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  21 <h2><span class="refentrytitle">Function template extract_or_throw</span></h2>
  22 <p>boost::log::extract_or_throw</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> DescriptorT<span class="special">,</span> <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="special">&gt;</span> <span class="keyword">class</span> ActorT<span class="special">&gt;</span>
  29   <a class="link" href="result_of/extract_or_throw.html" title="Struct template extract_or_throw">result_of::extract_or_throw</a><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">DescriptorT</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">,</span> <span class="identifier">DescriptorT</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span>
  30   <span class="identifier">extract_or_throw</span><span class="special">(</span><span class="identifier">expressions</span><span class="special">::</span><span class="identifier">attribute_keyword</span><span class="special">&lt;</span> <span class="identifier">DescriptorT</span><span class="special">,</span> <span class="identifier">ActorT</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span> keyword<span class="special">,</span>
  31                    <span class="identifier">record</span> <span class="keyword">const</span> <span class="special">&amp;</span> rec<span class="special">)</span><span class="special">;</span></pre></div>
  32 <div class="refsect1">
  33 <a name="idm45554749268512"></a><h2>Description</h2>
  34 <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 extracted.</p>
  35 <p>
  36
  37
  38 </p>
  39 <div class="variablelist"><table border="0" class="variablelist compact">
  40 <colgroup>
  41 <col align="left" valign="top">
  42 <col>
  43 </colgroup>
  44 <tbody>
  45 <tr>
  46 <td><p><span class="term">Parameters:</span></p></td>
  47 <td><div class="variablelist"><table border="0" class="variablelist compact">
  48 <colgroup>
  49 <col align="left" valign="top">
  50 <col>
  51 </colgroup>
  52 <tbody>
  53 <tr>
  54 <td><p><span class="term"><code class="computeroutput">keyword</code></span></p></td>
  55 <td><p>The keyword of the attribute value to extract. </p></td>
  56 </tr>
  57 <tr>
  58 <td><p><span class="term"><code class="computeroutput">rec</code></span></p></td>
  59 <td><p>A log record. The attribute value will be sought among those associated with the record. </p></td>
  60 </tr>
  61 </tbody>
  62 </table></div></td>
  63 </tr>
  64 <tr>
  65 <td><p><span class="term">Returns:</span></p></td>
  66 <td><p>The extracted value or a non-empty <code class="computeroutput">value_ref</code> that refers to the value. </p></td>
  67 </tr>
  68 <tr>
  69 <td><p><span class="term">Throws:</span></p></td>
  70 <td>An exception is thrown if the requested value cannot be extracted. </td>
  71 </tr>
  72 </tbody>
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  78 <td align="right"><div class="copyright-footer">Copyright &#169; 2007-2016 Andrey Semashev<p>
  79         Distributed under the Boost Software License, Version 1.0. (See accompanying
  80         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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