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- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_typetraits.reference.detected_or"></a><a class="link" href="detected_or.html" title="detected_or">detected_or</a>
- </h3></div></div></div>
- <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Default</span><span class="special">,</span> <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...></span> <span class="keyword">class</span> <span class="identifier">Op</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span>
- <span class="keyword">using</span> <span class="identifier">detected_or</span> <span class="special">=</span> <em class="replaceable"><code>see-below</code></em><span class="special">;</span>
- <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Default</span><span class="special">,</span> <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...></span> <span class="keyword">class</span> <span class="identifier">Op</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span>
- <span class="keyword">using</span> <span class="identifier">detected_or_t</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">detected_or</span><span class="special"><</span><span class="identifier">Default</span><span class="special">,</span> <span class="identifier">Op</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">...>::</span><span class="identifier">type</span><span class="special">;</span>
- </pre>
- <p>
- <span class="bold"><strong>Aliases:</strong></span> An unspecified type with two public
- member type definitions:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- <code class="computeroutput"><span class="identifier">value_t</span></code> is <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>
- if <code class="computeroutput"><span class="identifier">Op</span><span class="special"><</span><span class="identifier">Args</span><span class="special">...></span></code>
- is a valid template-id, otherwise <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>
- </li>
- <li class="listitem">
- <code class="computeroutput"><span class="identifier">type</span></code> is <code class="computeroutput"><span class="identifier">Op</span><span class="special"><</span><span class="identifier">Args</span><span class="special">...></span></code>
- if it is a valid template-id, otherwise <code class="computeroutput"><span class="identifier">Default</span></code>
- </li>
- </ul></div>
- <p>
- <span class="bold"><strong>C++ Standard Paper:</strong></span> <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf" target="_top">N4502</a>
- </p>
- <p>
- <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires C++11 variadic
- templates and C++11 template aliases.
- </p>
- <p>
- <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"><span class="preprocessor">#include</span>
- <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">detected_or</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
- </p>
- <p>
- <span class="bold"><strong>Examples:</strong></span>
- </p>
- <p>
- Suppose we wish to declare a type that represents the difference between
- two values of type T, it should be T::difference_type if such a type exists,
- or std::ptrdiff_t otherwise:
- </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">using</span> <span class="identifier">difference_t</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">::</span><span class="identifier">difference_type</span><span class="special">;</span>
- <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">using</span> <span class="identifier">difference_type</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">detected_or_t</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span><span class="special">,</span> <span class="identifier">difference_t</span><span class="special">,</span> <span class="identifier">T</span><span class="special">>;</span>
- </pre>
- <p>
- Now the type <code class="computeroutput"><span class="identifier">difference_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
- gives us what we need.
- </p>
- <p>
- See also: <a class="link" href="is_detected.html" title="is_detected">is_detected</a>,
- <a class="link" href="is_detected_convertible.html" title="is_detected_convertible">is_detected_convertible</a>,
- <a class="link" href="is_detected_exact.html" title="is_detected_exact">is_detected_exact</a>.
- </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 © 2000, 2011 Adobe Systems Inc, David Abrahams,
- Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
- Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
- Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
- Watanabe<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>
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