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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.is_detected"></a><a class="link" href="is_detected.html" title="is_detected">is_detected</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</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">is_detected</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">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">constexpr</span> <span class="keyword">bool</span> <span class="identifier">is_detected_v</span> <span class="special">=</span> <span class="identifier">is_detected</span><span class="special">&lt;</span><span class="identifier">Op</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">...&gt;::</span><span class="identifier">value</span><span class="special">;</span>
</pre>
<p>
        <span class="bold"><strong>Aliases:</strong></span> 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, aliases <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
        otherwise aliases <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
      </p>
<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">is_detected</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 "reset" a value of type T, if the type has a
        <code class="computeroutput"><span class="identifier">clear</span><span class="special">()</span></code>
        member function then we should call it, otherwise we should assign a default
        constructed value:
      </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">clear_t</span> <span class="special">=</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">declval</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;().</span><span class="identifier">clear</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">void</span> <span class="identifier">clear</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">value</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">if</span> <span class="keyword">constexpr</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_detected_v</span><span class="special">&lt;</span><span class="identifier">clear_t</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&gt;)</span> <span class="special">{</span>
        <span class="identifier">value</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
    <span class="special">}</span> <span class="keyword">else</span> <span class="special">{</span>
        <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">();</span>
    <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
        See also: <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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