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<title>Overloading template functions with float128_t</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="math_toolkit.float128.overloading"></a><a class="link" href="overloading.html" title="Overloading template functions with float128_t">Overloading template
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functions with float128_t</a>
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</h3></div></div></div>
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<p>
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An artifact of providing C++ standard library support for quadmath may mandate
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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>
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<span class="bold"><strong>before</strong></span> the inclusion of other headers.
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</p>
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<p>
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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>
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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>
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directive:
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</p>
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<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>
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<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>
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<span class="special">{</span>
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<span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">fabs</span><span class="special">;</span>
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<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>
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<span class="special">}</span>
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</pre>
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<p>
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In this function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code>
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may be found via <a href="http://en.wikipedia.org/wiki/Argument-dependent_name_lookup" target="_top">argument-dependent-lookup
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(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>
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overloads. There is a key difference between them however: an overload in
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the same namespace as T and found via ADL need <span class="emphasis"><em><span class="bold"><strong>not
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be defined at the time the function is declared</strong></span></em></span>. However,
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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>
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are fundamental types, so for these types we are relying on finding an overload
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declared in namespace <code class="computeroutput"><span class="identifier">std</span></code>.
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In that case however, <span class="emphasis"><em><span class="bold"><strong>all such overloads
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must be declared prior to the definition of function <code class="computeroutput"><span class="identifier">unsigned_compare</span></code>
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otherwise they are not considered</strong></span></em></span>.
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</p>
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<p>
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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>
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has been included <span class="bold"><strong>after</strong></span> the definition of
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the above function, the correct overload of <code class="computeroutput"><span class="identifier">fabs</span></code>,
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while present, is simply not considered as part of the overload set. So the
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compiler tries to downcast the <code class="computeroutput"><span class="identifier">float128_t</span></code>
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argument first to <code class="computeroutput"><span class="keyword">long</span> <span class="keyword">double</span></code>,
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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
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is ambiguous. However the compiler error message will appear cruelly inscrutable,
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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>
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is the clue to what is wrong.
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</p>
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<p>
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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>
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<span class="bold"><strong>before</strong></span> the inclusion of the any header containing
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generic floating point code (such as other Boost.Math headers, then the compiler
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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>
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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>.
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<td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal,
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Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
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Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani,
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Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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