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<title>The Overhead in the Number Class Wrapper</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_multiprecision.perf.overhead"></a><a class="link" href="overhead.html" title="The Overhead in the Number Class Wrapper">The Overhead in the
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Number Class Wrapper</a>
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</h3></div></div></div>
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<p>
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Using a simple <a href="http://www.boost.org/doc/libs/release/libs/multiprecision/doc/html/../../performance/arithmetic_backend.hpp" target="_top">backend
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class</a> that wraps any <a href="https://en.cppreference.com/w/cpp/language/types" target="_top">fundamental
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(built-in)</a> arithmetic type we can measure the overhead involved in
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wrapping a type inside the <code class="computeroutput"><span class="identifier">number</span></code>
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frontend, and the effect that turning on expression templates has. The following
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table compares the performance between <code class="computeroutput"><span class="keyword">double</span></code>
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and a <code class="computeroutput"><span class="keyword">double</span></code> wrapped inside
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class <code class="computeroutput"><span class="identifier">number</span></code>:
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</p>
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<div class="table">
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<a name="boost_multiprecision.perf.overhead.bessel_functions_16_digit_precis"></a><p class="title"><b>Table 1.12. Bessel Functions (16 digit precision)</b></p>
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<div class="table-contents"><table class="table" summary="Bessel Functions (16 digit precision)">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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Type
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</p>
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</th>
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<th>
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<p>
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Time
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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arithmetic_backend<double>
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</p>
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</td>
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<td>
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<p>
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2.09301 (0.00133409s)
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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arithmetic_backend<double> - no expression templates
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</p>
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</td>
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<td>
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<p>
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1 (0.000637403s)
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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double
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</p>
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</td>
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<td>
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<p>
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1.07956 (0.000688113s)
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<br class="table-break"><p>
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As you can see whether or not there is an overhead, and how large it is depends
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on the actual situation, but the overhead is in any cases small. Expression
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templates generally add a greater overhead the more complex the expression
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becomes due to the logic of figuring out how to best unpack and evaluate
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the expression, but of course this is also the situation where you save more
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temporaries. For a "trivial" backend like this, saving temporaries
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has no benefit, but for larger types it becomes a bigger win.
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</p>
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<p>
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The following table compares arithmetic using either <code class="computeroutput"><span class="keyword">long</span>
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<span class="keyword">long</span></code> or <code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">arithmetic_backend</span><span class="special"><</span><span class="keyword">long</span> <span class="keyword">long</span><span class="special">></span> <span class="special">></span></code> for the <a href="http://www.boost.org/doc/libs/release/libs/multiprecision/doc/html/../../performance/voronoi_performance.cpp" target="_top">voronoi-diagram
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builder test</a>:
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</p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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Type
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</p>
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</th>
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<th>
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<p>
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Relative time
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">int64_t</span></code>
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</p>
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</td>
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<td>
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<p>
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<span class="bold"><strong>1.0</strong></span>(0.0128646s)
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">number</span><span class="special"><</span><span class="identifier">arithmetic_backend</span><span class="special"><</span><span class="identifier">int64_t</span><span class="special">>,</span>
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<span class="identifier">et_off</span><span class="special">></span></code>
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</p>
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</td>
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<td>
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<p>
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1.005 (0.0129255s)
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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<p>
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This test involves mainly creating a lot of temporaries and performing a
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small amount of arithmetic on them, with very little difference in performance
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between the native and "wrapped" types.
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</p>
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<div class="table">
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<a name="boost_multiprecision.perf.overhead.platform"></a><p class="title"><b>Table 1.13. Platform Details</b></p>
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<div class="table-contents"><table class="table" summary="Platform Details">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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Platform
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</p>
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</th>
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<th>
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<p>
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Linux 5.3.0-24-generic, version #26-Ubuntu SMP Thu Nov 14 01:33:18
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UTC 2019, x86_64
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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Compiler
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</p>
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</td>
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<td>
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<p>
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GNU C++ version 9.2.1 20191008
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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Boost
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</p>
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</td>
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<td>
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<p>
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1.72.0
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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Run date
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</p>
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</td>
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<td>
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<p>
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Dec 13 2019
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<br class="table-break">
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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 © 2002-2020 John
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Maddock and Christopher Kormanyos<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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