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<title>Overloading template functions with float128_t</title>
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<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>
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<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>
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Holin, Bruno Lalande, John Maddock, Johan Råde, Gautam Sewani, Benjamin Sobotta,
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