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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_multiprecision.tut.floats.float128"></a><a class="link" href="float128.html" title="float128">float128</a>
</h4></div></div></div>
<p>
<code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</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">></span></code>
</p>
<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>
<span class="keyword">class</span> <span class="identifier">float128_backend</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">number</span><span class="special"><</span><span class="identifier">float128_backend</span><span class="special">,</span> <span class="identifier">et_off</span><span class="special">></span> <span class="identifier">float128</span><span class="special">;</span>
<span class="special">}}</span> <span class="comment">// namespaces</span>
</pre>
<p>
The <code class="computeroutput"><span class="identifier">float128</span></code> number type
is a very thin wrapper around GCC's <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
or Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code> data types
and provides an real-number type that is a drop-in replacement for the
native C++ floating-point types, but with a 113 bit mantissa, and compatible
with FORTRAN's 128-bit QUAD real.
</p>
<p>
All the usual standard library and <code class="computeroutput"><span class="identifier">numeric_limits</span></code>
support are available, performance should be equivalent to the underlying
native types: for example the LINPACK benchmarks for GCC's <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
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>.
</p>
<p>
As well as the usual conversions from arithmetic and string types, instances
of <code class="computeroutput"><span class="identifier">float128</span></code> are copy constructible
and assignable from GCC's <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
and Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code> data
types.
</p>
<p>
It's also possible to access the underlying <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
or <code class="computeroutput"><span class="identifier">_Quad</span></code> type via the
<code class="computeroutput"><span class="identifier">data</span><span class="special">()</span></code>
member function of <code class="computeroutput"><span class="identifier">float128_backend</span></code>.
</p>
<p>
Things you should know when using this type:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Default constructed <code class="computeroutput"><span class="identifier">float128</span></code>s
have the value zero.
</li>
<li class="listitem">
This backend supports rvalue-references and is move-aware, making instantiations
of <code class="computeroutput"><span class="identifier">number</span></code> on this backend
move aware.
</li>
<li class="listitem">
It is not possible to round-trip objects of this type to and from a
string and get back exactly the same value when compiled with Intel's
C++ compiler and using <code class="computeroutput"><span class="identifier">_Quad</span></code>
as the underlying type: this is a current limitation of our code. Round
tripping when using <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
as the underlying type is possible (both for GCC and Intel).
</li>
<li class="listitem">
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>
being thrown if the string can not be interpreted as a valid floating
point number.
</li>
<li class="listitem">
Division by zero results in an infinity being produced.
</li>
<li class="listitem">
Type <code class="computeroutput"><span class="identifier">float128</span></code> can be
used as a literal type (constexpr support).
</li>
<li class="listitem">
When using the Intel compiler, the underlying type defaults to <code class="computeroutput"><a class="link" href="float128.html" title="float128">float128</a></code>
if it's available and <code class="computeroutput"><span class="identifier">_Quad</span></code>
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>.
</li>
<li class="listitem">
When the underlying type is Intel's <code class="computeroutput"><span class="identifier">_Quad</span></code>
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>.
</li>
</ul></div>
<h6>
<a name="boost_multiprecision.tut.floats.float128.h0"></a>
<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
example:</a>
</h6>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</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">></span>
<span class="preprocessor">#include</span> <span class="special"><</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">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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>
<span class="comment">// Operations at 128-bit precision and full numeric_limits support:</span>
<span class="identifier">float128</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
<span class="comment">// There are 113-bits of precision:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</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">float128</span><span class="special">>::</span><span class="identifier">digits</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">// Or 34 decimal places:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</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">float128</span><span class="special">>::</span><span class="identifier">digits10</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">// We can use any C++ std lib function, lets print all the digits as well:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</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"><</span><span class="identifier">float128</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">)</span>
<span class="special"><<</span> <span class="identifier">log</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span> <span class="special"><<</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>
<span class="comment">// We can also use any function from Boost.Math:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</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"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">// And since we have an extended exponent range we can generate some really large </span>
<span class="comment">// numbers here (4.02387260077093773543702433923004111e+2564):</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</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"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">//</span>
<span class="comment">// We can declare constants using GCC or Intel's native types, and the Q suffix,</span>
<span class="comment">// these can be declared constexpr if required:</span>
<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>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<div class="footnotes">
<br><hr style="width:100; align:left;">
<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>
On 64-bit Ubuntu 11.10, GCC-4.8.0, Intel Core 2 Duo T5800.
</p></div>
</div>
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<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2002-2013 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>)
</p>
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