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   4 <title>Function template extract_or_default</title>
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  21 <h2><span class="refentrytitle">Function template extract_or_default</span></h2>
  22 <p>boost::log::extract_or_default</p>
  23 </div>
  24 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  25 <div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../attributes.html#header.boost.log.attributes.value_extraction_hpp" title="Header &lt;boost/log/attributes/value_extraction.hpp&gt;">boost/log/attributes/value_extraction.hpp</a>&gt;
  26
  27 </span>
  28 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> TagT <span class="special">=</span> <span class="keyword">void</span><span class="special">,</span> <span class="keyword">typename</span> DefaultT<span class="special">&gt;</span>
  29   <a class="link" href="result_of/extract_or_default.html" title="Struct template extract_or_default">result_of::extract_or_default</a><span class="special">&lt;</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">DefaultT</span><span class="special">,</span> <span class="identifier">TagT</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span>
  30   <span class="identifier">extract_or_default</span><span class="special">(</span><a class="link" href="attribute_value.html" title="Class attribute_value">attribute_value</a> <span class="keyword">const</span> <span class="special">&amp;</span> value<span class="special">,</span> <span class="identifier">DefaultT</span> <span class="keyword">const</span> <span class="special">&amp;</span> def_val<span class="special">)</span><span class="special">;</span></pre></div>
  31 <div class="refsect1">
  32 <a name="idm46436917955808"></a><h2>Description</h2>
  33 <p>The function extracts an attribute value from the view. The user has to explicitly specify the type or set of possible types of the attribute value to be visited.</p>
  34 <div class="note"><table border="0" summary="Note">
  35 <tr>
  36 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
  37 <th align="left">Note</th>
  38 </tr>
  39 <tr><td align="left" valign="top"><p>Caution must be exercised if the default value is a temporary object. Because the function returns a reference, if the temporary object is destroyed, the reference may become dangling.</p></td></tr>
  40 </table></div>
  41 <p>
  42
  43
  44 </p>
  45 <div class="variablelist"><table border="0" class="variablelist compact">
  46 <colgroup>
  47 <col align="left" valign="top">
  48 <col>
  49 </colgroup>
  50 <tbody>
  51 <tr>
  52 <td><p><span class="term">Parameters:</span></p></td>
  53 <td><div class="variablelist"><table border="0" class="variablelist compact">
  54 <colgroup>
  55 <col align="left" valign="top">
  56 <col>
  57 </colgroup>
  58 <tbody>
  59 <tr>
  60 <td><p><span class="term"><code class="computeroutput">def_val</code></span></p></td>
  61 <td><p>The default value </p></td>
  62 </tr>
  63 <tr>
  64 <td><p><span class="term"><code class="computeroutput">value</code></span></p></td>
  65 <td><p>Attribute value. </p></td>
  66 </tr>
  67 </tbody>
  68 </table></div></td>
  69 </tr>
  70 <tr>
  71 <td><p><span class="term">Returns:</span></p></td>
  72 <td><p>The extracted value, if found. The default value otherwise. </p></td>
  73 </tr>
  74 </tbody>
  75 </table></div>
  76 </div>
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  80 <td align="right"><div class="copyright-footer">Copyright &#169; 2007-2016 Andrey Semashev<p>
  81         Distributed under the Boost Software License, Version 1.0. (See accompanying
  82         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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