1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Overloading template functions with float128_t</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="../float128.html" title="Implementation of Float128 type"> 9<link rel="prev" href="../float128.html" title="Implementation of Float128 type"> 10<link rel="next" href="exp_function.html" title="Exponential function"> 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="../float128.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../float128.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="exp_function.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="math_toolkit.float128.overloading"></a><a class="link" href="overloading.html" title="Overloading template functions with float128_t">Overloading template 28 functions with float128_t</a> 29</h3></div></div></div> 30<p> 31 An artifact of providing C++ standard library support for quadmath may mandate 32 the inclusion of <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 33 <span class="bold"><strong>before</strong></span> the inclusion of other headers. 34 </p> 35<p> 36 Consider a function that calls <code class="computeroutput"><span class="identifier">fabs</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code> 37 and has previously injected <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">()</span></code> into local scope via a <code class="computeroutput"><span class="keyword">using</span></code> 38 directive: 39 </p> 40<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 41<span class="keyword">bool</span> <span class="identifier">unsigned_compare</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">b</span><span class="special">)</span> 42<span class="special">{</span> 43 <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">;</span> 44 <span class="keyword">return</span> <span class="identifier">fabs</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">fabs</span><span class="special">(</span><span class="identifier">b</span><span class="special">);</span> 45<span class="special">}</span> 46</pre> 47<p> 48 In this function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code> 49 may be found via <a href="http://en.wikipedia.org/wiki/Argument-dependent_name_lookup" target="_top">argument-dependent-lookup 50 (ADL)</a> or by calling one of the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span></code> 51 overloads. There is a key difference between them however: an overload in 52 the same namespace as T and found via ADL need <span class="emphasis"><em><span class="bold"><strong>not 53 be defined at the time the function is declared</strong></span></em></span>. However, 54 all the types declared in <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 55 are fundamental types, so for these types we are relying on finding an overload 56 declared in namespace <code class="computeroutput"><span class="identifier">std</span></code>. 57 In that case however, <span class="emphasis"><em><span class="bold"><strong>all such overloads 58 must be declared prior to the definition of function <code class="computeroutput"><span class="identifier">unsigned_compare</span></code> 59 otherwise they are not considered</strong></span></em></span>. 60 </p> 61<p> 62 In the event that <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 63 has been included <span class="bold"><strong>after</strong></span> the definition of 64 the above function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code>, 65 while present, is simply not considered as part of the overload set. So the 66 compiler tries to downcast the <code class="computeroutput"><span class="identifier">float128_t</span></code> 67 argument first to <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>, 68 then to <code class="computeroutput"><span class="keyword">double</span></code>, then to <code class="computeroutput"><span class="keyword">float</span></code>; the compilation fails because the result 69 is ambiguous. However the compiler error message will appear cruelly inscrutable, 70 at an apparently irrelevant line number and making no mention of <code class="computeroutput"><span class="identifier">float128</span></code>: the word <span class="emphasis"><em>ambiguous</em></span> 71 is the clue to what is wrong. 72 </p> 73<p> 74 Provided you <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 75 <span class="bold"><strong>before</strong></span> the inclusion of the any header containing 76 generic floating point code (such as other Boost.Math headers, then the compiler 77 will know about and use the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">float128_t</span><span class="special">)</span></code> 78 that we provide in <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">cstdfloat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>. 79 </p> 80</div> 81<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 82<td align="left"></td> 83<td align="right"><div class="copyright-footer">Copyright © 2006-2019 Nikhar 84 Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, 85 Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan 86 Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg, 87 Daryle Walker and Xiaogang Zhang<p> 88 Distributed under the Boost Software License, Version 1.0. (See accompanying 89 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>) 90 </p> 91</div></td> 92</tr></table> 93<hr> 94<div class="spirit-nav"> 95<a accesskey="p" href="../float128.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../float128.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="exp_function.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 96</div> 97</body> 98</html> 99
