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| 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 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> | |
| 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 © 2006-2010, 2012-2014 Nikhar Agrawal, | |
| 84 | Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert | |
| 85 | Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani, | |
| 86 | Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p> | |
| 87 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 88 | 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>) | |
| 89 | </p> | |
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