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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_multiprecision.tut.input_output"></a><a class="link" href="input_output.html" title="Input Output">Input Output</a>
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</h3></div></div></div>
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<h5>
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<a name="boost_multiprecision.tut.input_output.h0"></a>
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<span class="phrase"><a name="boost_multiprecision.tut.input_output.loopback_testing"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.loopback_testing">Loopback
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testing</a>
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</h5>
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<p>
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<span class="emphasis"><em>Loopback</em></span> or <span class="emphasis"><em>round-tripping</em></span> refers
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to writing out a value as a decimal digit string using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">iostream</span></code>,
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usually to a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code>, and then reading the string
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back in to another value, and confirming that the two values are identical.
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A trivial example using <code class="computeroutput"><span class="keyword">float</span></code>
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is:
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</p>
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<pre class="programlisting"><span class="keyword">float</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Value to round-trip.</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span> <span class="comment">// Read and write std::stringstream.</span>
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<span class="identifier">ss</span><span class="special">.</span><span class="identifier">precision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span>
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<span class="identifier">ss</span><span class="special">.</span><span class="identifier">flags</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">fmtflags</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">scientific</span><span class="special">));</span> <span class="comment">// Use scientific format.</span>
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<span class="identifier">ss</span> <span class="special"><<</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to string.</span>
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<span class="keyword">float</span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Expected.</span>
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<span class="identifier">ss</span> <span class="special">>></span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Read decimal digits string from stringstream.</span>
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<span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">write</span><span class="special">,</span> <span class="identifier">read</span><span class="special">);</span> <span class="comment">// Should be the same.</span>
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</pre>
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<p>
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and this can be run in a loop for all possible values of a 32-bit float.
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For other floating-point types <code class="computeroutput"><span class="identifier">T</span></code>,
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including built-in <code class="computeroutput"><span class="keyword">double</span></code>, it
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takes far too long to test all values, so a reasonable test strategy is to
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use a large number of random values.
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</p>
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<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">write</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span>
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<span class="identifier">ss</span><span class="special">.</span><span class="identifier">precision</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span>
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<span class="identifier">ss</span><span class="special">.</span><span class="identifier">flags</span><span class="special">(</span><span class="identifier">f</span><span class="special">);</span> <span class="comment">// Changed from default iostream format flags if desired.</span>
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<span class="identifier">ss</span> <span class="special"><<</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to stringstream.</span>
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<span class="identifier">T</span> <span class="identifier">read</span><span class="special">;</span>
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<span class="identifier">ss</span> <span class="special">>></span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Get read using operator>> from stringstream.</span>
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<span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span>
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<span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">());</span> <span class="comment">// Get read by converting from decimal digits string representation of write.</span>
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<span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span>
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<span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">write</span><span class="special">.</span><span class="identifier">str</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="identifier">f</span><span class="special">));</span> <span class="comment">// Get read using format specified when written.</span>
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<span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span>
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</pre>
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<p>
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The test at <a href="../../../../test/test_cpp_bin_float_io.cpp" target="_top">test_cpp_bin_float_io.cpp</a>
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allows any floating-point type to be <span class="emphasis"><em>round_tripped</em></span> using
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a wide range of fairly random values. It also includes tests compared a collection
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of <a href="../../../../test/string_data.ipp" target="_top">stringdata</a> test cases
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in a file.
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</p>
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<h5>
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<a name="boost_multiprecision.tut.input_output.h1"></a>
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<span class="phrase"><a name="boost_multiprecision.tut.input_output.comparing_with_output_using_buil"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.comparing_with_output_using_buil">Comparing
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with output using Built-in types</a>
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</h5>
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<p>
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One can make some comparisons with the output of
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</p>
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<pre class="programlisting"><span class="special"><</span><span class="identifier">number</span><span class="special"><</span><span class="identifier">cpp_bin_float</span><span class="special"><</span><span class="number">53</span><span class="special">,</span> <span class="identifier">digit_count_2</span><span class="special">></span> <span class="special">></span>
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</pre>
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<p>
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which has the same number of significant bits (53) as 64-bit double precision
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floating-point.
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</p>
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<p>
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However, although most outputs are identical, there are differences on some
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platforms caused by the implementation-dependent behaviours allowed by the
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C99 specification <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99
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ISO/IEC 9899:TC2</a>, incorporated by C++.
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</p>
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<div class="blockquote"><blockquote class="blockquote"><p>
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<span class="emphasis"><em>"For e, E, f, F, g, and G conversions, if the number of
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significant decimal digits is at most DECIMAL_DIG, then the result should
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be correctly rounded. If the number of significant decimal digits is more
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than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG
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digits, then the result should be an exact representation with trailing
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zeros. Otherwise, the source value is bounded by two adjacent decimal strings
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L < U, both having DECIMAL_DIG significant digits; the value of the
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resultant decimal string D should satisfy L<= D <= U, with the extra
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stipulation that the error should have a correct sign for the current rounding
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direction."</em></span>
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</p></blockquote></div>
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<p>
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So not only is correct rounding for the full number of digits not required,
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but even if the <span class="bold"><strong>optional</strong></span> recommended practice
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is followed, then the value of these last few digits is unspecified as long
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as the value is within certain bounds.
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</p>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Do not expect the output from different platforms to be <span class="bold"><strong>identical</strong></span>,
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but <code class="computeroutput"><span class="identifier">cpp_dec_float</span></code>, <code class="computeroutput"><span class="identifier">cpp_bin_float</span></code> (and other backends) outputs
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should be correctly rounded to the number of digits requested by the set
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precision and format.
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</p></td></tr>
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</table></div>
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<h5>
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<a name="boost_multiprecision.tut.input_output.h2"></a>
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<span class="phrase"><a name="boost_multiprecision.tut.input_output.macro_boost_mp_min_exponent_digi"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.macro_boost_mp_min_exponent_digi">Macro
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BOOST_MP_MIN_EXPONENT_DIGITS</a>
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</h5>
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<p>
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<a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99
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Standard</a> for format specifiers, 7.19.6 Formatted input/output functions
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requires:
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</p>
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<p>
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"The exponent always contains at least two digits, and only as many
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more digits as necessary to represent the exponent."
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</p>
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<p>
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So to conform to the C99 standard (incorporated by C++)
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</p>
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<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">2</span>
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</pre>
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<p>
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Confusingly, Microsoft (and MinGW) do not conform to this standard and provide
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<span class="bold"><strong>at least three digits</strong></span>, for example <code class="computeroutput"><span class="number">1e+001</span></code>. So if you want the output to match
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that from built-in floating-point types on compilers that use Microsofts
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runtime then use:
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</p>
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<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">3</span>
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</pre>
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<p>
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Also useful to get the minimum exponent field width is
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</p>
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<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">1</span>
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</pre>
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<p>
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producing a compact output like <code class="computeroutput"><span class="number">2e+4</span></code>,
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useful when conserving space is important.
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</p>
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<p>
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Larger values are also supported, for example, value 4 for <code class="computeroutput"><span class="number">2e+0004</span></code> which may be useful to ensure that
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columns line up.
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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 © 2002-2019 John Maddock
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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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