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4<title>Attributes</title>
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17</div>
18<div class="section">
19<div class="titlepage"><div><div><h3 class="title">
20<a name="log.detailed.attributes"></a><a class="link" href="attributes.html" title="Attributes">Attributes</a>
21</h3></div></div></div>
22<div class="toc"><dl class="toc">
23<dt><span class="section"><a href="attributes.html#log.detailed.attributes.constant">Constants</a></span></dt>
24<dt><span class="section"><a href="attributes.html#log.detailed.attributes.mutable_constant">Mutable constants</a></span></dt>
25<dt><span class="section"><a href="attributes.html#log.detailed.attributes.counter">Counters</a></span></dt>
26<dt><span class="section"><a href="attributes.html#log.detailed.attributes.clock">Wall clock</a></span></dt>
27<dt><span class="section"><a href="attributes.html#log.detailed.attributes.timer">Stop watch (timer)</a></span></dt>
28<dt><span class="section"><a href="attributes.html#log.detailed.attributes.named_scope">Named scopes</a></span></dt>
29<dt><span class="section"><a href="attributes.html#log.detailed.attributes.process_id">Current process
30        identifier</a></span></dt>
31<dt><span class="section"><a href="attributes.html#log.detailed.attributes.process_name">Current process
32        name</a></span></dt>
33<dt><span class="section"><a href="attributes.html#log.detailed.attributes.thread_id">Current thread identifier</a></span></dt>
34<dt><span class="section"><a href="attributes.html#log.detailed.attributes.function">Function objects
35        as attributes</a></span></dt>
36<dt><span class="section"><a href="attributes.html#log.detailed.attributes.related_components">Other attribute-related
37        components</a></span></dt>
38</dl></div>
39<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.attribute_hpp" title="Header &lt;boost/log/attributes/attribute.hpp&gt;">boost/log/attributes/attribute.hpp</a></code><span class="special">&gt;</span>
40<span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.attribute_cast_hpp" title="Header &lt;boost/log/attributes/attribute_cast.hpp&gt;">boost/log/attributes/attribute_cast.hpp</a></code><span class="special">&gt;</span>
41<span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.attribute_value_hpp" title="Header &lt;boost/log/attributes/attribute_value.hpp&gt;">boost/log/attributes/attribute_value.hpp</a></code><span class="special">&gt;</span>
42</pre>
43<p>
44        All attributes in the library are implemented using the <a href="http://c2.com/cgi/wiki?PimplIdiom" target="_top">pimpl
45        idiom</a>, or more specifically - shared pimpl idiom. Every attribute
46        provides an interface class which derives from the <code class="computeroutput"><a class="link" href="../../boost/log/attribute.html" title="Class attribute">attribute</a></code>
47        class and an implementation class that derives from <code class="computeroutput"><a class="link" href="../../boost/log/attribute/impl.html" title="Struct impl">impl</a></code>.
48        The interface class only holds a reference counted pointer to the actual
49        implementation of the attribute; this pointer is a member of the <code class="computeroutput"><a class="link" href="../../boost/log/attribute.html" title="Class attribute">attribute</a></code> class, so derived interface
50        classes don't have any data members. When the interface class is default
51        constructed, it creates the corresponding implementation object and initializes
52        the <code class="computeroutput"><a class="link" href="../../boost/log/attribute.html" title="Class attribute">attribute</a></code> base class
53        with a pointer to the implementation. Therefore the pimpl nature of attributes
54        is transparent for users in a typical workflow.
55      </p>
56<p>
57        The shared pimpl design comes significant in a few cases though. One such
58        case is copying the attribute. The copy operation is shallow, so multiple
59        interface objects may refer to a single implementation object. There is no
60        way to deep copy an attribute. Another case is default construction of <code class="computeroutput"><a class="link" href="../../boost/log/attribute.html" title="Class attribute">attribute</a></code> which creates an empty
61        object that does not refer to an implementation. Attributes in such empty
62        state should not be passed to the library but can be useful in some cases,
63        e.g. when a delayed variable initialization is needed.
64      </p>
65<p>
66        It is possible to upcast the attribute interface from <code class="computeroutput"><a class="link" href="../../boost/log/attribute.html" title="Class attribute">attribute</a></code>
67        to the actual interface class. To do this one has to apply <code class="computeroutput"><a class="link" href="../../boost/log/attribute_cast.html" title="Function template attribute_cast">attribute_cast</a></code>:
68      </p>
69<pre class="programlisting"><span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute</span> <span class="identifier">attr</span> <span class="special">=</span> <span class="special">...;</span>
70<span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span> <span class="identifier">const_attr</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_cast</span><span class="special">&lt;</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span> <span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">);</span>
71</pre>
72<p>
73        In this example, the cast will succeed (i.e. the <code class="computeroutput"><span class="identifier">const_attr</span></code>
74        will be non-empty) if the attribute <code class="computeroutput"><span class="identifier">attr</span></code>
75        was originally created as <code class="computeroutput"><span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span></code>. Since all data is stored in the implementation
76        object, no data is lost in the casting process.
77      </p>
78<p>
79        The main purpose of attributes is to generate attribute values. Values are
80        semantically distinct from the attributes. Such separation allows implementing
81        attributes that can return different values at different time points (like
82        clock-related attributes, for example) and, on the other hand, allows using
83        different values of the same attribute independently. The <code class="computeroutput"><a class="link" href="../../boost/log/attribute.html" title="Class attribute">attribute</a></code>
84        interface has a method named <code class="computeroutput"><span class="identifier">get_value</span></code>
85        that returns the actual attribute value. Attribute values are also implemented
86        using the shared pimpl approach, the interface class is <code class="computeroutput"><a class="link" href="../../boost/log/attribute_value.html" title="Class attribute_value">attribute_value</a></code>
87        and implementation classes derive from <code class="computeroutput"><a class="link" href="../../boost/log/attribute_value/impl.html" title="Struct impl">impl</a></code>.
88      </p>
89<p>
90        The attribute value object is mostly intended to store the actual attribute
91        value and implement type dispatching in order to be able to extract the stored
92        value. One should not confuse the attribute value object type and the stored
93        value type. The former is in most cases not needed by users and provides
94        type erasure, but the latter is needed to be able to extract the value. For
95        brevity we call the stored attribute value type simply the attribute value
96        type in this documentation.
