ceph/src/boost/libs/multiprecision/doc/html/boost_multiprecision/tut/floats/float128.html

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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_multiprecision.tut.floats.float128"></a><a class="link" href="float128.html" title="float128">float128</a>
  28 </h4></div></div></div>
  29 <p>
  30           <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">multiprecision</span><span class="special">/</span><span class="identifier">float128</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
  31         </p>
  32 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">multiprecision</span><span class="special">{</span>
  33
  34 <span class="keyword">class</span> <span class="identifier">float128_backend</span><span class="special">;</span>
  35
  36 <span class="keyword">typedef</span> <span class="identifier">number</span><span class="special">&lt;</span><span class="identifier">float128_backend</span><span class="special">,</span> <span class="identifier">et_off</span><span class="special">&gt;</span>    <span class="identifier">float128</span><span class="special">;</span>
  37
  38 <span class="special">}}</span> <span class="comment">// namespaces</span>
  39 </pre>
  40 <p>
  41           The <code class="computeroutput"><span class="identifier">float128</span></code> number type
  42           is a very thin wrapper around GCC's <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
  43           or Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code> data types
  44           and provides an real-number type that is a drop-in replacement for the
  45           native C++ floating-point types, but with a 113 bit mantissa, and compatible
  46           with FORTRAN's 128-bit QUAD real.
  47         </p>
  48 <p>
  49           All the usual standard library and <code class="computeroutput"><span class="identifier">numeric_limits</span></code>
  50           support are available, performance should be equivalent to the underlying
  51           native types: for example the LINPACK benchmarks for GCC's <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
  52           and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">::</span><span class="identifier">float128</span></code> both achieved 5.6 MFLOPS<a href="#ftn.boost_multiprecision.tut.floats.float128.f0" class="footnote"><sup class="footnote"><a name="boost_multiprecision.tut.floats.float128.f0"></a>[3]</sup></a>.
  53         </p>
  54 <p>
  55           As well as the usual conversions from arithmetic and string types, instances
  56           of <code class="computeroutput"><span class="identifier">float128</span></code> are copy constructible
  57           and assignable from GCC's <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
  58           and Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code> data
  59           types.
  60         </p>
  61 <p>
  62           It's also possible to access the underlying <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
  63           or <code class="computeroutput"><span class="identifier">_Quad</span></code> type via the
  64           <code class="computeroutput"><span class="identifier">data</span><span class="special">()</span></code>
  65           member function of <code class="computeroutput"><span class="identifier">float128_backend</span></code>.
  66         </p>
  67 <p>
  68           Things you should know when using this type:
  69         </p>
  70 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  71 <li class="listitem">
  72               Default constructed <code class="computeroutput"><span class="identifier">float128</span></code>s
  73               have the value zero.
  74             </li>
  75 <li class="listitem">
  76               This backend supports rvalue-references and is move-aware, making instantiations
  77               of <code class="computeroutput"><span class="identifier">number</span></code> on this backend
  78               move aware.
  79             </li>
  80 <li class="listitem">
  81               It is not possible to round-trip objects of this type to and from a
  82               string and get back exactly the same value when compiled with Intel's
  83               C++ compiler and using <code class="computeroutput"><span class="identifier">_Quad</span></code>
  84               as the underlying type: this is a current limitation of our code. Round
  85               tripping when using <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
  86               as the underlying type is possible (both for GCC and Intel).
  87             </li>
  88 <li class="listitem">
  89               Conversion from a string results in a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span></code>
  90               being thrown if the string can not be interpreted as a valid floating-point
  91               number.
  92             </li>
  93 <li class="listitem">
  94               Division by zero results in an infinity being produced.
  95             </li>
  96 <li class="listitem">
  97               Type <code class="computeroutput"><span class="identifier">float128</span></code> can be
  98               used as a literal type (constexpr support).
  99             </li>
 100 <li class="listitem">
 101               When using the Intel compiler, the underlying type defaults to <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
 102               if it's available and <code class="computeroutput"><span class="identifier">_Quad</span></code>
 103               if not. You can override the default by defining either <code class="computeroutput"><span class="identifier">BOOST_MP_USE_FLOAT128</span></code> or <code class="computeroutput"><span class="identifier">BOOST_MP_USE_QUAD</span></code>.
 104             </li>
 105 <li class="listitem">
 106               When the underlying type is Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code>
 107               type, the code must be compiled with the compiler option <code class="computeroutput"><span class="special">-</span><span class="identifier">Qoption</span><span class="special">,</span><span class="identifier">cpp</span><span class="special">,--</span><span class="identifier">extended_float_type</span></code>.
 108             </li>
 109 </ul></div>
 110 <h6>
 111 <a name="boost_multiprecision.tut.floats.float128.h0"></a>
 112           <span class="phrase"><a name="boost_multiprecision.tut.floats.float128.float128_example"></a></span><a class="link" href="float128.html#boost_multiprecision.tut.floats.float128.float128_example">float128
 113           example:</a>
 114         </h6>
 115 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">multiprecision</span><span class="special">/</span><span class="identifier">float128</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 116 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">special_functions</span><span class="special">/</span><span class="identifier">gamma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 117 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 118
 119 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 120 <span class="special">{</span>
 121    <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">multiprecision</span><span class="special">;</span>
 122
 123    <span class="comment">// Operations at 128-bit precision and full numeric_limits support:</span>
 124    <span class="identifier">float128</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
 125    <span class="comment">// There are 113-bits of precision:</span>
 126    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">float128</span><span class="special">&gt;::</span><span class="identifier">digits</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 127    <span class="comment">// Or 34 decimal places:</span>
 128    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="identifier">float128</span><span class="special">&gt;::</span><span class="identifier">digits10</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 129    <span class="comment">// We can use any C++ std lib function, lets print all the digits as well:</span>
 130    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">setprecision</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">float128</span><span class="special">&gt;::</span><span class="identifier">max_digits10</span><span class="special">)</span>
 131       <span class="special">&lt;&lt;</span> <span class="identifier">log</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// print log(2) = 0.693147180559945309417232121458176575</span>
 132    <span class="comment">// We can also use any function from Boost.Math:</span>
 133    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 134    <span class="comment">// And since we have an extended exponent range we can generate some really large </span>
 135    <span class="comment">// numbers here (4.02387260077093773543702433923004111e+2564):</span>
 136    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">float128</span><span class="special">(</span><span class="number">1000</span><span class="special">))</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 137    <span class="comment">//</span>
 138    <span class="comment">// We can declare constants using GCC or Intel's native types, and the Q suffix,</span>
 139    <span class="comment">// these can be declared constexpr if required:</span>
 140
 141    <span class="keyword">constexpr</span> <span class="identifier">float128</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="number">3.1415926535897932384626433832795028841971693993751058</span><span class="identifier">Q</span><span class="special">;</span>
 142
 143    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 144 <span class="special">}</span>
 145 </pre>
 146 <div class="footnotes">
 147 <br><hr style="width:100; align:left;">
 148 <div id="ftn.boost_multiprecision.tut.floats.float128.f0" class="footnote"><p><a href="#boost_multiprecision.tut.floats.float128.f0" class="para"><sup class="para">[3] </sup></a>
 149             On 64-bit Ubuntu 11.10, GCC-4.8.0, Intel Core 2 Duo T5800.
 150           </p></div>
 151 </div>
 152 </div>
 153 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 154 <td align="left"></td>
 155 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2013 John Maddock and Christopher Kormanyos<p>
 156         Distributed under the Boost Software License, Version 1.0. (See accompanying
 157         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>)
 158       </p>
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