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<title>Hints on using float128 (and __float128)</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="math_toolkit.float128_hints"></a><a class="link" href="float128_hints.html" title="Hints on using float128 (and __float128)">Hints on using float128 (and
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__float128)</a>
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</h2></div></div></div>
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<h6>
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<a name="math_toolkit.float128_hints.h0"></a>
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<span class="phrase"><a name="math_toolkit.float128_hints.different_float128"></a></span><a class="link" href="float128_hints.html#math_toolkit.float128_hints.different_float128">__float128
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versus float128</a>
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</h6>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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__float128 is the (optionally) compiler supplied hardware type, it's an
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C-ish extension to C++ and there is only minimal support for it in normal
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C++ (no IO streams or <code class="computeroutput"><span class="identifier">numeric_limits</span></code>
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support, function names in libquadmath all have different names to the
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<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span></code>
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ones etc.) So you can program type <code class="computeroutput"><span class="identifier">__float128</span></code>
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directly, but it's harder work.
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</li>
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<li class="listitem">
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Type <code class="computeroutput"><span class="identifier">float128</span></code> uses __float128
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and makes it C++ and generic code friendly, with all the usual standard
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<code class="computeroutput"><span class="identifier">iostream</span></code>, <code class="computeroutput"><span class="identifier">numeric_limits</span></code>, <code class="computeroutput"><span class="identifier">complex</span></code>
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in namspace <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span></code>
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available, so strongly recommended for C++ use.
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</li>
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</ul></div>
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<h6>
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<a name="math_toolkit.float128_hints.h1"></a>
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<span class="phrase"><a name="math_toolkit.float128_hints.hints_and_tips"></a></span><a class="link" href="float128_hints.html#math_toolkit.float128_hints.hints_and_tips">Hints
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and tips</a>
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</h6>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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Make sure you declare variables with the correct type, here <code class="computeroutput"><span class="identifier">float128</span></code>.
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</li>
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<li class="listitem">
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Make sure that if you pass a variable to a function then it is casted to
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<code class="computeroutput"><span class="identifier">float128</span></code>.
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</li>
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<li class="listitem">
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Make sure you declare literals with the correct suffix - otherwise they'll
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be treated as type <code class="computeroutput"><span class="keyword">double</span></code>
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with catastrophic loss of precision. So make sure they have a Q suffix
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for 128-bit floating-point literals.
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</li>
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<li class="listitem">
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All the std library functions, cmath functions, plus all the constants,
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and special functions from Boost.Math should then just work.
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</li>
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<li class="listitem">
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Make sure std lib functions are called <span class="bold"><strong>unqualified</strong></span>
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so that the correct overload is found via <a href="http://en.cppreference.com/w/cpp/language/adl" target="_top">Argument
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Dependent Lookup (ADL)</a>. So write sqrt(variable) and not std::sqrt(variable).
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</li>
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<li class="listitem">
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In general, try not to reinvent stuff - using constants from Boost.Math
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is probably less error prone than declaring your own, likewise the special
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functions etc.
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</li>
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</ul></div>
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<p>
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Some examples of what can go horribly and silently wrong are at <a href="../../../example/float128_example.cpp" target="_top">float128_example.cpp</a>.
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</p>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal,
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Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
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Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani,
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Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<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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