218 lines
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218 lines
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>async_connect (4 of 6 overloads)</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
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<link rel="home" href="../../../index.html" title="Asio">
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<link rel="up" href="../async_connect.html" title="async_connect">
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<link rel="prev" href="overload3.html" title="async_connect (3 of 6 overloads)">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../../asio.png"></td></tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="overload3.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../async_connect.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="overload5.html"><img src="../../../next.png" alt="Next"></a>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="asio.reference.async_connect.overload4"></a><a class="link" href="overload4.html" title="async_connect (4 of 6 overloads)">async_connect
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(4 of 6 overloads)</a>
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</h4></div></div></div>
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<p>
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Asynchronously establishes a socket connection by trying each endpoint
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in a sequence.
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span>
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<span class="keyword">typename</span> <a class="link" href="../Protocol.html" title="Protocol requirements">Protocol</a><span class="special">,</span>
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<span class="keyword">typename</span> <a class="link" href="../Executor1.html" title="Executor requirements">Executor</a><span class="special">,</span>
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<span class="keyword">typename</span> <a class="link" href="../EndpointSequence.html" title="Endpoint sequence requirements">EndpointSequence</a><span class="special">,</span>
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<span class="keyword">typename</span> <a class="link" href="../ConnectCondition.html" title="Connect condition requirements">ConnectCondition</a><span class="special">,</span>
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<span class="keyword">typename</span> <a class="link" href="../RangeConnectToken.html" title="Range connect token requirements">RangeConnectToken</a> <span class="special">=</span> <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a><span class="special">></span>
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<a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> <span class="identifier">async_connect</span><span class="special">(</span>
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<span class="identifier">basic_socket</span><span class="special"><</span> <span class="identifier">Protocol</span><span class="special">,</span> <span class="identifier">Executor</span> <span class="special">></span> <span class="special">&</span> <span class="identifier">s</span><span class="special">,</span>
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<span class="keyword">const</span> <span class="identifier">EndpointSequence</span> <span class="special">&</span> <span class="identifier">endpoints</span><span class="special">,</span>
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<span class="identifier">ConnectCondition</span> <span class="identifier">connect_condition</span><span class="special">,</span>
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<span class="identifier">RangeConnectToken</span> <span class="special">&&</span> <span class="identifier">token</span> <span class="special">=</span> <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
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<span class="keyword">typename</span> <span class="identifier">constraint</span><span class="special"><</span> <span class="identifier">is_endpoint_sequence</span><span class="special"><</span> <span class="identifier">EndpointSequence</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">=</span> <span class="number">0</span><span class="special">);</span>
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</pre>
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<p>
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This function attempts to connect a socket to one of a sequence of endpoints.
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It does this by repeated calls to the socket's <code class="computeroutput"><span class="identifier">async_connect</span></code>
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member function, once for each endpoint in the sequence, until a connection
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is successfully established. It is an initiating function for an <a class="link" href="../../overview/model/async_ops.html" title="Asynchronous Operations">asynchronous operation</a>,
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and always returns immediately.
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</p>
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<h6>
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<a name="asio.reference.async_connect.overload4.h0"></a>
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<span><a name="asio.reference.async_connect.overload4.parameters"></a></span><a class="link" href="overload4.html#asio.reference.async_connect.overload4.parameters">Parameters</a>
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</h6>
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<div class="variablelist">
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<p class="title"><b></b></p>
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<dl>
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<dt><span class="term">s</span></dt>
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<dd><p>
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The socket to be connected. If the socket is already open, it will
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be closed.
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</p></dd>
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<dt><span class="term">endpoints</span></dt>
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<dd><p>
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A sequence of endpoints.
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</p></dd>
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<dt><span class="term">connect_condition</span></dt>
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<dd>
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<p>
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A function object that is called prior to each connection attempt.
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The signature of the function object must be:
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</p>
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<pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">connect_condition</span><span class="special">(</span>
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<span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">ec</span><span class="special">,</span>
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<span class="keyword">const</span> <span class="keyword">typename</span> <span class="identifier">Protocol</span><span class="special">::</span><span class="identifier">endpoint</span><span class="special">&</span> <span class="identifier">next</span><span class="special">);</span>
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</pre>
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<p>
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The <code class="computeroutput"><span class="identifier">ec</span></code> parameter
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contains the result from the most recent connect operation. Before
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the first connection attempt, <code class="computeroutput"><span class="identifier">ec</span></code>
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is always set to indicate success. The <code class="computeroutput"><span class="identifier">next</span></code>
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parameter is the next endpoint to be tried. The function object should
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return true if the next endpoint should be tried, and false if it
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should be skipped.