97      </p>
98<div class="section">
99<div class="titlepage"><div><div><h4 class="title">
100<a name="log.detailed.attributes.constant"></a><a class="link" href="attributes.html#log.detailed.attributes.constant" title="Constants">Constants</a>
101</h4></div></div></div>
102<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.constant_hpp" title="Header &lt;boost/log/attributes/constant.hpp&gt;">boost/log/attributes/constant.hpp</a></code><span class="special">&gt;</span>
103</pre>
104<p>
105          The most simple and frequently used attribute type is a constant value
106          of some type. This kind of attribute is implemented with the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/constant.html" title="Class template constant">constant</a></code> class template.
107          The template is parametrized with the attribute value type. The constant
108          value should be passed to the attribute constructor. Here is an example:
109        </p>
110<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
111<span class="special">{</span>
112    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger</span> <span class="identifier">lg</span><span class="special">;</span>
113
114    <span class="comment">// Register a constant attribute that always yields value -5</span>
115    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"MyInteger"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;(-</span><span class="number">5</span><span class="special">));</span>
116
117    <span class="comment">// Register another constant attribute. Make it a string this time.</span>
118    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"MyString"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;(</span><span class="string">"Hello world!"</span><span class="special">));</span>
119
120    <span class="comment">// There is also a convenience generator function. "MyInteger2" is constant&lt; int &gt; here.</span>
121    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"MyInteger2"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">make_constant</span><span class="special">(</span><span class="number">10</span><span class="special">));</span>
122<span class="special">}</span>
123</pre>
124<p>
125          That's it, there's nothing much you can do with a constant attribute. Constants
126          are very useful when one wants to highlight some log records or just pass
127          some data to a sink backend (e.g. pass statistical parameters to the collector).
128        </p>
129</div>
130<div class="section">
131<div class="titlepage"><div><div><h4 class="title">
132<a name="log.detailed.attributes.mutable_constant"></a><a class="link" href="attributes.html#log.detailed.attributes.mutable_constant" title="Mutable constants">Mutable constants</a>
133</h4></div></div></div>
134<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><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></code><span class="special">&gt;</span>
135</pre>
136<p>
137          This kind of attribute is an extension for the <a class="link" href="attributes.html#log.detailed.attributes.constant" title="Constants">constant
138          attribute</a>. In addition to being able to store some value, the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/mutable_constant.html" title="Class template mutable_constant">mutable_constant</a></code>
139          class template has two distinctions:
140        </p>
141<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
142<li class="listitem">
143              it allows modification of the stored value without re-registering the
144              attribute
145            </li>
146<li class="listitem">
147              it allows synchronization of the stores and reads of the stored value
148            </li>
149</ul></div>
150<p>
151          In order to change the stored value of the attribute, one must call the
152          <code class="computeroutput"><span class="identifier">set</span></code> method:
153        </p>
154<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
155<span class="special">{</span>
156    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger</span> <span class="identifier">lg</span><span class="special">;</span>
157
158    <span class="comment">// Register a mutable constant attribute that always yields value -5</span>
159    <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">mutable_constant</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span> <span class="identifier">attr</span><span class="special">(-</span><span class="number">5</span><span class="special">);</span>
160    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"MyInteger"</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span>
161    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has MyInteger == -5"</span><span class="special">;</span>
162
163    <span class="comment">// Change the attribute value</span>
164    <span class="identifier">attr</span><span class="special">.</span><span class="identifier">set</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
165    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has MyInteger == 100"</span><span class="special">;</span>
166<span class="special">}</span>
167</pre>
168<p>
169          In multithreaded applications the <code class="computeroutput"><span class="identifier">set</span></code>
170          method calls must be serialized with the <code class="computeroutput"><span class="identifier">get_value</span></code>
171          calls (which, generally speaking, happen on every log record being made).
172          By default <code class="computeroutput"><a class="link" href="../../boost/log/attributes/mutable_constant.html" title="Class template mutable_constant">mutable_constant</a></code>
173          does not serialize calls in any way, assuming that the user will do so
174          externally. However, the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/mutable_constant.html" title="Class template mutable_constant">mutable_constant</a></code>
175          template provides three additional template arguments: synchronization
176          primitive type, scoped exclusive lock type and scoped shareable lock type.
177          If a synchronization primitive type is specified, the scoped exclusive
178          lock type is a mandatory parameter. If the scoped shareable lock type is
179          not specified, the attribute will fall back to the exclusive lock instead
180          of shared locks. For example:
181        </p>
182<pre class="programlisting"><span class="comment">// This mutable constant will always lock exclusively</span>
183<span class="comment">// either for reading or storing the value</span>
184<span class="keyword">typedef</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">mutable_constant</span><span class="special">&lt;</span>
185    <span class="keyword">int</span><span class="special">,</span>                                        <span class="comment">// attribute value type</span>
186    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mutex</span><span class="special">,</span>                               <span class="comment">// synchronization primitive</span>
187    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lock_guard</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mutex</span> <span class="special">&gt;</span>           <span class="comment">// exclusive lock type</span>
188<span class="special">&gt;</span> <span class="identifier">exclusive_mc</span><span class="special">;</span>
189<span class="identifier">exclusive_mc</span> <span class="identifier">my_int1</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
190
191<span class="comment">// This mutable constant will use shared clocking for reading the value</span>
192<span class="comment">// and exclusive locking for storing</span>
193<span class="keyword">typedef</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">mutable_constant</span><span class="special">&lt;</span>
194    <span class="keyword">int</span><span class="special">,</span>                                        <span class="comment">// attribute value type</span>
195    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_mutex</span><span class="special">,</span>                        <span class="comment">// synchronization primitive</span>
196    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique_lock</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_mutex</span> <span class="special">&gt;,</span>  <span class="comment">// exclusive lock type</span>
197    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_lock</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_mutex</span> <span class="special">&gt;</span>   <span class="comment">// shared lock type</span>
198<span class="special">&gt;</span> <span class="identifier">shared_mc</span><span class="special">;</span>
199<span class="identifier">shared_mc</span> <span class="identifier">my_int2</span><span class="special">(</span><span class="number">20</span><span class="special">);</span>
200
201<span class="identifier">BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">,</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">)</span>
202<span class="special">{</span>
203    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span> <span class="identifier">lg</span><span class="special">;</span>
204    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"MyInteger1"</span><span class="special">,</span> <span class="identifier">my_int1</span><span class="special">);</span>
205    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"MyInteger2"</span><span class="special">,</span> <span class="identifier">my_int2</span><span class="special">);</span>
206
207    <span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
208<span class="special">}</span>
209
210<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
211<span class="special">{</span>
212    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">&amp;</span> <span class="identifier">lg</span> <span class="special">=</span> <span class="identifier">get_my_logger</span><span class="special">();</span>
213
214    <span class="comment">// This is safe, even if executed in multiple threads</span>
215    <span class="identifier">my_int1</span><span class="special">.</span><span class="identifier">set</span><span class="special">(</span><span class="number">200</span><span class="special">);</span>
216    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has MyInteger1 == 200"</span><span class="special">;</span>
217
218    <span class="identifier">my_int2</span><span class="special">.</span><span class="identifier">set</span><span class="special">(</span><span class="number">300</span><span class="special">);</span>
219    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has MyInteger2 == 300"</span><span class="special">;</span>
220<span class="special">}</span>
221</pre>
222<p>
223          Mutable constants are often used as auxiliary attributes inside loggers
224          to store attributes that may change on some events. As opposed to regular
225          constants, which would require re-registering in case of value modification,
226          mutable constants allow modifying the value in-place.
