1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Hints on using float128 (and __float128)</title> 5<link rel="stylesheet" href="../math.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Math Toolkit 2.12.0"> 8<link rel="up" href="../cstdfloat.html" title="Chapter 3. Specified-width floating-point typedefs"> 9<link rel="prev" href="examples.html" title="Examples"> 10<link rel="next" href="float128.html" title="Implementation of Float128 type"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="examples.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../cstdfloat.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="float128.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<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 28 __float128)</a> 29</h2></div></div></div> 30<h6> 31<a name="math_toolkit.float128_hints.h0"></a> 32 <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 33 versus float128</a> 34 </h6> 35<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 36<li class="listitem"> 37 __float128 is the (optionally) compiler supplied hardware type, it's an 38 C-ish extension to C++ and there is only minimal support for it in normal 39 C++ (no IO streams or <code class="computeroutput"><span class="identifier">numeric_limits</span></code> 40 support, function names in libquadmath all have different names to the 41 <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span></code> 42 ones etc.) So you can program type <code class="computeroutput"><span class="identifier">__float128</span></code> 43 directly, but it's harder work. 44 </li> 45<li class="listitem"> 46 Type <code class="computeroutput"><span class="identifier">float128</span></code> uses __float128 47 and makes it C++ and generic code friendly, with all the usual standard 48 <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> 49 in namspace <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span></code> 50 available, so strongly recommended for C++ use. 51 </li> 52</ul></div> 53<h6> 54<a name="math_toolkit.float128_hints.h1"></a> 55 <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 56 and tips</a> 57 </h6> 58<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 59<li class="listitem"> 60 Make sure you declare variables with the correct type, here <code class="computeroutput"><span class="identifier">float128</span></code>. 61 </li> 62<li class="listitem"> 63 Make sure that if you pass a variable to a function then it is casted to 64 <code class="computeroutput"><span class="identifier">float128</span></code>. 65 </li> 66<li class="listitem"> 67 Make sure you declare literals with the correct suffix - otherwise they'll 68 be treated as type <code class="computeroutput"><span class="keyword">double</span></code> 69 with catastrophic loss of precision. So make sure they have a Q suffix 70 for 128-bit floating-point literals. 71 </li> 72<li class="listitem"> 73 All the std library functions, cmath functions, plus all the constants, 74 and special functions from Boost.Math should then just work. 75 </li> 76<li class="listitem"> 77 Make sure std lib functions are called <span class="bold"><strong>unqualified</strong></span> 78 so that the correct overload is found via <a href="http://en.cppreference.com/w/cpp/language/adl" target="_top">Argument 79 Dependent Lookup (ADL)</a>. So write sqrt(variable) and not std::sqrt(variable). 80 </li> 81<li class="listitem"> 82 In general, try not to reinvent stuff - using constants from Boost.Math 83 is probably less error prone than declaring your own, likewise the special 84 functions etc. 85 </li> 86</ul></div> 87<p> 88 Some examples of what can go horribly and silently wrong are at <a href="../../../example/float128_example.cpp" target="_top">float128_example.cpp</a>. 89 </p> 90</div> 91<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 92<td align="left"></td> 93<td align="right"><div class="copyright-footer">Copyright © 2006-2019 Nikhar 94 Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, 95 Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan 96 Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg, 97 Daryle Walker and Xiaogang Zhang<p> 98 Distributed under the Boost Software License, Version 1.0. (See accompanying 99 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>) 100 </p> 101</div></td> 102</tr></table> 103<hr> 104<div class="spirit-nav"> 105<a accesskey="p" href="examples.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../cstdfloat.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="float128.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 106</div> 107</body> 108</html> 109
