doc/html/boost_asio/reference/async_connect/overload4.html


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>async_connect (4 of 6 overloads)</title>
<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../../../boost_asio.html" title="Boost.Asio">
<link rel="up" href="../async_connect.html" title="async_connect">
<link rel="prev" href="overload3.html" title="async_connect (3 of 6 overloads)">
<link rel="next" href="overload5.html" title="async_connect (5 of 6 overloads)">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
<td align="center"><a href="../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<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>
</div>
<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 6 overloads)">async_connect
        (4 of 6 overloads)</a>
</h4></div></div></div>
<p>
          Asynchronously establishes a socket connection by trying each endpoint
          in a sequence.
        </p>
<pre class="programlisting">template&lt;
    typename <a class="link" href="../Protocol.html" title="Protocol requirements">Protocol</a>,
    typename <a class="link" href="../EndpointSequence.html" title="Endpoint sequence requirements">EndpointSequence</a>,
    typename <a class="link" href="../ConnectCondition.html" title="Connect condition requirements">ConnectCondition</a>,
    typename <a class="link" href="../RangeConnectHandler.html" title="Range connect handler requirements">RangeConnectHandler</a>&gt;
<a class="link" href="../asynchronous_operations/automatic_deduction_of_initiating_function_return_type.html" title="Automatic deduction of initiating function return type"><span class="emphasis"><em>DEDUCED</em></span></a> async_connect(
    basic_socket&lt; Protocol &gt; &amp; s,
    const EndpointSequence &amp; endpoints,
    ConnectCondition connect_condition,
    RangeConnectHandler &amp;&amp; handler,
    typename enable_if&lt; is_endpoint_sequence&lt; EndpointSequence &gt;::value &gt;::type *  = 0);
</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">async_connect</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">endpoints</span></dt>
<dd><p>
                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">bool connect_condition(
    const boost::system::error_code&amp; ec,
    const typename Protocol::endpoint&amp; next);
</pre>
<p>
                The <code class="computeroutput">ec</code> parameter contains the result from the most recent
                connect operation. Before the first connection attempt, <code class="computeroutput">ec</code>
                is always set to indicate success. The <code class="computeroutput">next</code> parameter
                is the next endpoint to be tried. The function object should return
                true if the next endpoint should be tried, and false if it should
                be skipped.
              </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">void handler(
  // Result of operation. if the sequence is empty, set to
  // boost::asio::error::not_found. Otherwise, contains the
  // error from the last connection attempt.
  const boost::system::error_code&amp; error,

  // On success, an iterator denoting the successfully
  // connected endpoint. Otherwise, the end iterator.
  Iterator iterator
);
</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">boost::asio::io_context::post()</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">struct my_connect_condition
{
  bool operator()(
      const boost::system::error_code&amp; ec,
      const::tcp::endpoint&amp; next)
  {
    if (ec) std::cout &lt;&lt; "Error: " &lt;&lt; ec.message() &lt;&lt; std::endl;
    std::cout &lt;&lt; "Trying: " &lt;&lt; next &lt;&lt; std::endl;
    return true;
  }
};
</pre>
<p>
          It would be used with the <code class="computeroutput">boost::asio::connect</code> function as
          follows:
        </p>
<pre class="programlisting">tcp::resolver r(io_context);
tcp::resolver::query q("host", "service");
tcp::socket s(io_context);

// ...

r.async_resolve(q, resolve_handler);

// ...

void resolve_handler(
    const boost::system::error_code&amp; ec,
    tcp::resolver::results_type results)
{
  if (!ec)
  {
    boost::asio::async_connect(s, results,
        my_connect_condition(),
        connect_handler);
  }
}

// ...

void connect_handler(
    const boost::system::error_code&amp; ec,
    const tcp::endpoint&amp; endpoint)
{
  if (ec)
  {
    // An error occurred.
  }
  else
  {
    std::cout &lt;&lt; "Connected to: " &lt;&lt; endpoint &lt;&lt; std::endl;
  }
}
</pre>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2017 Christopher M. Kohlhoff<p>
        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>)
      </p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<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>
</div>
</body>
</html>