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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_connect.overload4"></a><a class="link" href="overload4.html" title="async_connect (4 of 6 overloads)">async_connect
  28         (4 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 <a class="link" href="../EndpointSequence.html" title="Endpoint sequence requirements">EndpointSequence</a>,
  38     typename <a class="link" href="../ConnectCondition.html" title="Connect condition requirements">ConnectCondition</a>,
  39     typename <a class="link" href="../RangeConnectHandler.html" title="Range connect handler requirements">RangeConnectHandler</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     const EndpointSequence &amp; endpoints,
  43     ConnectCondition connect_condition,
  44     RangeConnectHandler &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>,
  45     typename enable_if&lt; is_endpoint_sequence&lt; EndpointSequence &gt;::value &gt;::type *  = 0);
  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.overload4.h0"></a>
  55           <span class="phrase"><a name="boost_asio.reference.async_connect.overload4.parameters"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_connect.overload4.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">endpoints</span></dt>
  66 <dd><p>
  67                 A sequence of endpoints.
  68               </p></dd>
  69 <dt><span class="term">connect_condition</span></dt>
  70 <dd>
  71 <p>
  72                 A function object that is called prior to each connection attempt.
  73                 The signature of the function object must be:
  74 </p>
  75 <pre class="programlisting">bool connect_condition(
  76     const boost::system::error_code&amp; ec,
  77     const typename Protocol::endpoint&amp; next);
  78 </pre>
  79 <p>
  80                 The <code class="computeroutput">ec</code> parameter contains the result from the most recent
  81                 connect operation. Before the first connection attempt, <code class="computeroutput">ec</code>
  82                 is always set to indicate success. The <code class="computeroutput">next</code> parameter
  83                 is the next endpoint to be tried. The function object should return
  84                 true if the next endpoint should be tried, and false if it should
  85                 be skipped.
  86               </p>
  87 </dd>
  88 <dt><span class="term">handler</span></dt>
  89 <dd>
  90 <p>
  91                 The handler to be called when the connect operation completes. Copies
  92                 will be made of the handler as required. The function signature of
  93                 the handler must be:
  94 </p>
  95 <pre class="programlisting">void handler(
  96   // Result of operation. if the sequence is empty, set to
  97   // boost::asio::error::not_found. Otherwise, contains the
  98   // error from the last connection attempt.
  99   const boost::system::error_code&amp; error,
 100
 101   // On success, an iterator denoting the successfully
 102   // connected endpoint. Otherwise, the end iterator.
 103   Iterator iterator
 104 );
 105 </pre>
 106 <p>
 107                 Regardless of whether the asynchronous operation completes immediately
 108                 or not, the handler will not be invoked from within this function.
 109                 On immediate completion, invocation of the handler will be performed
 110                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 111               </p>
 112 </dd>
 113 </dl>
 114 </div>
 115 <h6>
 116 <a name="boost_asio.reference.async_connect.overload4.h1"></a>
 117           <span class="phrase"><a name="boost_asio.reference.async_connect.overload4.example"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_connect.overload4.example">Example</a>
 118         </h6>
 119 <p>
 120           The following connect condition function object can be used to output information
 121           about the individual connection attempts:
 122         </p>
 123 <pre class="programlisting">struct my_connect_condition
 124 {
 125   bool operator()(
 126       const boost::system::error_code&amp; ec,
 127       const::tcp::endpoint&amp; next)
 128   {
 129     if (ec) std::cout &lt;&lt; "Error: " &lt;&lt; ec.message() &lt;&lt; std::endl;
 130     std::cout &lt;&lt; "Trying: " &lt;&lt; next &lt;&lt; std::endl;
 131     return true;
 132   }
 133 };
 134 </pre>
 135 <p>
 136           It would be used with the <code class="computeroutput">boost::asio::connect</code> function as
 137           follows:
 138         </p>
 139 <pre class="programlisting">tcp::resolver r(my_context);
 140 tcp::resolver::query q("host", "service");
 141 tcp::socket s(my_context);
 142
 143 // ...
 144
 145 r.async_resolve(q, resolve_handler);
 146
 147 // ...
 148
 149 void resolve_handler(
 150     const boost::system::error_code&amp; ec,
 151     tcp::resolver::results_type results)
 152 {
 153   if (!ec)
 154   {
 155     boost::asio::async_connect(s, results,
 156         my_connect_condition(),
 157         connect_handler);
 158   }
 159 }
 160
 161 // ...
 162
 163 void connect_handler(
 164     const boost::system::error_code&amp; ec,
 165     const tcp::endpoint&amp; endpoint)
 166 {
 167   if (ec)
 168   {
 169     // An error occurred.
 170   }
 171   else
 172   {
 173     std::cout &lt;&lt; "Connected to: " &lt;&lt; endpoint &lt;&lt; std::endl;
 174   }
 175 }
 176 </pre>
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