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   4 <title>Class template mutable_constant</title>
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  18 <div class="refentry">
  19 <a name="boost.log.attributes.mutable_constant"></a><div class="titlepage"></div>
  20 <div class="refnamediv">
  21 <h2><span class="refentrytitle">Class template mutable_constant</span></h2>
  22 <p>boost::log::attributes::mutable_constant &#8212; A class of an attribute that holds a single constant value with ability to change it. </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.mutable_constant_hpp" title="Header &lt;boost/log/attributes/mutable_constant.hpp&gt;">boost/log/attributes/mutable_constant.hpp</a>&gt;
  26
  27 </span><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> MutexT <span class="special">=</span> <span class="keyword">void</span><span class="special">,</span> <span class="keyword">typename</span> ScopedWriteLockT <span class="special">=</span> <span class="keyword">auto</span><span class="special">,</span>
  28          <span class="keyword">typename</span> ScopedReadLockT <span class="special">=</span> <span class="keyword">auto</span><span class="special">&gt;</span>
  29 <span class="keyword">class</span> <a class="link" href="mutable_constant.html" title="Class template mutable_constant">mutable_constant</a> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">attribute</span> <span class="special">{</span>
  30 <span class="keyword">public</span><span class="special">:</span>
  31   <span class="comment">// types</span>
  32   <span class="keyword">typedef</span> <span class="identifier">T</span> <a name="boost.log.attributes.mutable_constant.value_type"></a><span class="identifier">value_type</span><span class="special">;</span>  <span class="comment">// The attribute value type. </span>
  33
  34   <span class="comment">// member classes/structs/unions</span>
  35
  36   <span class="comment">// Factory implementation.</span>
  37
  38   <span class="keyword">class</span> <a class="link" href="mutable_constant/impl.html" title="Class impl">impl</a> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">attribute</span><span class="special">::</span><span class="identifier">impl</span> <span class="special">{</span>
  39   <span class="keyword">public</span><span class="special">:</span>
  40     <span class="comment">// <a class="link" href="mutable_constant/impl.html#boost.log.attributes.mutable_constant.implconstruct-copy-destruct">construct/copy/destruct</a></span>
  41     <span class="keyword">explicit</span> <a class="link" href="mutable_constant/impl.html#idp62098736-bb"><span class="identifier">impl</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  42     <span class="keyword">explicit</span> <a class="link" href="mutable_constant/impl.html#idp62100416-bb"><span class="identifier">impl</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;&amp;</span><span class="special">)</span><span class="special">;</span>
  43
  44     <span class="comment">// <a class="link" href="mutable_constant/impl.html#idp62091648-bb">public member functions</a></span>
  45     <span class="keyword">virtual</span> <a class="link" href="../attribute_value.html" title="Class attribute_value">attribute_value</a> <a class="link" href="mutable_constant/impl.html#idp62092208-bb"><span class="identifier">get_value</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span>
  46     <span class="keyword">void</span> <a class="link" href="mutable_constant/impl.html#idp62094384-bb"><span class="identifier">set</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  47     <span class="keyword">void</span> <a class="link" href="mutable_constant/impl.html#idp62095920-bb"><span class="identifier">set</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;&amp;</span><span class="special">)</span><span class="special">;</span>
  48     <span class="identifier">value_type</span> <a class="link" href="mutable_constant/impl.html#idp62097456-bb"><span class="identifier">get</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  49   <span class="special">}</span><span class="special">;</span>
  50
  51   <span class="comment">// <a class="link" href="mutable_constant.html#boost.log.attributes.mutable_constantconstruct-copy-destruct">construct/copy/destruct</a></span>
  52   <span class="keyword">explicit</span> <a class="link" href="mutable_constant.html#idp62109664-bb"><span class="identifier">mutable_constant</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  53   <span class="keyword">explicit</span> <a class="link" href="mutable_constant.html#idp62111360-bb"><span class="identifier">mutable_constant</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;&amp;</span><span class="special">)</span><span class="special">;</span>
  54   <span class="keyword">explicit</span> <a class="link" href="mutable_constant.html#idp62113056-bb"><span class="identifier">mutable_constant</span></a><span class="special">(</span><a class="link" href="cast_source.html" title="Class cast_source">cast_source</a> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  55
  56   <span class="comment">// <a class="link" href="mutable_constant.html#idp62103248-bb">public member functions</a></span>
  57   <span class="keyword">void</span> <a class="link" href="mutable_constant.html#idp62103808-bb"><span class="identifier">set</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="special">;</span>
  58   <span class="keyword">void</span> <a class="link" href="mutable_constant.html#idp62105872-bb"><span class="identifier">set</span></a><span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;&amp;</span><span class="special">)</span><span class="special">;</span>
  59   <span class="identifier">value_type</span> <a class="link" href="mutable_constant.html#idp62107840-bb"><span class="identifier">get</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  60
  61   <span class="comment">// <a class="link" href="mutable_constant.html#idp62115024-bb">protected member functions</a></span>
