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26 <div class=
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27 <a name=
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28 functions with float128_t
</a>
29 </h3></div></div></div>
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.
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>
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>
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>.
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.
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>.
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"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 R
åde, 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>)
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