227        </p>
228</div>
229<div class="section">
230<div class="titlepage"><div><div><h4 class="title">
231<a name="log.detailed.attributes.counter"></a><a class="link" href="attributes.html#log.detailed.attributes.counter" title="Counters">Counters</a>
232</h4></div></div></div>
233<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.counter_hpp" title="Header &lt;boost/log/attributes/counter.hpp&gt;">boost/log/attributes/counter.hpp</a></code><span class="special">&gt;</span>
234</pre>
235<p>
236          Counters are one of the simplest attributes that generate a new value each
237          time requested. Counters are often used to identify log records or to count
238          some events, e.g. accepted network connections. The class template <code class="computeroutput"><a class="link" href="../../boost/log/attributes/counter.html" title="Class template counter">counter</a></code> provides such
239          functionality. This template is parametrized with the counter value type,
240          which should support arithmetic operations, such as <code class="computeroutput"><span class="keyword">operator</span>
241          <span class="special">+</span></code> and <code class="computeroutput"><span class="keyword">operator</span>
242          <span class="special">-</span></code>. The counter attribute allows
243          specification of the initial value and step (which can be negative) on
244          construction.
245        </p>
246<pre class="programlisting"><span class="identifier">BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">,</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">)</span>
247<span class="special">{</span>
248    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span> <span class="identifier">lg</span><span class="special">;</span>
249
250    <span class="comment">// This counter will count lines, starting from 0</span>
251    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"LineCounter"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">counter</span><span class="special">&lt;</span> <span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="special">&gt;());</span>
252
253    <span class="comment">// This counter will count backwards, starting from 100 with step -5</span>
254    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"CountDown"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">counter</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;(</span><span class="number">100</span><span class="special">,</span> <span class="special">-</span><span class="number">5</span><span class="special">));</span>
255
256    <span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
257<span class="special">}</span>
258
259<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
260<span class="special">{</span>
261    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">&amp;</span> <span class="identifier">lg</span> <span class="special">=</span> <span class="identifier">get_my_logger</span><span class="special">();</span>
262    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has LineCounter == 0, CountDown == 100"</span><span class="special">;</span>
263    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has LineCounter == 1, CountDown == 95"</span><span class="special">;</span>
264    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has LineCounter == 2, CountDown == 90"</span><span class="special">;</span>
265<span class="special">}</span>
266</pre>
267<div class="note"><table border="0" summary="Note">
268<tr>
269<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
270<th align="left">Note</th>
271</tr>
272<tr><td align="left" valign="top"><p>
273            Don't expect that the log records with the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/counter.html" title="Class template counter">counter</a></code>
274            attribute will always have ascending or descending counter values in
275            the resulting log. In multithreaded applications counter values acquired
276            by different threads may come to a sink in any order. See <a class="link" href="../rationale/why_weak_record_ordering.html" title="Why log records are weakly ordered in a multithreaded application?">Rationale</a>
277            for a more detailed explanation on why it can happen. For this reason
278            it is more accurate to say that the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/counter.html" title="Class template counter">counter</a></code>
279            attribute generates an identifier in an ascending or descending order
280            rather than that it counts log records in either order.
281          </p></td></tr>
282</table></div>
283</div>
284<div class="section">
285<div class="titlepage"><div><div><h4 class="title">
286<a name="log.detailed.attributes.clock"></a><a class="link" href="attributes.html#log.detailed.attributes.clock" title="Wall clock">Wall clock</a>
287</h4></div></div></div>
288<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.clock_hpp" title="Header &lt;boost/log/attributes/clock.hpp&gt;">boost/log/attributes/clock.hpp</a></code><span class="special">&gt;</span>
289</pre>
290<p>
291          One of the "must-have" features of any logging library is support
292          for attaching a time stamp to every log record. The library provides two
293          attributes for this purpose: <code class="computeroutput"><span class="identifier">utc_clock</span></code>
294          and <code class="computeroutput"><span class="identifier">local_clock</span></code>. The former
295          returns the current UTC time and the latter returns the current local time.
296          In either case the returned time stamp is acquired with the maximum precision
297          for the target platform. The attribute value is <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">posix_time</span><span class="special">::</span><span class="identifier">ptime</span></code>
298          (see <a href="http://www.boost.org/doc/libs/release/doc/html/date_time.html" target="_top">Boost.DateTime</a>).
