1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_connect (4 of 6 overloads)</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../async_connect.html" title="async_connect"> 9<link rel="prev" href="overload3.html" title="async_connect (3 of 6 overloads)"> 10<link rel="next" href="overload5.html" title="async_connect (5 of 6 overloads)"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="overload3.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_connect.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="overload5.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 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< 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>> 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< Protocol, Executor > & s, 42 const EndpointSequence & endpoints, 43 ConnectCondition connect_condition, 44 RangeConnectHandler && 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< is_endpoint_sequence< EndpointSequence >::value >::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& ec, 77 const typename Protocol::endpoint& 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& 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& ec, 127 const::tcp::endpoint& next) 128 { 129 if (ec) std::cout << "Error: " << ec.message() << std::endl; 130 std::cout << "Trying: " << next << 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); 140tcp::resolver::query q("host", "service"); 141tcp::socket s(my_context); 142 143// ... 144 145r.async_resolve(q, resolve_handler); 146 147// ... 148 149void resolve_handler( 150 const boost::system::error_code& 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 163void connect_handler( 164 const boost::system::error_code& ec, 165 const tcp::endpoint& endpoint) 166{ 167 if (ec) 168 { 169 // An error occurred. 170 } 171 else 172 { 173 std::cout << "Connected to: " << endpoint << std::endl; 174 } 175} 176</pre> 177</div> 178<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 179<td align="left"></td> 180<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 181 Kohlhoff<p> 182 Distributed under the Boost Software License, Version 1.0. (See accompanying 183 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>) 184 </p> 185</div></td> 186</tr></table> 187<hr> 188<div class="spirit-nav"> 189<a accesskey="p" href="overload3.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_connect.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="overload5.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 190</div> 191</body> 192</html> 193
