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<title>async_connect (4 of 4 overloads)</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.async_connect.overload4"></a><a class="link" href="overload4.html" title="async_connect (4 of 4 overloads)">async_connect
(4 of 4 overloads)</a>
</h4></div></div></div>
<p>
Asynchronously establishes a socket connection by trying each endpoint
in a sequence.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span>
<span class="keyword">typename</span> <a class="link" href="../Protocol.html" title="Protocol requirements">Protocol</a><span class="special">,</span>
<span class="keyword">typename</span> <a class="link" href="../SocketService.html" title="Socket service requirements">SocketService</a><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">ConnectCondition</span><span class="special">,</span>
<span class="keyword">typename</span> <a class="link" href="../ComposedConnectHandler.html" title="Composed connect handler requirements">ComposedConnectHandler</a><span class="special">></span>
<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>
<span class="identifier">basic_socket</span><span class="special"><</span> <span class="identifier">Protocol</span><span class="special">,</span> <span class="identifier">SocketService</span> <span class="special">></span> <span class="special">&</span> <span class="identifier">s</span><span class="special">,</span>
<span class="identifier">Iterator</span> <span class="identifier">begin</span><span class="special">,</span>
<span class="identifier">Iterator</span> <span class="identifier">end</span><span class="special">,</span>
<span class="identifier">ConnectCondition</span> <span class="identifier">connect_condition</span><span class="special">,</span>
<span class="identifier">ComposedConnectHandler</span> <span class="identifier">handler</span><span class="special">);</span>
</pre>
<p>
This function attempts to connect a socket to one of a sequence of endpoints.
It does this by repeated calls to the socket's <code class="computeroutput"><span class="identifier">async_connect</span></code>
member function, once for each endpoint in the sequence, until a connection
is successfully established.
</p>
<h6>
<a name="boost_asio.reference.async_connect.overload4.h0"></a>
<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>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">s</span></dt>
<dd><p>
The socket to be connected. If the socket is already open, it will
be closed.
</p></dd>
<dt><span class="term">begin</span></dt>
<dd><p>
An iterator pointing to the start of a sequence of endpoints.
</p></dd>
<dt><span class="term">end</span></dt>
<dd><p>
An iterator pointing to the end of a sequence of endpoints.
</p></dd>
<dt><span class="term">connect_condition</span></dt>
<dd>
<p>
A function object that is called prior to each connection attempt.
The signature of the function object must be:
</p>
<pre class="programlisting"><span class="identifier">Iterator</span> <span class="identifier">connect_condition</span><span class="special">(</span>
<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">&</span> <span class="identifier">ec</span><span class="special">,</span>
<span class="identifier">Iterator</span> <span class="identifier">next</span><span class="special">);</span>
</pre>
<p>
The <code class="computeroutput"><span class="identifier">ec</span></code> parameter
contains the result from the most recent connect operation. Before
the first connection attempt, <code class="computeroutput"><span class="identifier">ec</span></code>
is always set to indicate success. The <code class="computeroutput"><span class="identifier">next</span></code>
parameter is an iterator pointing to the next endpoint to be tried.
The function object should return the next iterator, but is permitted
to return a different iterator so that endpoints may be skipped.
The implementation guarantees that the function object will never
be called with the end iterator.
</p>
</dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
The handler to be called when the connect operation completes. Copies
will be made of the handler as required. The function signature of
the handler must be:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
<span class="comment">// Result of operation. if the sequence is empty, set to</span>
<span class="comment">// boost::asio::error::not_found. Otherwise, contains the</span>
<span class="comment">// error from the last connection attempt.</span>
<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">&</span> <span class="identifier">error</span><span class="special">,</span>
<span class="comment">// On success, an iterator denoting the successfully</span>
<span class="comment">// connected endpoint. Otherwise, the end iterator.</span>
<span class="identifier">Iterator</span> <span class="identifier">iterator</span>
<span class="special">);</span>
</pre>
<p>
Regardless of whether the asynchronous operation completes immediately
or not, the handler will not be invoked from within this function.
Invocation of the handler will be performed in a manner equivalent
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>.
</p>
</dd>
</dl>
</div>
<h6>
<a name="boost_asio.reference.async_connect.overload4.h1"></a>
<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>
</h6>
<p>
The following connect condition function object can be used to output information
about the individual connection attempts:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">my_connect_condition</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span>
<span class="identifier">Iterator</span> <span class="keyword">operator</span><span class="special">()(</span>
<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">&</span> <span class="identifier">ec</span><span class="special">,</span>
<span class="identifier">Iterator</span> <span class="identifier">next</span><span class="special">)</span>
<span class="special">{</span>
<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"><<</span> <span class="string">"Error: "</span> <span class="special"><<</span> <span class="identifier">ec</span><span class="special">.</span><span class="identifier">message</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Trying: "</span> <span class="special"><<</span> <span class="identifier">next</span><span class="special">-></span><span class="identifier">endpoint</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">next</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
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>
function as follows:
</p>
<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>
<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>
<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>
<span class="comment">// ...</span>
<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>
<span class="comment">// ...</span>
<span class="keyword">void</span> <span class="identifier">resolve_handler</span><span class="special">(</span>
<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">&</span> <span class="identifier">ec</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> <span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(!</span><span class="identifier">ec</span><span class="special">)</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> <span class="identifier">end</span><span class="special">;</span>
<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> <span class="identifier">end</span><span class="special">,</span>
<span class="identifier">my_connect_condition</span><span class="special">(),</span>
<span class="identifier">connect_handler</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="comment">// ...</span>
<span class="keyword">void</span> <span class="identifier">connect_handler</span><span class="special">(</span>
<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">&</span> <span class="identifier">ec</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> <span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">ec</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// An error occurred.</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Connected to: "</span> <span class="special"><<</span> <span class="identifier">i</span><span class="special">-></span><span class="identifier">endpoint</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
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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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