299          The usage is quite straightforward:
300        </p>
301<pre class="programlisting"><span class="identifier">BOOST_LOG_DECLARE_GLOBAL_LOGGER</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">,</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">)</span>
302
303<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
304<span class="special">{</span>
305    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">core</span><span class="special">::</span><span class="identifier">get</span><span class="special">()-&gt;</span><span class="identifier">add_global_attribute</span><span class="special">(</span>
306        <span class="string">"TimeStamp"</span><span class="special">,</span>
307        <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">local_clock</span><span class="special">());</span>
308
309    <span class="comment">// Now every log record ever made will have a time stamp attached</span>
310    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">&amp;</span> <span class="identifier">lg</span> <span class="special">=</span> <span class="identifier">get_my_logger</span><span class="special">();</span>
311    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"This record has a time stamp"</span><span class="special">;</span>
312<span class="special">}</span>
313</pre>
314</div>
315<div class="section">
316<div class="titlepage"><div><div><h4 class="title">
317<a name="log.detailed.attributes.timer"></a><a class="link" href="attributes.html#log.detailed.attributes.timer" title="Stop watch (timer)">Stop watch (timer)</a>
318</h4></div></div></div>
319<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.timer_hpp" title="Header &lt;boost/log/attributes/timer.hpp&gt;">boost/log/attributes/timer.hpp</a></code><span class="special">&gt;</span>
320</pre>
321<p>
322          The <code class="computeroutput"><a class="link" href="../../boost/log/attributes/timer.html" title="Class timer">timer</a></code> attribute
323          is very useful when there is a need to estimate the duration of some prolonged
324          process. The attribute returns the time elapsed since the attribute construction.
325          The attribute value type is <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">posix_time</span><span class="special">::</span><span class="identifier">ptime</span><span class="special">::</span><span class="identifier">time_duration_type</span></code>
326          (see <a href="http://www.boost.org/doc/libs/release/doc/html/date_time.html" target="_top">Boost.DateTime</a>).
327        </p>
328<pre class="programlisting"><span class="comment">// The class represents a single peer-to-peer connection</span>
329<span class="keyword">class</span> <span class="identifier">network_connection</span>
330<span class="special">{</span>
331    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger</span> <span class="identifier">m_logger</span><span class="special">;</span>
332
333<span class="keyword">public</span><span class="special">:</span>
334    <span class="identifier">network_connection</span><span class="special">()</span>
335    <span class="special">{</span>
336        <span class="identifier">m_logger</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"Duration"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span><span class="special">());</span>
337        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_logger</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Connection established"</span><span class="special">;</span>
338    <span class="special">}</span>
339    <span class="special">~</span><span class="identifier">network_connection</span><span class="special">()</span>
340    <span class="special">{</span>
341        <span class="comment">// This log record will show the whole life time duration of the connection</span>
342        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_logger</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Connection closed"</span><span class="special">;</span>
343    <span class="special">}</span>
344<span class="special">};</span>
345</pre>
346<p>
347          The attribute provides high resolution of the time estimation and can even
348          be used as a simple in-place performance profiling tool.
349        </p>
350<div class="tip"><table border="0" summary="Tip">
351<tr>
352<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
353<th align="left">Tip</th>
354</tr>
355<tr><td align="left" valign="top"><p>
356            The <code class="computeroutput"><a class="link" href="../../boost/log/attributes/timer.html" title="Class timer">timer</a></code>
357            attribute can even be used to profile the code in different modules without
358            recompiling them. The trick is to wrap an expensive call to a foreign
359            module with the thread-specific <code class="computeroutput"><a class="link" href="../../boost/log/attributes/timer.html" title="Class timer">timer</a></code>
360            <a class="link" href="attributes.html#log.detailed.attributes.related_components.scoped_attributes" title="Scoped attributes">scoped
361            attribute</a>, which will markup all log records made from within
362            the module with time readings.
363          </p></td></tr>
364</table></div>
365</div>
366<div class="section">
367<div class="titlepage"><div><div><h4 class="title">
368<a name="log.detailed.attributes.named_scope"></a><a class="link" href="attributes.html#log.detailed.attributes.named_scope" title="Named scopes">Named scopes</a>
369</h4></div></div></div>
370<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.named_scope_hpp" title="Header &lt;boost/log/attributes/named_scope.hpp&gt;">boost/log/attributes/named_scope.hpp</a></code><span class="special">&gt;</span>
371
372<span class="comment">// Supporting headers</span>
373<span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../other_libraries_support_layer.html#header.boost.log.support.exception_hpp" title="Header &lt;boost/log/support/exception.hpp&gt;">boost/log/support/exception.hpp</a></code><span class="special">&gt;</span>
374</pre>
375<p>
376          The logging library supports maintaining scope stack tracking during the
377          application's execution. This stack may either be written to log or be
378          used for other needs (for example, to save the exact call sequence that
379          led to an exception when throwing one). Each stack element contains the
380          following information (see the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/named_scope_entry.html" title="Struct named_scope_entry">named_scope_entry</a></code>
381          structure template definition):
382        </p>
383<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
384<li class="listitem">
385              Scope name. It can be defined by the user or generated by the compiler,
386              but in any case it <span class="underline">must be a constant string
387              literal</span> (see <a class="link" href="../rationale.html#log.rationale.why_str_lit" title="Why string literals as scope names?">Rationale</a>).
388            </li>
389<li class="listitem">
390              Source file name, where the scope begins. It is usually a result of
391              the standard <code class="computeroutput"><span class="identifier">__FILE__</span></code>
392              macro expansion. Like the scope name, the file name <span class="underline">must
393              be a constant string literal</span>.
394            </li>
395<li class="listitem">
396              Line number in the source file. Usually it is a result of the standard
397              <code class="computeroutput"><span class="identifier">__LINE__</span></code> macro expansion.
398            </li>
399</ul></div>
400<p>
401          The scope stack is implemented as a thread-specific global storage internally.
402          There is the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/named_scope.html" title="Class named_scope">named_scope</a></code>
403          attribute that allows hooking this stack into the logging pipeline. This
404          attribute generates value of the nested type <code class="computeroutput"><span class="identifier">named_scope</span><span class="special">::</span><span class="identifier">scope_stack</span></code>
405          which is the instance of the scope stack. The attribute can be registered
406          in the following way:
407        </p>
408<pre class="programlisting"><span class="identifier">logging</span><span class="special">::</span><span class="identifier">core</span><span class="special">::</span><span class="identifier">get</span><span class="special">()-&gt;</span><span class="identifier">add_global_attribute</span><span class="special">(</span><span class="string">"Scope"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">named_scope</span><span class="special">());</span>
409</pre>
410<p>
411          Note that it is perfectly valid to register the attribute globally because
412          the scope stack is thread-local anyway. This will also implicitly add scope
413          tracking to all threads of the application, which is often exactly what
414          is needed.
