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<title>Input Output</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_multiprecision.tut.input_output"></a><a class="link" href="input_output.html" title="Input Output">Input Output</a>
</h3></div></div></div>
<h5>
<a name="boost_multiprecision.tut.input_output.h0"></a>
<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
testing</a>
</h5>
<p>
<span class="emphasis"><em>Loopback</em></span> or <span class="emphasis"><em>round-tripping</em></span> refers
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>,
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
back in to another value, and confirming that the two values are identical.
A trivial example using <code class="computeroutput"><span class="keyword">float</span></code>
is:
</p>
<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>
<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>
<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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span>
<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>
<span class="identifier">ss</span> <span class="special">&lt;&lt;</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to string.</span>
<span class="keyword">float</span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Expected.</span>
<span class="identifier">ss</span> <span class="special">&gt;&gt;</span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Read decimal digits string from stringstream.</span>
<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>
</pre>
<p>
and this can be run in a loop for all possible values of a 32-bit float.
For other floating-point types <code class="computeroutput"><span class="identifier">T</span></code>,
including built-in <code class="computeroutput"><span class="keyword">double</span></code>, it
takes far too long to test all values, so a reasonable test strategy is to
use a large number of random values.
</p>
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">write</span><span class="special">;</span>
<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="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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span>
<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>
<span class="identifier">ss</span> <span class="special">&lt;&lt;</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to stringstream.</span>
<span class="identifier">T</span> <span class="identifier">read</span><span class="special">;</span>
<span class="identifier">ss</span> <span class="special">&gt;&gt;</span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Get read using operator&gt;&gt; from stringstream.</span>
<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>
<span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</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>
<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>
<span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</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>
<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>
</pre>
<p>
The test at <a href="../../../../test/test_cpp_bin_float_io.cpp" target="_top">test_cpp_bin_float_io.cpp</a>
allows any floating-point type to be <span class="emphasis"><em>round_tripped</em></span> using
a wide range of fairly random values. It also includes tests compared a collection
of <a href="../../../../test/string_data.ipp" target="_top">stringdata</a> test cases
in a file.
</p>
<h5>
<a name="boost_multiprecision.tut.input_output.h1"></a>
<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
with output using Built-in types</a>
</h5>
<p>
One can make some comparisons with the output of
</p>
<pre class="programlisting"><span class="special">&lt;</span><span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">cpp_bin_float</span><span class="special">&lt;</span><span class="number">53</span><span class="special">,</span> <span class="identifier">digit_count_2</span><span class="special">&gt;</span> <span class="special">&gt;</span>
</pre>
<p>
which has the same number of significant bits (53) as 64-bit double precision
floating-point.
</p>
<p>
However, although most outputs are identical, there are differences on some
platforms caused by the implementation-dependent behaviours allowed by the
C99 specification <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99
ISO/IEC 9899:TC2</a>, incorporated by C++.
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="emphasis"><em>"For e, E, f, F, g, and G conversions, if the number of
significant decimal digits is at most DECIMAL_DIG, then the result should
be correctly rounded. If the number of significant decimal digits is more
than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG
digits, then the result should be an exact representation with trailing
zeros. Otherwise, the source value is bounded by two adjacent decimal strings
L &lt; U, both having DECIMAL_DIG significant digits; the value of the
resultant decimal string D should satisfy L&lt;= D &lt;= U, with the extra
stipulation that the error should have a correct sign for the current rounding
direction."</em></span>
</p></blockquote></div>
<p>
So not only is correct rounding for the full number of digits not required,
but even if the <span class="bold"><strong>optional</strong></span> recommended practice
is followed, then the value of these last few digits is unspecified as long
as the value is within certain bounds.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Do not expect the output from different platforms to be <span class="bold"><strong>identical</strong></span>,
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
should be correctly rounded to the number of digits requested by the set
precision and format.
</p></td></tr>
</table></div>
<h5>
<a name="boost_multiprecision.tut.input_output.h2"></a>
<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
BOOST_MP_MIN_EXPONENT_DIGITS</a>
</h5>
<p>
<a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99
Standard</a> for format specifiers, 7.19.6 Formatted input/output functions
requires:
</p>
<p>
"The exponent always contains at least two digits, and only as many
more digits as necessary to represent the exponent."
</p>
<p>
So to conform to the C99 standard (incorporated by C++)
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">2</span>
</pre>
<p>
Confusingly, Microsoft (and MinGW) do not conform to this standard and provide
<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
that from built-in floating-point types on compilers that use Microsofts
runtime then use:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">3</span>
</pre>
<p>
Also useful to get the minimum exponent field width is
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">1</span>
</pre>
<p>
producing a compact output like <code class="computeroutput"><span class="number">2e+4</span></code>,
useful when conserving space is important.
</p>
<p>
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
columns line up.
</p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2019 John Maddock
and Christopher Kormanyos<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>)
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