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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_connect.overload3"></a><a class="link" href="overload3.html" title="async_connect (3 of 4 overloads)">async_connect
  28         (3 of 4 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"><span class="keyword">template</span><span class="special">&lt;</span>
  35     <span class="keyword">typename</span> <a class="link" href="../Protocol.html" title="Protocol requirements">Protocol</a><span class="special">,</span>
  36     <span class="keyword">typename</span> <a class="link" href="../SocketService.html" title="Socket service requirements">SocketService</a><span class="special">,</span>
  37     <span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">,</span>
  38     <span class="keyword">typename</span> <span class="identifier">ConnectCondition</span><span class="special">,</span>
  39     <span class="keyword">typename</span> <a class="link" href="../ComposedConnectHandler.html" title="Composed connect handler requirements">ComposedConnectHandler</a><span class="special">&gt;</span>
  40 <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.return_type_of_an_initiating_function"><span class="emphasis"><em>void-or-deduced</em></span></a> <span class="identifier">async_connect</span><span class="special">(</span>
  41     <span class="identifier">basic_socket</span><span class="special">&lt;</span> <span class="identifier">Protocol</span><span class="special">,</span> <span class="identifier">SocketService</span> <span class="special">&gt;</span> <span class="special">&amp;</span> <span class="identifier">s</span><span class="special">,</span>
  42     <span class="identifier">Iterator</span> <span class="identifier">begin</span><span class="special">,</span>
  43     <span class="identifier">ConnectCondition</span> <span class="identifier">connect_condition</span><span class="special">,</span>
  44     <span class="identifier">ComposedConnectHandler</span> <span class="identifier">handler</span><span class="special">);</span>
  45 </pre>
  46 <p>
  47           This function attempts to connect a socket to one of a sequence of endpoints.
  48           It does this by repeated calls to the socket's <code class="computeroutput"><span class="identifier">async_connect</span></code>
  49           member function, once for each endpoint in the sequence, until a connection
  50           is successfully established.
  51         </p>
  52 <h6>
  53 <a name="boost_asio.reference.async_connect.overload3.h0"></a>
  54           <span class="phrase"><a name="boost_asio.reference.async_connect.overload3.parameters"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_connect.overload3.parameters">Parameters</a>
  55         </h6>
  56 <div class="variablelist">
  57 <p class="title"><b></b></p>
  58 <dl class="variablelist">
  59 <dt><span class="term">s</span></dt>
  60 <dd><p>
  61                 The socket to be connected. If the socket is already open, it will
  62                 be closed.
  63               </p></dd>
  64 <dt><span class="term">begin</span></dt>
  65 <dd><p>
  66                 An iterator pointing to the start of a sequence of endpoints.
  67               </p></dd>
  68 <dt><span class="term">connect_condition</span></dt>
  69 <dd>
  70 <p>
  71                 A function object that is called prior to each connection attempt.
  72                 The signature of the function object must be:
  73 </p>
  74 <pre class="programlisting"><span class="identifier">Iterator</span> <span class="identifier">connect_condition</span><span class="special">(</span>
  75     <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">ec</span><span class="special">,</span>
  76     <span class="identifier">Iterator</span> <span class="identifier">next</span><span class="special">);</span>
  77 </pre>
  78 <p>
  79                 The <code class="computeroutput"><span class="identifier">ec</span></code> parameter
  80                 contains the result from the most recent connect operation. Before
  81                 the first connection attempt, <code class="computeroutput"><span class="identifier">ec</span></code>
  82                 is always set to indicate success. The <code class="computeroutput"><span class="identifier">next</span></code>
  83                 parameter is an iterator pointing to the next endpoint to be tried.
  84                 The function object should return the next iterator, but is permitted
  85                 to return a different iterator so that endpoints may be skipped.
  86                 The implementation guarantees that the function object will never
  87                 be called with the end iterator.
  88               </p>
  89 </dd>
  90 <dt><span class="term">handler</span></dt>
  91 <dd>
  92 <p>
  93                 The handler to be called when the connect operation completes. Copies
  94                 will be made of the handler as required. The function signature of
  95                 the handler must be:
  96 </p>
  97 <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
  98   <span class="comment">// Result of operation. if the sequence is empty, set to</span>
  99   <span class="comment">// boost::asio::error::not_found. Otherwise, contains the</span>
 100   <span class="comment">// error from the last connection attempt.</span>
 101   <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span>
 102
 103   <span class="comment">// On success, an iterator denoting the successfully</span>
 104   <span class="comment">// connected endpoint. Otherwise, the end iterator.</span>
 105   <span class="identifier">Iterator</span> <span class="identifier">iterator</span>
 106 <span class="special">);</span>
 107 </pre>
 108 <p>
 109                 Regardless of whether the asynchronous operation completes immediately
 110                 or not, the handler will not be invoked from within this function.
