| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Overloading template functions with float128_t</title> |
| <link rel="stylesheet" href="../../math.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.77.1"> |
| <link rel="home" href="../../index.html" title="Math Toolkit 2.2.0"> |
| <link rel="up" href="../float128.html" title="Implementation of Float128 type"> |
| <link rel="prev" href="../float128.html" title="Implementation of Float128 type"> |
| <link rel="next" href="exp_function.html" title="Exponential function"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <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> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="math_toolkit.float128.overloading_template_functions_w"></a><a class="link" href="overloading_template_functions_w.html" title="Overloading template functions with float128_t">Overloading |
| template functions with float128_t</a> |
| </h3></div></div></div> |
| <p> |
| An artifact of providing C++ standard library support for quadmath may mandate |
| 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> |
| <span class="bold"><strong>before</strong></span> the inclusion of other headers. |
| </p> |
| <p> |
| 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> |
| 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> |
| directive: |
| </p> |
| <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> |
| <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> |
| <span class="special">{</span> |
| <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">;</span> |
| <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| In this function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code> |
| may be found via <a href="http://en.wikipedia.org/wiki/Argument-dependent_name_lookup" target="_top">argument-dependent-lookup |
| (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> |
| overloads. There is a key difference between them however: an overload in |
| the same namespace as T and found via ADL need <span class="emphasis"><em><span class="bold"><strong>not |
| be defined at the time the function is declared</strong></span></em></span>. However, |
| 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> |
| are fundamental types, so for these types we are relying on finding an overload |
| declared in namespace <code class="computeroutput"><span class="identifier">std</span></code>. |
| In that case however, <span class="emphasis"><em><span class="bold"><strong>all such overloads |
| must be declared prior to the definition of function <code class="computeroutput"><span class="identifier">unsigned_compare</span></code> |
| otherwise they are not considered</strong></span></em></span>. |
| </p> |
| <p> |
| 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> |
| has been included <span class="bold"><strong>after</strong></span> the definition of |
| the above function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code>, |
| while present, is simply not considered as part of the overload set. So the |
| compiler tries to downcast the <code class="computeroutput"><span class="identifier">float128_t</span></code> |
| argument first to <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>, |
| 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 |
| is ambiguous. However the compiler error message will appear cruelly inscrutable, |
| at an apparently irelevant 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> |
| is the clue to what is wrong. |
| </p> |
| <p> |
| 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> |
| <span class="bold"><strong>before</strong></span> the inclusion of the any header containing |
| generic floating point code (such as other Boost.Math headers, then the compiler |
| 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> |
| 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>. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal, |
| Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert |
| Holin, Bruno Lalande, John Maddock, Johan Råde, Gautam Sewani, Benjamin Sobotta, |
| Thijs van den Berg, Daryle Walker and Xiaogang Zhang<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> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <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> |
| </div> |
| </body> |
| </html> |