415        </p>
416<p>
417          Now we can mark execution scopes with the macros <code class="computeroutput"><span class="identifier">BOOST_LOG_FUNCTION</span></code>
418          and <code class="computeroutput"><span class="identifier">BOOST_LOG_NAMED_SCOPE</span></code>
419          (the latter accepts the scope name as its argument). These macros automatically
420          add source position information to each scope entry. An example follows:
421        </p>
422<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">n</span><span class="special">)</span>
423<span class="special">{</span>
424    <span class="comment">// Mark the scope of the function foo</span>
425    <span class="identifier">BOOST_LOG_FUNCTION</span><span class="special">();</span>
426
427    <span class="keyword">switch</span> <span class="special">(</span><span class="identifier">n</span><span class="special">)</span>
428    <span class="special">{</span>
429    <span class="keyword">case</span> <span class="number">0</span><span class="special">:</span>
430        <span class="special">{</span>
431            <span class="comment">// Mark the current scope</span>
432            <span class="identifier">BOOST_LOG_NAMED_SCOPE</span><span class="special">(</span><span class="string">"case 0"</span><span class="special">);</span>
433            <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Some log record"</span><span class="special">;</span>
434            <span class="identifier">bar</span><span class="special">();</span> <span class="comment">// call some function</span>
435        <span class="special">}</span>
436        <span class="keyword">break</span><span class="special">;</span>
437
438    <span class="keyword">case</span> <span class="number">1</span><span class="special">:</span>
439        <span class="special">{</span>
440            <span class="comment">// Mark the current scope</span>
441            <span class="identifier">BOOST_LOG_NAMED_SCOPE</span><span class="special">(</span><span class="string">"case 1"</span><span class="special">);</span>
442            <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Some log record"</span><span class="special">;</span>
443            <span class="identifier">bar</span><span class="special">();</span> <span class="comment">// call some function</span>
444        <span class="special">}</span>
445        <span class="keyword">break</span><span class="special">;</span>
446
447    <span class="keyword">default</span><span class="special">:</span>
448        <span class="special">{</span>
449            <span class="comment">// Mark the current scope</span>
450            <span class="identifier">BOOST_LOG_NAMED_SCOPE</span><span class="special">(</span><span class="string">"default"</span><span class="special">);</span>
451            <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Some log record"</span><span class="special">;</span>
452            <span class="identifier">bar</span><span class="special">();</span> <span class="comment">// call some function</span>
453        <span class="special">}</span>
454        <span class="keyword">break</span><span class="special">;</span>
455    <span class="special">}</span>
456<span class="special">}</span>
457</pre>
458<p>
459          After executing <code class="computeroutput"><span class="identifier">foo</span></code> we
460          will be able to see in the log that the <code class="computeroutput"><span class="identifier">bar</span></code>
461          function was called from <code class="computeroutput"><span class="identifier">foo</span></code>
462          and, more precisely, from the case statement that corresponds to the value
463          of <code class="computeroutput"><span class="identifier">n</span></code>. This may be very
464          useful when tracking down subtle bugs that show up only when <code class="computeroutput"><span class="identifier">bar</span></code> is called from a specific location
465          (e.g. if <code class="computeroutput"><span class="identifier">bar</span></code> is being passed
466          invalid arguments in that particular location).
467        </p>
468<div class="note"><table border="0" summary="Note">
469<tr>
470<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
471<th align="left">Note</th>
472</tr>
473<tr><td align="left" valign="top"><p>
474            The <code class="computeroutput"><span class="identifier">BOOST_LOG_FUNCTION</span></code>
475            macro uses compiler-specific extensions to generate the scope name from
476            the enclosing function. C++11 defines a standard macro <code class="computeroutput"><span class="identifier">__func__</span></code> for this purpose, but it is
477            not universally supported. Additionally, format of the string is not
478            standardized and may vary from one compiler to another. For this reason
479            it is generally advised to use <code class="computeroutput"><span class="identifier">BOOST_LOG_NAMED_SCOPE</span></code>
480            instead of <code class="computeroutput"><span class="identifier">BOOST_LOG_FUNCTION</span></code>
481            to ensure consistent and portable behavior.
482          </p></td></tr>
483</table></div>
484<p>
485          Another good use case is attaching the scope stack information to an exception.
486          With the help of <a href="http://www.boost.org/doc/libs/release/libs/exception/doc/boost-exception.html" target="_top">Boost.Exception</a>,
487          this is possible:
488        </p>
489<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">bar</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span>
490<span class="special">{</span>
491    <span class="identifier">BOOST_LOG_FUNCTION</span><span class="special">();</span>
492
493    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span> <span class="special">&lt;</span> <span class="number">0</span><span class="special">)</span>
494    <span class="special">{</span>
495        <span class="comment">// Attach a copy of the current scope stack to the exception</span>
496        <span class="keyword">throw</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_error_info</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">range_error</span><span class="special">(</span><span class="string">"x must not be negative"</span><span class="special">))</span>
497            <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">current_scope</span><span class="special">();</span>
498    <span class="special">}</span>
499<span class="special">}</span>
500
501<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
502<span class="special">{</span>
503    <span class="identifier">BOOST_LOG_FUNCTION</span><span class="special">();</span>
504
505    <span class="keyword">try</span>
506    <span class="special">{</span>
507        <span class="identifier">bar</span><span class="special">(-</span><span class="number">1</span><span class="special">);</span>
508    <span class="special">}</span>
509    <span class="keyword">catch</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">range_error</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
510    <span class="special">{</span>
511        <span class="comment">// Acquire the scope stack from the exception object</span>
512        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"bar call failed: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="string">", scopes stack:\n"</span>
513            <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">get_error_info</span><span class="special">&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">current_scope_info</span> <span class="special">&gt;(</span><span class="identifier">e</span><span class="special">);</span>
514    <span class="special">}</span>
515<span class="special">}</span>
516</pre>
517<div class="note"><table border="0" summary="Note">
518<tr>
519<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
520<th align="left">Note</th>
521</tr>
522<tr><td align="left" valign="top"><p>
523            In order this code to compile, the <a href="http://www.boost.org/doc/libs/release/libs/exception/doc/boost-exception.html" target="_top">Boost.Exception</a>
524            support header has to be included.
