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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">&lt;</span><span class="keyword">class</span> <span class="identifier">Default</span><span class="special">,</span> <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">...&gt;</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">&gt;</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">&lt;</span><span class="keyword">class</span> <span class="identifier">Default</span><span class="special">,</span> <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">...&gt;</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">&gt;</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">&lt;</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">...&gt;::</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">&lt;</span><span class="identifier">Args</span><span class="special">...&gt;</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">&lt;</span><span class="identifier">Args</span><span class="special">...&gt;</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">&lt;</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">&gt;</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">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</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">&lt;</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">&gt;;</span>
</pre>
<p>
        Now the type <code class="computeroutput"><span class="identifier">difference_type</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
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<td align="right"><div class="copyright-footer">Copyright &#169; 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>)
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