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   4 <title>Struct template attribute_keyword</title>
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  19 <a name="boost.log.expressions.attribute_keyword"></a><div class="titlepage"></div>
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  21 <h2><span class="refentrytitle">Struct template attribute_keyword</span></h2>
  22 <p>boost::log::expressions::attribute_keyword &#8212; This class implements an expression template keyword. </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="../../../expressions.html#header.boost.log.expressions.keyword_fwd_hpp" title="Header &lt;boost/log/expressions/keyword_fwd.hpp&gt;">boost/log/expressions/keyword_fwd.hpp</a>&gt;
  26
  27 </span><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>
  28 <span class="keyword">struct</span> <a class="link" href="attribute_keyword.html" title="Struct template attribute_keyword">attribute_keyword</a> <span class="special">{</span>
  29
  30   <span class="comment">// <a class="link" href="attribute_keyword.html#idm45711347564272-bb">public static functions</a></span>
  31   <span class="keyword">static</span> <span class="identifier">attribute_name</span> <a class="link" href="attribute_keyword.html#idm45711347563712-bb"><span class="identifier">get_name</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
  32   <span class="keyword">static</span> <span class="identifier">or_none_result_type</span> <a class="link" href="attribute_keyword.html#idm45711347562288-bb"><span class="identifier">or_none</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
  33   <span class="keyword">static</span> <span class="identifier">or_throw_result_type</span> <a class="link" href="attribute_keyword.html#idm45711347560816-bb"><span class="identifier">or_throw</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
  34   <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> DefaultT<span class="special">&gt;</span>
  35     <span class="keyword">static</span> <a class="link" href="attribute_actor.html" title="Class template attribute_actor">attribute_actor</a><span class="special">&lt;</span> <span class="identifier">value_type</span><span class="special">,</span> <span class="identifier">fallback_to_default</span><span class="special">&lt;</span> <span class="identifier">DefaultT</span> <span class="special">&gt;</span><span class="special">,</span> <span class="identifier">descriptor_type</span><span class="special">,</span> <span class="identifier">ActorT</span> <span class="special">&gt;</span>
  36     <a class="link" href="attribute_keyword.html#idm45711347559344-bb"><span class="identifier">or_default</span></a><span class="special">(</span><span class="identifier">DefaultT</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  37 <span class="special">}</span><span class="special">;</span></pre></div>
  38 <div class="refsect1">
  39 <a name="idm45554743821344"></a><h2>Description</h2>
  40 <p>This class implements an expression template keyword. It is used to start template expressions involving attribute values. </p>
  41 <div class="refsect2">
  42 <a name="idm45554743820528"></a><h3>
  43 <a name="idm45711347564272-bb"></a><code class="computeroutput">attribute_keyword</code> public static functions</h3>
  44 <div class="orderedlist"><ol class="orderedlist" type="1">
  45 <li class="listitem">
  46 <pre class="literallayout"><span class="keyword">static</span> <span class="identifier">attribute_name</span> <a name="idm45711347563712-bb"></a><span class="identifier">get_name</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>Returns attribute name. </li>
  47 <li class="listitem">
  48 <pre class="literallayout"><span class="keyword">static</span> <span class="identifier">or_none_result_type</span> <a name="idm45711347562288-bb"></a><span class="identifier">or_none</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>Generates an expression that extracts the attribute value or a default value. </li>
  49 <li class="listitem">
  50 <pre class="literallayout"><span class="keyword">static</span> <span class="identifier">or_throw_result_type</span> <a name="idm45711347560816-bb"></a><span class="identifier">or_throw</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>Generates an expression that extracts the attribute value or throws an exception. </li>
  51 <li class="listitem">
  52 <pre class="literallayout"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> DefaultT<span class="special">&gt;</span>
  53   <span class="keyword">static</span> <a class="link" href="attribute_actor.html" title="Class template attribute_actor">attribute_actor</a><span class="special">&lt;</span> <span class="identifier">value_type</span><span class="special">,</span> <span class="identifier">fallback_to_default</span><span class="special">&lt;</span> <span class="identifier">DefaultT</span> <span class="special">&gt;</span><span class="special">,</span> <span class="identifier">descriptor_type</span><span class="special">,</span> <span class="identifier">ActorT</span> <span class="special">&gt;</span>
  54   <a name="idm45711347559344-bb"></a><span class="identifier">or_default</span><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>Generates an expression that extracts the attribute value or a default value. </li>
  55 </ol></div>
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