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</p>
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</dd>
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<dt><span class="term">token</span></dt>
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<dd>
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<p>
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The <a class="link" href="../../overview/model/completion_tokens.html" title="Completion Tokens">completion
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token</a> that will be used to produce a completion handler, which
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will be called when the connect completes. Potential completion tokens
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include <a class="link" href="../use_future.html" title="use_future"><code class="computeroutput"><span class="identifier">use_future</span></code></a>,
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<a class="link" href="../use_awaitable.html" title="use_awaitable"><code class="computeroutput"><span class="identifier">use_awaitable</span></code></a>,
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<a class="link" href="../yield_context.html" title="yield_context"><code class="computeroutput"><span class="identifier">yield_context</span></code></a>,
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or a function object with the correct completion signature. The function
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signature of the completion handler must be:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
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<span class="comment">// Result of operation. if the sequence is empty, set to</span>
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<span class="comment">// asio::error::not_found. Otherwise, contains the</span>
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<span class="comment">// error from the last connection attempt.</span>
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<span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">error</span><span class="special">,</span>
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<span class="comment">// On success, an iterator denoting the successfully</span>
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<span class="comment">// connected endpoint. Otherwise, the end iterator.</span>
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<span class="identifier">Iterator</span> <span class="identifier">iterator</span>
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<span class="special">);</span>
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</pre>
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<p>
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Regardless of whether the asynchronous operation completes immediately
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or not, the completion handler will not be invoked from within this
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function. On immediate completion, invocation of the handler will
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be performed in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput"><span class="identifier">post</span></code></a>.
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</p>
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</dd>
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</dl>
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</div>
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<h6>
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<a name="asio.reference.async_connect.overload4.h1"></a>
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<span><a name="asio.reference.async_connect.overload4.completion_signature"></a></span><a class="link" href="overload4.html#asio.reference.async_connect.overload4.completion_signature">Completion
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Signature</a>
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</h6>
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<pre class="programlisting"><span class="keyword">void</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Protocol</span><span class="special">::</span><span class="identifier">endpoint</span><span class="special">)</span>
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</pre>
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<h6>
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<a name="asio.reference.async_connect.overload4.h2"></a>
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<span><a name="asio.reference.async_connect.overload4.example"></a></span><a class="link" href="overload4.html#asio.reference.async_connect.overload4.example">Example</a>
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</h6>
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<p>
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The following connect condition function object can be used to output information
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about the individual connection attempts:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">my_connect_condition</span>
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<span class="special">{</span>
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<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span>
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<span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">ec</span><span class="special">,</span>
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<span class="keyword">const</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">endpoint</span><span class="special">&</span> <span class="identifier">next</span><span class="special">)</span>
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<span class="special">{</span>
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<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>
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<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">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="special">};</span>
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</pre>
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<p>
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It would be used with the <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">connect</span></code>
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function as follows:
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</p>
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<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">my_context</span><span class="special">);</span>
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<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>
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<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">my_context</span><span class="special">);</span>
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<span class="comment">// ...</span>
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<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>
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<span class="comment">// ...</span>
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<span class="keyword">void</span> <span class="identifier">resolve_handler</span><span class="special">(</span>
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<span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">ec</span><span class="special">,</span>
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<span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">results_type</span> <span class="identifier">results</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="keyword">if</span> <span class="special">(!</span><span class="identifier">ec</span><span class="special">)</span>
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<span class="special">{</span>
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<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">results</span><span class="special">,</span>
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<span class="identifier">my_connect_condition</span><span class="special">(),</span>
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<span class="identifier">connect_handler</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="special">}</span>
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<span class="comment">// ...</span>
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<span class="keyword">void</span> <span class="identifier">connect_handler</span><span class="special">(</span>
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<span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">ec</span><span class="special">,</span>
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<span class="keyword">const</span> <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">endpoint</span><span class="special">&</span> <span class="identifier">endpoint</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">ec</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="comment">// An error occurred.</span>
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<span class="special">}</span>
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<span class="keyword">else</span>
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<span class="special">{</span>
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<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">endpoint</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="special">}</span>
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</pre>
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<h6>
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<a name="asio.reference.async_connect.overload4.h3"></a>
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<span><a name="asio.reference.async_connect.overload4.per_operation_cancellation"></a></span><a class="link" href="overload4.html#asio.reference.async_connect.overload4.per_operation_cancellation">Per-Operation
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Cancellation</a>
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</h6>
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<p>
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This asynchronous operation supports cancellation for the following <a class="link" href="../cancellation_type.html" title="cancellation_type"><code class="computeroutput"><span class="identifier">cancellation_type</span></code></a>
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values:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">terminal</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">partial</span></code>
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</li>
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</ul></div>
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<p>
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if they are also supported by the socket's <code class="computeroutput"><span class="identifier">async_connect</span></code>
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operation.
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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</div>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="overload3.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../async_connect.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="overload5.html"><img src="../../../next.png" alt="Next"></a>
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</div>
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</body>
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</html>
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