525          </p></td></tr>
526</table></div>
527<div class="note"><table border="0" summary="Note">
528<tr>
529<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
530<th align="left">Note</th>
531</tr>
532<tr><td align="left" valign="top"><p>
533            We do not inject the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/named_scope.html" title="Class named_scope">named_scope</a></code>
534            attribute into the exception. Since scope stacks are maintained globally,
535            throwing an exception will cause stack unwinding and, as a result, will
536            truncate the global stack. Instead we create a copy of the scope stack
537            by calling <code class="computeroutput"><a class="link" href="../../boost/log/current_scope.html" title="Function current_scope">current_scope</a></code> at the
538            throw site. This copy will be kept intact even if the global stack instance
539            changes during the stack unwinding.
540          </p></td></tr>
541</table></div>
542</div>
543<div class="section">
544<div class="titlepage"><div><div><h4 class="title">
545<a name="log.detailed.attributes.process_id"></a><a class="link" href="attributes.html#log.detailed.attributes.process_id" title="Current process identifier">Current process
546        identifier</a>
547</h4></div></div></div>
548<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.current_process_id_hpp" title="Header &lt;boost/log/attributes/current_process_id.hpp&gt;">boost/log/attributes/current_process_id.hpp</a></code><span class="special">&gt;</span>
549</pre>
550<p>
551          It is often useful to know the process identifier that produces the log,
552          especially if the log can eventually combine the output of different processes.
553          The <code class="computeroutput"><a class="link" href="../../boost/log/attributes/current_process_id.html" title="Class current_process_id">current_process_id</a></code>
554          attribute is a constant that formats into the current process identifier.
555          The value type of the attribute can be determined by the <code class="computeroutput"><span class="identifier">current_process_id</span><span class="special">::</span><span class="identifier">value_type</span></code> typedef.
556        </p>
557<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
558<span class="special">{</span>
559    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">core</span><span class="special">::</span><span class="identifier">get</span><span class="special">()-&gt;</span><span class="identifier">add_global_attribute</span><span class="special">(</span>
560        <span class="string">"ProcessID"</span><span class="special">,</span>
561        <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">current_process_id</span><span class="special">());</span>
562<span class="special">}</span>
563</pre>
564</div>
565<div class="section">
566<div class="titlepage"><div><div><h4 class="title">
567<a name="log.detailed.attributes.process_name"></a><a class="link" href="attributes.html#log.detailed.attributes.process_name" title="Current process name">Current process
568        name</a>
569</h4></div></div></div>
570<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.current_process_name_hpp" title="Header &lt;boost/log/attributes/current_process_name.hpp&gt;">boost/log/attributes/current_process_name.hpp</a></code><span class="special">&gt;</span>
571</pre>
572<p>
573          The <code class="computeroutput"><a class="link" href="../../boost/log/attributes/current_process_name.html" title="Class current_process_name">current_process_name</a></code>
574          produces <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> values with the executable name
575          of the current process.
576        </p>
577<div class="note"><table border="0" summary="Note">
578<tr>
579<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
580<th align="left">Note</th>
581</tr>
582<tr><td align="left" valign="top"><p>
583            This attribute is not universally portable, although Windows, Linux and
584            OS X are supported. The attribute may work on other POSIX systems as
585            well, but it was not tested. If the process name cannot be obtained the
586            attribute will generate a string with the process id.
587          </p></td></tr>
588</table></div>
589<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
590<span class="special">{</span>
591    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">core</span><span class="special">::</span><span class="identifier">get</span><span class="special">()-&gt;</span><span class="identifier">add_global_attribute</span><span class="special">(</span>
592        <span class="string">"Process"</span><span class="special">,</span>
593        <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">current_process_name</span><span class="special">());</span>
594<span class="special">}</span>
595</pre>
596</div>
597<div class="section">
598<div class="titlepage"><div><div><h4 class="title">
599<a name="log.detailed.attributes.thread_id"></a><a class="link" href="attributes.html#log.detailed.attributes.thread_id" title="Current thread identifier">Current thread identifier</a>
600</h4></div></div></div>
601<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.current_thread_id_hpp" title="Header &lt;boost/log/attributes/current_thread_id.hpp&gt;">boost/log/attributes/current_thread_id.hpp</a></code><span class="special">&gt;</span>
602</pre>
603<p>
604          Multithreaded builds of the library also support the <code class="computeroutput"><a class="link" href="../../boost/log/attributes/current_thread_id.html" title="Class current_thread_id">current_thread_id</a></code>
605          attribute with value type <code class="computeroutput"><span class="identifier">current_thread_id</span><span class="special">::</span><span class="identifier">value_type</span></code>.
606          The attribute will generate values specific to the calling thread. The
607          usage is similar to the process id.
608        </p>
609<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
610<span class="special">{</span>
611    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">core</span><span class="special">::</span><span class="identifier">get</span><span class="special">()-&gt;</span><span class="identifier">add_global_attribute</span><span class="special">(</span>
612        <span class="string">"ThreadID"</span><span class="special">,</span>
613        <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">current_thread_id</span><span class="special">());</span>
614<span class="special">}</span>
615</pre>
616<div class="tip"><table border="0" summary="Tip">
617<tr>
618<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
619<th align="left">Tip</th>
620</tr>
621<tr><td align="left" valign="top"><p>
622            You may have noticed that the attribute is registered globally. This
623            will not result in all threads having the same ThreadID in log records
624            as the attribute will always return a thread-specific value. The additional
625            benefit is that you don't have to do a thing in the thread initialization
626            routines to have the thread-specific attribute value in log records.
627          </p></td></tr>
628</table></div>
629</div>
630<div class="section">
631<div class="titlepage"><div><div><h4 class="title">
632<a name="log.detailed.attributes.function"></a><a class="link" href="attributes.html#log.detailed.attributes.function" title="Function objects as attributes">Function objects
633        as attributes</a>
634</h4></div></div></div>
635<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.function_hpp" title="Header &lt;boost/log/attributes/function.hpp&gt;">boost/log/attributes/function.hpp</a></code><span class="special">&gt;</span>
636</pre>
637<p>
638          This attribute is a simple wrapper around a user-defined function object.
639          Each attempt to acquire the attribute value results in the function object
640          call. The result of the call is returned as the attribute value (this implies
641          that the function must not return <code class="computeroutput"><span class="keyword">void</span></code>).