 111                 Invocation of the handler will be performed in a manner equivalent
 112                 to using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">post</span><span class="special">()</span></code>.
 113               </p>
 114 </dd>
 115 </dl>
 116 </div>
 117 <h6>
 118 <a name="boost_asio.reference.async_connect.overload3.h1"></a>
 119           <span class="phrase"><a name="boost_asio.reference.async_connect.overload3.remarks"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_connect.overload3.remarks">Remarks</a>
 120         </h6>
 121 <p>
 122           This overload assumes that a default constructed object of type <code class="computeroutput"><span class="identifier">Iterator</span></code> represents the end of the sequence.
 123           This is a valid assumption for iterator types such as <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">iterator</span></code>.
 124         </p>
 125 <h6>
 126 <a name="boost_asio.reference.async_connect.overload3.h2"></a>
 127           <span class="phrase"><a name="boost_asio.reference.async_connect.overload3.example"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_connect.overload3.example">Example</a>
 128         </h6>
 129 <p>
 130           The following connect condition function object can be used to output information
 131           about the individual connection attempts:
 132         </p>
 133 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">my_connect_condition</span>
 134 <span class="special">{</span>
 135   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
 136   <span class="identifier">Iterator</span> <span class="keyword">operator</span><span class="special">()(</span>
 137       <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">ec</span><span class="special">,</span>
 138       <span class="identifier">Iterator</span> <span class="identifier">next</span><span class="special">)</span>
 139   <span class="special">{</span>
 140     <span class="keyword">if</span> <span class="special">(</span><span class="identifier">ec</span><span class="special">)</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Error: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">ec</span><span class="special">.</span><span class="identifier">message</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 141     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Trying: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">next</span><span class="special">-&gt;</span><span class="identifier">endpoint</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 142     <span class="keyword">return</span> <span class="identifier">next</span><span class="special">;</span>
 143   <span class="special">}</span>
 144 <span class="special">};</span>
 145 </pre>
 146 <p>
 147           It would be used with the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">connect</span></code>
 148           function as follows:
 149         </p>
 150 <pre class="programlisting"><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span> <span class="identifier">r</span><span class="special">(</span><span class="identifier">io_service</span><span class="special">);</span>
 151 <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">query</span> <span class="identifier">q</span><span class="special">(</span><span class="string">"host"</span><span class="special">,</span> <span class="string">"service"</span><span class="special">);</span>
 152 <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">io_service</span><span class="special">);</span>
 153
 154 <span class="comment">// ...</span>
 155
 156 <span class="identifier">r</span><span class="special">.</span><span class="identifier">async_resolve</span><span class="special">(</span><span class="identifier">q</span><span class="special">,</span> <span class="identifier">resolve_handler</span><span class="special">);</span>
 157
 158 <span class="comment">// ...</span>
 159
 160 <span class="keyword">void</span> <span class="identifier">resolve_handler</span><span class="special">(</span>
 161     <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">ec</span><span class="special">,</span>
 162     <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">i</span><span class="special">)</span>
 163 <span class="special">{</span>
 164   <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">ec</span><span class="special">)</span>
 165   <span class="special">{</span>
 166     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_connect</span><span class="special">(</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">i</span><span class="special">,</span>
 167         <span class="identifier">my_connect_condition</span><span class="special">(),</span>
 168         <span class="identifier">connect_handler</span><span class="special">);</span>
 169   <span class="special">}</span>
 170 <span class="special">}</span>
 171
 172 <span class="comment">// ...</span>
 173
 174 <span class="keyword">void</span> <span class="identifier">connect_handler</span><span class="special">(</span>
 175     <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">ec</span><span class="special">,</span>
 176     <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">i</span><span class="special">)</span>
 177 <span class="special">{</span>
 178   <span class="keyword">if</span> <span class="special">(</span><span class="identifier">ec</span><span class="special">)</span>
 179   <span class="special">{</span>
 180     <span class="comment">// An error occurred.</span>
 181   <span class="special">}</span>
 182   <span class="keyword">else</span>
 183   <span class="special">{</span>
 184     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Connected to: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span><span class="special">-&gt;</span><span class="identifier">endpoint</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 185   <span class="special">}</span>
 186 <span class="special">}</span>
 187 </pre>
 188 </div>
 189 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 190 <td align="left"></td>
 191 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2014 Christopher M. Kohlhoff<p>
 192         Distributed under the Boost Software License, Version 1.0. (See accompanying
 193         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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