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  19 <a name="boost.log.fallback_to_none"></a><div class="titlepage"></div>
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  21 <h2><span class="refentrytitle">Struct fallback_to_none</span></h2>
  22 <p>boost::log::fallback_to_none</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.fallback_policy_hpp" title="Header &lt;boost/log/attributes/fallback_policy.hpp&gt;">boost/log/attributes/fallback_policy.hpp</a>&gt;
  26
  27 </span>
  28 <span class="keyword">struct</span> <a class="link" href="fallback_to_none.html" title="Struct fallback_to_none">fallback_to_none</a> <span class="special">{</span>
  29
  30   <span class="keyword">enum</span> <a name="boost.log.fallback_to_none.@0"></a>@0 <span class="special">{</span> guaranteed_result = = false <span class="special">}</span><span class="special">;</span>
  31
  32   <span class="comment">// <a class="link" href="fallback_to_none.html#idp62004160-bb">public static functions</a></span>
  33   <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> FunT<span class="special">&gt;</span> <span class="keyword">static</span> <span class="keyword">bool</span> <a class="link" href="fallback_to_none.html#idp62004720-bb"><span class="identifier">apply_default</span></a><span class="special">(</span><span class="identifier">FunT</span> <span class="special">&amp;</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> FunT<span class="special">&gt;</span> <span class="keyword">static</span> <span class="keyword">bool</span> <a class="link" href="fallback_to_none.html#idp62007856-bb"><span class="identifier">apply_default</span></a><span class="special">(</span><span class="identifier">FunT</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  35   <span class="keyword">static</span> <span class="keyword">void</span> <a class="link" href="fallback_to_none.html#idp62010992-bb"><span class="identifier">on_invalid_type</span></a><span class="special">(</span><span class="identifier">typeindex</span><span class="special">::</span><span class="identifier">type_index</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  36   <span class="keyword">static</span> <span class="keyword">void</span> <a class="link" href="fallback_to_none.html#idp62013328-bb"><span class="identifier">on_missing_value</span></a><span class="special">(</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="idp106755824"></a><h2>Description</h2>
  40 <p>The <code class="computeroutput"><a class="link" href="fallback_to_none.html" title="Struct fallback_to_none">fallback_to_none</a></code> policy results in returning an empty value reference if the attribute value cannot be extracted. </p>
  41 <div class="refsect2">
  42 <a name="idp106757920"></a><h3>
  43 <a name="idp62004160-bb"></a><code class="computeroutput">fallback_to_none</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">template</span><span class="special">&lt;</span><span class="keyword">typename</span> FunT<span class="special">&gt;</span> <span class="keyword">static</span> <span class="keyword">bool</span> <a name="idp62004720-bb"></a><span class="identifier">apply_default</span><span class="special">(</span><span class="identifier">FunT</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span></pre>
  47 <p>The method is called in order to apply a function object to the default value. </p>
  48 </li>
  49 <li class="listitem">
  50 <pre class="literallayout"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> FunT<span class="special">&gt;</span> <span class="keyword">static</span> <span class="keyword">bool</span> <a name="idp62007856-bb"></a><span class="identifier">apply_default</span><span class="special">(</span><span class="identifier">FunT</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span></pre>
  51 <p>The method is called in order to apply a function object to the default value. </p>
  52 </li>
  53 <li class="listitem">
  54 <pre class="literallayout"><span class="keyword">static</span> <span class="keyword">void</span> <a name="idp62010992-bb"></a><span class="identifier">on_invalid_type</span><span class="special">(</span><span class="identifier">typeindex</span><span class="special">::</span><span class="identifier">type_index</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span></pre>
  55 <p>The method is called when value extraction failed because the attribute value has different type than requested. </p>
  56 </li>
  57 <li class="listitem">
  58 <pre class="literallayout"><span class="keyword">static</span> <span class="keyword">void</span> <a name="idp62013328-bb"></a><span class="identifier">on_missing_value</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>
  59 <p>The method is called when value extraction failed because the attribute value was not found. </p>
  60 </li>
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  67 <td align="right"><div class="copyright-footer">Copyright &#169; 2007-2016 Andrey Semashev<p>
  68         Distributed under the Boost Software License, Version 1.0. (See accompanying
  69         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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