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188 lines
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<title>async_read_until (7 of 12 overloads)</title>
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<div class="titlepage"><div><div><h4 class="title">
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<a name="boost_asio.reference.async_read_until.overload7"></a><a class="link" href="overload7.html" title="async_read_until (7 of 12 overloads)">async_read_until
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(7 of 12 overloads)</a>
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</h4></div></div></div>
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<p>
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Start an asynchronous operation to read data into a streambuf until some
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part of its data matches a regular expression.
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</p>
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<pre class="programlisting">template<
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typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
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typename Allocator,
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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.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>>
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<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(
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AsyncReadStream & s,
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boost::asio::basic_streambuf< Allocator > & b,
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const boost::regex & expr,
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ReadHandler && handler = <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>);
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</pre>
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<p>
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This function is used to asynchronously read data into the specified streambuf
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until the streambuf's get area contains some data that matches a regular
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expression. The function call always returns immediately. The asynchronous
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operation will continue until one of the following conditions is true:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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A substring of the streambuf's get area matches the regular expression.
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</li>
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<li class="listitem">
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An error occurred.
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</li>
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</ul></div>
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<p>
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This operation is implemented in terms of zero or more calls to the stream's
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async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
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If the streambuf's get area already contains data that matches the regular
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expression, this asynchronous operation completes immediately. The program
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must ensure that the stream performs no other read operations (such as
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async_read, async_read_until, the stream's async_read_some function, or
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any other composed operations that perform reads) until this operation
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completes.
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</p>
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<h6>
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<a name="boost_asio.reference.async_read_until.overload7.h0"></a>
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<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>
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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 class="variablelist">
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<dt><span class="term">s</span></dt>
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<dd><p>
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The stream from which the data is to be read. The type must support
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the AsyncReadStream concept.
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</p></dd>
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<dt><span class="term">b</span></dt>
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<dd><p>
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A streambuf object into which the data will be read. Ownership of
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the streambuf is retained by the caller, which must guarantee that
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it remains valid until the handler is called.
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</p></dd>
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<dt><span class="term">expr</span></dt>
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<dd><p>
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The regular expression.
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</p></dd>
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<dt><span class="term">handler</span></dt>
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<dd>
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<p>
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The handler to be called when the read operation completes. Copies
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will be made of the handler as required. The function signature of
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the handler must be:
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</p>
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<pre class="programlisting">void handler(
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// Result of operation.
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const boost::system::error_code& error,
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// The number of bytes in the streambuf's get
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// area up to and including the substring
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// that matches the regular. expression.
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// 0 if an error occurred.
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std::size_t bytes_transferred
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);
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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 handler will not be invoked from within this function.
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On immediate completion, invocation of the handler will be performed
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in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</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="boost_asio.reference.async_read_until.overload7.h1"></a>
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<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>
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</h6>
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<p>
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After a successful async_read_until operation, the streambuf may contain
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additional data beyond that which matched the regular expression. An application
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will typically leave that data in the streambuf for a subsequent async_read_until
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operation to examine.
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</p>
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<h6>
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<a name="boost_asio.reference.async_read_until.overload7.h2"></a>
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<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>
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</h6>
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<p>
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To asynchronously read data into a streambuf until a CR-LF sequence is
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encountered:
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</p>
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<pre class="programlisting">boost::asio::streambuf b;
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...
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void handler(const boost::system::error_code& e, std::size_t size)
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{
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if (!e)
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{
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std::istream is(&b);
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std::string line;
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std::getline(is, line);
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...
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}
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}
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...
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boost::asio::async_read_until(s, b, boost::regex("\r\n"), handler);
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</pre>
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<p>
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After the <code class="computeroutput">async_read_until</code> operation completes successfully,
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the buffer <code class="computeroutput">b</code> contains the data which matched the regular expression:
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</p>
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<pre class="programlisting">{ 'a', 'b', ..., 'c', '\r', '\n', 'd', 'e', ... }
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</pre>
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<p>
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The call to <code class="computeroutput">std::getline</code> then extracts the data up to and
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including the newline (which is discarded), so that the string <code class="computeroutput">line</code>
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contains:
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</p>
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<pre class="programlisting">{ 'a', 'b', ..., 'c', '\r' }
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</pre>
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<p>
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The remaining data is left in the buffer <code class="computeroutput">b</code> as follows:
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</p>
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<pre class="programlisting">{ 'd', 'e', ... }
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</pre>
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<p>
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This data may be the start of a new line, to be extracted by a subsequent
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<code class="computeroutput">async_read_until</code> operation.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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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></td>
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</tr></table>
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