcp::socket&amp; socket()
  {
    return socket_;
  }
</pre>
<p>
        In the function <code class="computeroutput">start()</code>, we call asio::async_write() to serve
        the data to the client. Note that we are using asio::async_write(), rather
        than <a class="link" href="../reference/basic_stream_socket/async_write_some.html" title="basic_stream_socket::async_write_some">ip::tcp::socket::async_write_some()</a>,
        to ensure that the entire block of data is sent.
      </p>
<pre class="programlisting">  void start()
  {
</pre>
<p>
        The data to be sent is stored in the class member <code class="computeroutput">message_</code> as
        we need to keep the data valid until the asynchronous operation is complete.
      </p>
<pre class="programlisting">    message_ = make_daytime_string();
</pre>
<p>
        When initiating the asynchronous operation, and if using <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a>
        , 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 <code class="computeroutput">handle_write()</code>.
      </p>
<pre class="programlisting">    asio::async_write(socket_, asio::buffer(message_),
        boost::bind(&amp;tcp_connection::handle_write, shared_from_this(),
          asio::placeholders::error,
          asio::placeholders::bytes_transferred));
</pre>
<p>
        Any further actions for this client connection are now the responsibility
        of <code class="computeroutput">handle_write()</code>.
      </p>
<pre class="programlisting">  }

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

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

  tcp::socket socket_;
  std::string message_;
};
</pre>
<h5>
<a name="asio.tutorial.tutdaytime3.h3"></a>
        <span><a name="asio.tutorial.tutdaytime3.removing_unused_handler_parameters"></a></span><a class="link" href="tutdaytime3.html#asio.tutorial.tutdaytime3.removing_unused_handler_parameters">Removing
        unused handler parameters</a>
      </h5>
<p>
        You may have noticed that the <code class="computeroutput">error</code>, and <code class="computeroutput">bytes_transferred</code>
        parameters are not used in the body of the <code class="computeroutput">handle_write()</code> function.
        If parameters are not needed, it is possible to remove them from the function
        so that it looks like:
      </p>
<pre class="programlisting">  void handle_write()
  {
  }
</pre>
<p>
        The asio::async_write() call used to initiate the call can then be changed
        to just:
      </p>
<pre class="programlisting">  asio::async_write(socket_, asio::buffer(message_),
      boost::bind(&amp;tcp_connection::handle_write, shared_from_this()));
</pre>
<p>
        See the <a class="link" href="tutdaytime3/src.html" title="Source listing for Daytime.3">full source listing</a>
      </p>
<p>
        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
      </p>
<p>
        Previous: <a class="link" href="tutdaytime2.html" title="Daytime.2 - A synchronous TCP daytime server">Daytime.2 - A synchronous
        TCP daytime server</a>
      </p>
<p>
        Next: <a class="link" href="tutdaytime4.html" title="Daytime.4 - A synchronous UDP daytime client">Daytime.4 - A synchronous
        UDP daytime client</a>
      </p>
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      Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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