642          The function object attribute can be constructed with the <code class="computeroutput"><span class="identifier">make_function</span></code> helper function, like this:
643        </p>
644<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
645<span class="special">{</span>
646    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">core</span><span class="special">::</span><span class="identifier">get</span><span class="special">()-&gt;</span><span class="identifier">add_global_attribute</span><span class="special">(</span><span class="string">"MyRandomAttr"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">make_function</span><span class="special">(&amp;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">rand</span><span class="special">));</span>
647<span class="special">}</span>
648</pre>
649<p>
650          Auto-generated function objects, like the ones defined in <a href="http://www.boost.org/doc/libs/release/libs/bind/bind.html" target="_top">Boost.Bind</a>
651          or STL, are also supported.
652        </p>
653<div class="note"><table border="0" summary="Note">
654<tr>
655<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
656<th align="left">Note</th>
657</tr>
658<tr><td align="left" valign="top"><p>
659            Some deficient compilers may not support <code class="computeroutput"><span class="identifier">result_of</span></code>
660            construct properly. This metafunction is used in the <code class="computeroutput"><span class="identifier">make_function</span></code>
661            function to automatically detect the return type of the function object.
662            If <code class="computeroutput"><span class="identifier">result_of</span></code> breaks or
663            detects incorrect type, one can try to explicitly specify the return
664            type of the function object as a template argument to the <code class="computeroutput"><span class="identifier">make_function</span></code> function.
665          </p></td></tr>
666</table></div>
667</div>
668<div class="section">
669<div class="titlepage"><div><div><h4 class="title">
670<a name="log.detailed.attributes.related_components"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components" title="Other attribute-related components">Other attribute-related
671        components</a>
672</h4></div></div></div>
673<div class="toc"><dl class="toc">
674<dt><span class="section"><a href="attributes.html#log.detailed.attributes.related_components.attribute_name">Attribute
675          names</a></span></dt>
676<dt><span class="section"><a href="attributes.html#log.detailed.attributes.related_components.attribute_set">Attribute
677          set</a></span></dt>
678<dt><span class="section"><a href="attributes.html#log.detailed.attributes.related_components.attribute_value_set">Attribute
679          value set</a></span></dt>
680<dt><span class="section"><a href="attributes.html#log.detailed.attributes.related_components.value_processing">Attribute
681          value extraction and visitation</a></span></dt>
682<dt><span class="section"><a href="attributes.html#log.detailed.attributes.related_components.scoped_attributes">Scoped
683          attributes</a></span></dt>
684</dl></div>
685<div class="section">
686<div class="titlepage"><div><div><h5 class="title">
687<a name="log.detailed.attributes.related_components.attribute_name"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components.attribute_name" title="Attribute names">Attribute
688          names</a>
689</h5></div></div></div>
690<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.attribute_name_hpp" title="Header &lt;boost/log/attributes/attribute_name.hpp&gt;">boost/log/attributes/attribute_name.hpp</a></code><span class="special">&gt;</span>
691</pre>
692<p>
693            Attribute names are represented with <code class="computeroutput"><a class="link" href="../../boost/log/attribute_name.html" title="Class attribute_name">attribute_name</a></code>
694            objects which are used as keys in associative containers of attributes
695            used by the library. The <code class="computeroutput"><a class="link" href="../../boost/log/attribute_name.html" title="Class attribute_name">attribute_name</a></code>
696            object can be created from a string, so most of the time its use is transparent.
697          </p>
698<p>
699            The name is not stored as a string within the <code class="computeroutput"><a class="link" href="../../boost/log/attribute_name.html" title="Class attribute_name">attribute_name</a></code>
700            object. Instead, a process-wide unique identifier is generated and associated
701            with the particular name. This association is preserved until the process
702            termination, so every time the <code class="computeroutput"><a class="link" href="../../boost/log/attribute_name.html" title="Class attribute_name">attribute_name</a></code>
703            object is created for the same name it obtains the same identifier. The
704            association is not stable across the different runs of the application
705            though.
706          </p>
707<div class="warning"><table border="0" summary="Warning">
708<tr>
709<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
710<th align="left">Warning</th>
711</tr>
712<tr><td align="left" valign="top"><p>
713              Since the association between string names and identifiers involves
714              some state allocation, it is not advised to use externally provided
715              or known to be changing strings for attribute names. Even if the name
716              is not used in any log records, the association is preserved anyway.
717              Continuously constructing <code class="computeroutput"><a class="link" href="../../boost/log/attribute_name.html" title="Class attribute_name">attribute_name</a></code>
718              objects with unique string names may manifest itself as a memory leak.
719            </p></td></tr>
720</table></div>
721<p>
722            Working with identifiers is much more efficient than with strings. For
723            example, copying does not involve dynamic memory allocation and comparison
724            operators are very lightweight. On the other hand, it is easy to get
725            a human-readable attribute name for presentation, if needed.
726          </p>
727<p>
728            The <code class="computeroutput"><a class="link" href="../../boost/log/attribute_name.html" title="Class attribute_name">attribute_name</a></code>
729            class supports an empty (uninitialized) state when default constructed.
730            In this state the name object is not equal to any other initialized name
731            object. Uninitialized attribute names should not be passed to the library
732            but can be useful in some contexts (e.g. when a delayed initialization
733            is desired).
734          </p>
735</div>
736<div class="section">
737<div class="titlepage"><div><div><h5 class="title">
738<a name="log.detailed.attributes.related_components.attribute_set"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components.attribute_set" title="Attribute set">Attribute
739          set</a>
740</h5></div></div></div>
741<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.attribute_set_hpp" title="Header &lt;boost/log/attributes/attribute_set.hpp&gt;">boost/log/attributes/attribute_set.hpp</a></code><span class="special">&gt;</span>
742</pre>
743<p>
744            Attribute set is an unordered associative container that maps <a class="link" href="attributes.html#log.detailed.attributes.related_components.attribute_name" title="Attribute names">attribute
745            names</a> to <a class="link" href="attributes.html" title="Attributes">attributes</a>.
