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<title>async_read_until (8 of 12 overloads)</title>
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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="boost_asio.reference.async_read_until.overload8"></a><a class="link" href="overload8.html" title="async_read_until (8 of 12 overloads)">async_read_until
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(8 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 a function
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object indicates a match.
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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 MatchCondition,
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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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MatchCondition match_condition,
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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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typename constraint< is_match_condition< MatchCondition >::value >::type = 0);
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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 a user-defined match condition function object, when applied to the
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data contained in the streambuf, indicates a successful match. The function
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call always returns immediately. The asynchronous operation will continue
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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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The match condition function object returns a std::pair where the second
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element evaluates to true.
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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 match condition function object already indicates a match, this
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asynchronous operation completes immediately. The program must ensure that
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the stream performs no other read operations (such as async_read, async_read_until,
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the stream's async_read_some function, or any other composed operations
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that perform reads) until this operation completes.
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</p>
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<h6>
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<a name="boost_asio.reference.async_read_until.overload8.h0"></a>
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<span class="phrase"><a name="boost_asio.reference.async_read_until.overload8.parameters"></a></span><a class="link" href="overload8.html#boost_asio.reference.async_read_until.overload8.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.
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</p></dd>
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<dt><span class="term">match_condition</span></dt>
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<dd>
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<p>
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The function object to be called to determine whether a match exists.
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The signature of the function object must be:
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</p>
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<pre class="programlisting">pair<iterator, bool> match_condition(iterator begin, iterator end);
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</pre>
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<p>
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where <code class="computeroutput">iterator</code> represents the type:
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</p>
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<pre class="programlisting">buffers_iterator<basic_streambuf<Allocator>::const_buffers_type>
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</pre>
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<p>
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The iterator parameters <code class="computeroutput">begin</code> and <code class="computeroutput">end</code> define
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the range of bytes to be scanned to determine whether there is a
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match. The <code class="computeroutput">first</code> member of the return value is an iterator
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marking one-past-the-end of the bytes that have been consumed by
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the match function. This iterator is used to calculate the <code class="computeroutput">begin</code>
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parameter for any subsequent invocation of the match condition. The
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<code class="computeroutput">second</code> member of the return value is true if a match
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has been found, false otherwise.
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</p>
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</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 that have been fully consumed by the
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// match function. O 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.overload8.h1"></a>
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<span class="phrase"><a name="boost_asio.reference.async_read_until.overload8.remarks"></a></span><a class="link" href="overload8.html#boost_asio.reference.async_read_until.overload8.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 function object. 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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<p>
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The default implementation of the <code class="computeroutput">is_match_condition</code> type
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trait evaluates to true for function pointers and function objects with
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a <code class="computeroutput">result_type</code> typedef. It must be specialised for other user-defined
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function objects.
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</p>
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<h6>
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<a name="boost_asio.reference.async_read_until.overload8.h2"></a>
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<span class="phrase"><a name="boost_asio.reference.async_read_until.overload8.examples"></a></span><a class="link" href="overload8.html#boost_asio.reference.async_read_until.overload8.examples">Examples</a>
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</h6>
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<p>
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To asynchronously read data into a streambuf until whitespace is encountered:
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</p>
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<pre class="programlisting">typedef boost::asio::buffers_iterator<
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boost::asio::streambuf::const_buffers_type> iterator;
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std::pair<iterator, bool>
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match_whitespace(iterator begin, iterator end)
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{
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iterator i = begin;
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while (i != end)
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if (std::isspace(*i++))
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return std::make_pair(i, true);
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return std::make_pair(i, false);
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}
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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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boost::asio::streambuf b;
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boost::asio::async_read_until(s, b, match_whitespace, handler);
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</pre>
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<p>
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To asynchronously read data into a streambuf until a matching character
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is found:
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</p>
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<pre class="programlisting">class match_char
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{
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public:
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explicit match_char(char c) : c_(c) {}
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template <typename Iterator>
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std::pair<Iterator, bool> operator()(
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Iterator begin, Iterator end) const
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{
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Iterator i = begin;
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while (i != end)
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if (c_ == *i++)
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return std::make_pair(i, true);
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return std::make_pair(i, false);
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}
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private:
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char c_;
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};
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namespace asio {
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template <> struct is_match_condition<match_char>
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: public boost::true_type {};
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} // namespace asio
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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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boost::asio::streambuf b;
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boost::asio::async_read_until(s, b, match_char('a'), handler);
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</pre>
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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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<hr>
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