  62   <a class="link" href="mutable_constant/impl.html" title="Class impl">impl</a> <span class="special">*</span> <a class="link" href="mutable_constant.html#idp62115600-bb"><span class="identifier">get_impl</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  63 <span class="special">}</span><span class="special">;</span></pre></div>
  64 <div class="refsect1">
  65 <a name="idp107353264"></a><h2>Description</h2>
  66 <p>The <a class="link" href="mutable_constant.html" title="Class template mutable_constant">mutable_constant</a> attribute stores a single value of type, specified as the first template argument. This value is returned on each attribute value acquisition.</p>
  67 <p>The attribute also allows to modify the stored value, even if the attribute is registered in an attribute set. In order to ensure thread safety of such modifications the <code class="computeroutput"><a class="link" href="mutable_constant.html" title="Class template mutable_constant">mutable_constant</a></code> class is also parametrized with three additional template arguments: mutex type, scoped write and scoped read lock types. If not specified, the lock types are automatically deduced based on the mutex type.</p>
  68 <p>The implementation may avoid using these types to actually create and use the mutex, if a more efficient synchronization method is available (such as atomic operations on the value type). By default no synchronization is done. </p>
  69 <div class="refsect2">
  70 <a name="idp107357408"></a><h3>
  71 <a name="boost.log.attributes.mutable_constantconstruct-copy-destruct"></a><code class="computeroutput">mutable_constant</code>
  72         public
  73        construct/copy/destruct</h3>
  74 <div class="orderedlist"><ol class="orderedlist" type="1">
  75 <li class="listitem">
  76 <pre class="literallayout"><span class="keyword">explicit</span> <a name="idp62109664-bb"></a><span class="identifier">mutable_constant</span><span class="special">(</span><span class="identifier">value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span> value<span class="special">)</span><span class="special">;</span></pre>
  77 <p>Constructor with the stored value initialization </p>
  78 </li>
  79 <li class="listitem">
  80 <pre class="literallayout"><span class="keyword">explicit</span> <a name="idp62111360-bb"></a><span class="identifier">mutable_constant</span><span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;&amp;</span> value<span class="special">)</span><span class="special">;</span></pre>
  81 <p>Constructor with the stored value initialization </p>
  82 </li>
  83 <li class="listitem">
  84 <pre class="literallayout"><span class="keyword">explicit</span> <a name="idp62113056-bb"></a><span class="identifier">mutable_constant</span><span class="special">(</span><a class="link" href="cast_source.html" title="Class cast_source">cast_source</a> <span class="keyword">const</span> <span class="special">&amp;</span> source<span class="special">)</span><span class="special">;</span></pre>
  85 <p>Constructor for casting support </p>
  86 </li>
  87 </ol></div>
  88 </div>
  89 <div class="refsect2">
  90 <a name="idp107379424"></a><h3>
  91 <a name="idp62103248-bb"></a><code class="computeroutput">mutable_constant</code> public member functions</h3>
  92 <div class="orderedlist"><ol class="orderedlist" type="1">
  93 <li class="listitem">
  94 <pre class="literallayout"><span class="keyword">void</span> <a name="idp62103808-bb"></a><span class="identifier">set</span><span class="special">(</span><span class="identifier">value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span> value<span class="special">)</span><span class="special">;</span></pre>
  95 <p>The method sets a new attribute value. The implementation exclusively locks the mutex in order to protect the value assignment. </p>
  96 </li>
  97 <li class="listitem">
  98 <pre class="literallayout"><span class="keyword">void</span> <a name="idp62105872-bb"></a><span class="identifier">set</span><span class="special">(</span><span class="identifier">value_type</span> <span class="special">&amp;&amp;</span> value<span class="special">)</span><span class="special">;</span></pre>
  99 <p>The method sets a new attribute value. </p>
 100 </li>
 101 <li class="listitem">
 102 <pre class="literallayout"><span class="identifier">value_type</span> <a name="idp62107840-bb"></a><span class="identifier">get</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
 103 <p>The method acquires the current attribute value. The implementation non-exclusively locks the mutex in order to protect the value acquisition. </p>
 104 </li>
 105 </ol></div>
 106 </div>
 107 <div class="refsect2">
 108 <a name="idp107399824"></a><h3>
 109 <a name="idp62115024-bb"></a><code class="computeroutput">mutable_constant</code> protected member functions</h3>
 110 <div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
 111 <pre class="literallayout"><a class="link" href="mutable_constant/impl.html" title="Class impl">impl</a> <span class="special">*</span> <a name="idp62115600-bb"></a><span class="identifier">get_impl</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>
 112 <p>
 113 </p>
 114 <div class="variablelist"><table border="0" class="variablelist compact">
 115 <colgroup>
 116 <col align="left" valign="top">
 117 <col>
 118 </colgroup>
 119 <tbody><tr>
 120 <td><p><span class="term">Returns:</span></p></td>
 121 <td><p>Pointer to the factory implementation </p></td>
 122 </tr></tbody>
 123 </table></div>
 124 </li></ol></div>
 125 </div>
 126 <div class="refsect2">
 127 <a name="idp107495024"></a><h3>Specializations</h3>
 128 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><a class="link" href="mutable_consta_idp62118016.html" title="Class template mutable_constant&lt;T, void, void, void&gt;">Class template mutable_constant&lt;T, void, void, void&gt;</a></p></li></ul></div>
 129 </div>
 130 </div>
 131 </div>
 132 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 133 <td align="left"></td>
 134 <td align="right"><div class="copyright-footer">Copyright &#169; 2007-2016 Andrey Semashev<p>
 135         Distributed under the Boost Software License, Version 1.0. (See accompanying
 136         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>).
 137       </p>
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