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<title>async_read_until (7 of 8 overloads)</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.async_read_until.overload7"></a><a class="link" href="overload7.html" title="async_read_until (7 of 8 overloads)">async_read_until
(7 of 8 overloads)</a>
</h4></div></div></div>
<p>
Start an asynchronous operation to read data into a streambuf until some
part of its data matches a regular expression.
</p>
<pre class="programlisting">template<
typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
typename Allocator,
typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a>>
<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_read_until(
AsyncReadStream & s,
boost::asio::basic_streambuf< Allocator > & b,
const boost::regex & expr,
ReadHandler && handler);
</pre>
<p>
This function is used to asynchronously read data into the specified streambuf
until the streambuf's get area contains some data that matches a regular
expression. The function call always returns immediately. The asynchronous
operation will continue until one of the following conditions is true:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
A substring of the streambuf's get area matches the regular expression.
</li>
<li class="listitem">
An error occurred.
</li>
</ul></div>
<p>
This operation is implemented in terms of zero or more calls to the stream's
async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
If the streambuf's get area already contains data that matches the regular
expression, this asynchronous operation completes immediately. The program
must ensure that the stream performs no other read operations (such as
async_read, async_read_until, the stream's async_read_some function, or
any other composed operations that perform reads) until this operation
completes.
</p>
<h6>
<a name="boost_asio.reference.async_read_until.overload7.h0"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_until.overload7.parameters"></a></span><a class="link" href="overload7.html#boost_asio.reference.async_read_until.overload7.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 stream from which the data is to be read. The type must support
the AsyncReadStream concept.
</p></dd>
<dt><span class="term">b</span></dt>
<dd><p>
A streambuf object into which the data will be read. Ownership of
the streambuf is retained by the caller, which must guarantee that
it remains valid until the handler is called.
</p></dd>
<dt><span class="term">expr</span></dt>
<dd><p>
The regular expression.
</p></dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
The handler to be called when the read 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.
const boost::system::error_code& error,
// The number of bytes in the streambuf's get
// area up to and including the substring
// that matches the regular. expression.
// 0 if an error occurred.
std::size_t bytes_transferred
);
</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_read_until.overload7.h1"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_until.overload7.remarks"></a></span><a class="link" href="overload7.html#boost_asio.reference.async_read_until.overload7.remarks">Remarks</a>
</h6>
<p>
After a successful async_read_until operation, the streambuf may contain
additional data beyond that which matched the regular expression. An application
will typically leave that data in the streambuf for a subsequent async_read_until
operation to examine.
</p>
<h6>
<a name="boost_asio.reference.async_read_until.overload7.h2"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_until.overload7.example"></a></span><a class="link" href="overload7.html#boost_asio.reference.async_read_until.overload7.example">Example</a>
</h6>
<p>
To asynchronously read data into a streambuf until a CR-LF sequence is
encountered:
</p>
<pre class="programlisting">boost::asio::streambuf b;
...
void handler(const boost::system::error_code& e, std::size_t size)
{
if (!e)
{
std::istream is(&b);
std::string line;
std::getline(is, line);
...
}
}
...
boost::asio::async_read_until(s, b, boost::regex("\r\n"), handler);
</pre>
<p>
After the <code class="computeroutput">async_read_until</code> operation completes successfully,
the buffer <code class="computeroutput">b</code> contains the data which matched the regular expression:
</p>
<pre class="programlisting">{ 'a', 'b', ..., 'c', '\r', '\n', 'd', 'e', ... }
</pre>
<p>
The call to <code class="computeroutput">std::getline</code> then extracts the data up to and
including the newline (which is discarded), so that the string <code class="computeroutput">line</code>
contains:
</p>
<pre class="programlisting">{ 'a', 'b', ..., 'c', '\r' }
</pre>
<p>
The remaining data is left in the buffer <code class="computeroutput">b</code> as follows:
</p>
<pre class="programlisting">{ 'd', 'e', ... }
</pre>
<p>
This data may be the start of a new line, to be extracted by a subsequent
<code class="computeroutput">async_read_until</code> operation.
</p>
</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 © 2003-2018 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>
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