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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">
<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
    __float128)</a>
</h2></div></div></div>
<h6>
<a name="math_toolkit.float128_hints.h0"></a>
      <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
      versus float128</a>
    </h6>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          __float128 is the (optionally) compiler supplied hardware type, it's an
          C-ish extension to C++ and there is only minimal support for it in normal
          C++ (no IO streams or <code class="computeroutput"><span class="identifier">numeric_limits</span></code>
          support, function names in libquadmath all have different names to the
          <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span></code>
          ones etc.) So you can program type <code class="computeroutput"><span class="identifier">__float128</span></code>
          directly, but it's harder work.
        </li>
<li class="listitem">
          Type <code class="computeroutput"><span class="identifier">float128</span></code> uses __float128
          and makes it C++ and generic code friendly, with all the usual standard
          <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>
          in namspace <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span></code>
          available, so strongly recommended for C++ use.
        </li>
</ul></div>
<h6>
<a name="math_toolkit.float128_hints.h1"></a>
      <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
      and tips</a>
    </h6>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Make sure you declare variables with the correct type, here <code class="computeroutput"><span class="identifier">float128</span></code>.
        </li>
<li class="listitem">
          Make sure that if you pass a variable to a function then it is casted to
          <code class="computeroutput"><span class="identifier">float128</span></code>.
        </li>
<li class="listitem">
          Make sure you declare literals with the correct suffix - otherwise they'll
          be treated as type <code class="computeroutput"><span class="keyword">double</span></code>
          with catastrophic loss of precision. So make sure they have a Q suffix
          for 128-bit floating-point literals.
        </li>
<li class="listitem">
          All the std library functions, cmath functions, plus all the constants,
          and special functions from Boost.Math should then just work.
        </li>
<li class="listitem">
          Make sure std lib functions are called <span class="bold"><strong>unqualified</strong></span>
          so that the correct overload is found via <a href="http://en.cppreference.com/w/cpp/language/adl" target="_top">Argument
          Dependent Lookup (ADL)</a>. So write sqrt(variable) and not std::sqrt(variable).
        </li>
<li class="listitem">
          In general, try not to reinvent stuff - using constants from Boost.Math
          is probably less error prone than declaring your own, likewise the special
          functions etc.
        </li>
</ul></div>
<p>
      Some examples of what can go horribly and silently wrong are at <a href="../../../example/float128_example.cpp" target="_top">float128_example.cpp</a>.
    </p>
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