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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_connect.overload6"></a><a class="link" href="overload6.html" title="async_connect (6 of 6 overloads)">async_connect
  28         (6 of 6 overloads)</a>
  29 </h4></div></div></div>
  30 <p>
  31           Asynchronously establishes a socket connection by trying each endpoint
  32           in a sequence.
  33         </p>
  34 <pre class="programlisting">template&lt;
  35     typename <a class="link" href="../Protocol.html" title="Protocol requirements">Protocol</a>,
  36     typename <a class="link" href="../Executor1.html" title="Executor requirements">Executor</a>,
  37     typename Iterator,
  38     typename <a class="link" href="../ConnectCondition.html" title="Connect condition requirements">ConnectCondition</a>,
  39     typename <a class="link" href="../IteratorConnectHandler.html" title="Iterator connect handler requirements">IteratorConnectHandler</a> = <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>&gt;
  40 <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_connect(
  41     basic_socket&lt; Protocol, Executor &gt; &amp; s,
  42     Iterator begin,
  43     Iterator end,
  44     ConnectCondition connect_condition,
  45     IteratorConnectHandler &amp;&amp; handler = <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>);
  46 </pre>
  47 <p>
  48           This function attempts to connect a socket to one of a sequence of endpoints.
  49           It does this by repeated calls to the socket's <code class="computeroutput">async_connect</code>
  50           member function, once for each endpoint in the sequence, until a connection
  51           is successfully established.
  52         </p>
  53 <h6>
  54 <a name="boost_asio.reference.async_connect.overload6.h0"></a>
  55           <span class="phrase"><a name="boost_asio.reference.async_connect.overload6.parameters"></a></span><a class="link" href="overload6.html#boost_asio.reference.async_connect.overload6.parameters">Parameters</a>
  56         </h6>
  57 <div class="variablelist">
  58 <p class="title"><b></b></p>
  59 <dl class="variablelist">
  60 <dt><span class="term">s</span></dt>
  61 <dd><p>
  62                 The socket to be connected. If the socket is already open, it will
  63                 be closed.
  64               </p></dd>
  65 <dt><span class="term">begin</span></dt>
  66 <dd><p>
  67                 An iterator pointing to the start of a sequence of endpoints.
  68               </p></dd>
  69 <dt><span class="term">end</span></dt>
  70 <dd><p>
  71                 An iterator pointing to the end of a sequence of endpoints.
  72               </p></dd>
  73 <dt><span class="term">connect_condition</span></dt>
  74 <dd>
  75 <p>
  76                 A function object that is called prior to each connection attempt.
  77                 The signature of the function object must be:
  78 </p>
  79 <pre class="programlisting">bool connect_condition(
  80     const boost::system::error_code&amp; ec,
  81     const typename Protocol::endpoint&amp; next);
  82 </pre>
  83 <p>
  84                 The <code class="computeroutput">ec</code> parameter contains the result from the most recent
  85                 connect operation. Before the first connection attempt, <code class="computeroutput">ec</code>
  86                 is always set to indicate success. The <code class="computeroutput">next</code> parameter
  87                 is the next endpoint to be tried. The function object should return
  88                 true if the next endpoint should be tried, and false if it should
  89                 be skipped.
  90               </p>
  91 </dd>
  92 <dt><span class="term">handler</span></dt>
  93 <dd>
  94 <p>
  95                 The handler to be called when the connect operation completes. Copies
  96                 will be made of the handler as required. The function signature of
  97                 the handler must be:
  98 </p>
  99 <pre class="programlisting">void handler(
 100   // Result of operation. if the sequence is empty, set to
 101   // boost::asio::error::not_found. Otherwise, contains the
 102   // error from the last connection attempt.
 103   const boost::system::error_code&amp; error,
 104
 105   // On success, an iterator denoting the successfully
 106   // connected endpoint. Otherwise, the end iterator.
 107   Iterator iterator
 108 );
 109 </pre>
 110 <p>
 111                 Regardless of whether the asynchronous operation completes immediately
 112                 or not, the handler will not be invoked from within this function.
 113                 On immediate completion, invocation of the handler will be performed
 114                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 115               </p>
 116 </dd>
 117 </dl>
 118 </div>
 119 <h6>
 120 <a name="boost_asio.reference.async_connect.overload6.h1"></a>
 121           <span class="phrase"><a name="boost_asio.reference.async_connect.overload6.example"></a></span><a class="link" href="overload6.html#boost_asio.reference.async_connect.overload6.example">Example</a>
 122         </h6>
 123 <p>
 124           The following connect condition function object can be used to output information
 125           about the individual connection attempts:
 126         </p>
 127 <pre class="programlisting">struct my_connect_condition
 128 {
 129   bool operator()(
 130       const boost::system::error_code&amp; ec,
 131       const::tcp::endpoint&amp; next)
 132   {
 133     if (ec) std::cout &lt;&lt; "Error: " &lt;&lt; ec.message() &lt;&lt; std::endl;
 134     std::cout &lt;&lt; "Trying: " &lt;&lt; next &lt;&lt; std::endl;
 135     return true;
 136   }
 137 };
 138 </pre>
 139 <p>
 140           It would be used with the <code class="computeroutput">boost::asio::connect</code> function as
 141           follows:
 142         </p>
 143 <pre class="programlisting">tcp::resolver r(my_context);
 144 tcp::resolver::query q("host", "service");
 145 tcp::socket s(my_context);
 146
 147 // ...
 148
 149 r.async_resolve(q, resolve_handler);
 150
 151 // ...
 152
 153 void resolve_handler(
 154     const boost::system::error_code&amp; ec,
 155     tcp::resolver::iterator i)
 156 {
 157   if (!ec)
 158   {
 159     tcp::resolver::iterator end;
 160     boost::asio::async_connect(s, i, end,
 161         my_connect_condition(),
 162         connect_handler);
 163   }
 164 }
 165
 166 // ...
 167
 168 void connect_handler(
 169     const boost::system::error_code&amp; ec,
 170     tcp::resolver::iterator i)
 171 {
 172   if (ec)
 173   {
 174     // An error occurred.
 175   }
 176   else
 177   {
 178     std::cout &lt;&lt; "Connected to: " &lt;&lt; i-&gt;endpoint() &lt;&lt; std::endl;
 179   }
 180 }
 181 </pre>
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 185 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2019 Christopher M. Kohlhoff<p>
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