746            It is used in <a class="link" href="sources.html" title="Logging sources">loggers</a> and
747            the <a class="link" href="../detailed.html#log.detailed.core.core" title="Logging core">logging core</a> to store
748            source-specific, thread-specific and global attributes. The interface
749            is very similar to STL associative containers and is described in the
750            <code class="computeroutput"><a class="link" href="../../boost/log/attribute_set.html" title="Class attribute_set">attribute_set</a></code>
751            class reference.
752          </p>
753</div>
754<div class="section">
755<div class="titlepage"><div><div><h5 class="title">
756<a name="log.detailed.attributes.related_components.attribute_value_set"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components.attribute_value_set" title="Attribute value set">Attribute
757          value set</a>
758</h5></div></div></div>
759<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.attribute_value_set_hpp" title="Header &lt;boost/log/attributes/attribute_value_set.hpp&gt;">boost/log/attributes/attribute_value_set.hpp</a></code><span class="special">&gt;</span>
760</pre>
761<p>
762            Attribute value set is an unordered associative container that maps
763            <a class="link" href="attributes.html#log.detailed.attributes.related_components.attribute_name" title="Attribute names">attribute
764            names</a> to <a class="link" href="attributes.html" title="Attributes">attribute values</a>.
765            This container is used in log <a class="link" href="../detailed.html#log.detailed.core.record" title="Logging records">records</a>
766            to represent attribute values. Unlike conventional containers, <code class="computeroutput"><a class="link" href="../../boost/log/attribute_value_set.html" title="Class attribute_value_set">attribute_value_set</a></code>
767            does not support removing or modifying elements after being inserted.
768            This warrants that the attribute values that participated filtering will
769            not disappear from the log record in the middle of the processing.
770          </p>
771<p>
772            Additionally, the set can be constructed from three <a class="link" href="attributes.html#log.detailed.attributes.related_components.attribute_set" title="Attribute set">attribute
773            sets</a>, which are interpreted as the sets of source-specific, thread-specific
774            and global attributes. The constructor adopts attribute values from the
775            three attribute sets into a single set of attribute values. After construction,
776            <code class="computeroutput"><a class="link" href="../../boost/log/attribute_value_set.html" title="Class attribute_value_set">attribute_value_set</a></code>
777            is considered to be in an unfrozen state. This means that the container
778            may keep references to the elements of the attribute sets used as the
779            source for the value set construction. While in this state, neither the
780            attribute sets nor the value set must not be modified in any way as this
781            may make the value set corrupted. The value set can be used for reading
782            in this state, its lookup operations will perform as usual. The value
783            set can be frozen by calling the <code class="computeroutput"><span class="identifier">freeze</span></code>
784            method; the set will no longer be attached to the original attribute
785            sets and will be available for further insertions after this call. The
786            library will ensure that the value set is always frozen when a log record
787            is returned from the logging core; the set is <span class="underline">not</span>
788            frozen during filtering though.
789          </p>
790<div class="tip"><table border="0" summary="Tip">
791<tr>
792<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
793<th align="left">Tip</th>
794</tr>
795<tr><td align="left" valign="top"><p>
796              In the unfrozen state the value set may not have all attribute values
797              acquired from the attributes. It will only acquire the values as requested
798              by filters. After freezing the container has all attribute values.
799              This transition allows to optimize the library so that attribute values
800              are only acquired when needed.
801            </p></td></tr>
802</table></div>
803<p>
804            For further details on the container interface please consult the <code class="computeroutput"><a class="link" href="../../boost/log/attribute_value_set.html" title="Class attribute_value_set">attribute_value_set</a></code>
805            reference.
806          </p>
807</div>
808<div class="section">
809<div class="titlepage"><div><div><h5 class="title">
810<a name="log.detailed.attributes.related_components.value_processing"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components.value_processing" title="Attribute value extraction and visitation">Attribute
811          value extraction and visitation</a>
812</h5></div></div></div>
813<p>
814            Since attribute values do not expose the stored value in the interface,
815            an API is needed to acquire the stored value. The library provides two
816            APIs for this purpose: value visitation and extraction.
817          </p>
818<div class="section">
819<div class="titlepage"><div><div><h6 class="title">
820<a name="log.detailed.attributes.related_components.value_processing.visitation"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components.value_processing.visitation" title="Value visitation">Value
821            visitation</a>
822</h6></div></div></div>
823<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.value_visitation_fwd_hpp" title="Header &lt;boost/log/attributes/value_visitation_fwd.hpp&gt;">boost/log/attributes/value_visitation_fwd.hpp</a></code><span class="special">&gt;</span>
824<span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.value_visitation_hpp" title="Header &lt;boost/log/attributes/value_visitation.hpp&gt;">boost/log/attributes/value_visitation.hpp</a></code><span class="special">&gt;</span>
825</pre>
826<p>
827              Attribute value visitation implements the visitor design pattern, hence
828              the naming. The user has to provide a unary function object (a visitor)
829              which will be invoked on the stored attribute value. The caller also
830              has to provide the expected type or set of possible types of the stored
831              value. Obviously, the visitor must be capable of receiving an argument
832              of the expected type. Visitation will only succeed if the stored type
833              matches the expectation.
834            </p>
835<p>
836              In order to apply the visitor, one should call the <code class="computeroutput"><a class="link" href="../../boost/log/visit_idm46079578815792.html" title="Function template visit">visit</a></code> function on
837              the attribute value. Let's see an example:
838            </p>
839<p>
840</p>
841<pre class="programlisting"><span class="comment">// Our attribute value visitor</span>
842<span class="keyword">struct</span> <span class="identifier">print_visitor</span>
843<span class="special">{</span>
844    <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
845
846    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">val</span><span class="special">)</span> <span class="keyword">const</span>
847    <span class="special">{</span>
848        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visited value is int: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
849    <span class="special">}</span>
850
851    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">val</span><span class="special">)</span> <span class="keyword">const</span>
852    <span class="special">{</span>
853        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visited value is string: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
854    <span class="special">}</span>
855<span class="special">};</span>
856
857<span class="keyword">void</span> <span class="identifier">print_value</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_value</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</span>
858<span class="special">{</span>
859    <span class="comment">// Define the set of expected types of the stored value</span>
860    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
861
862    <span class="comment">// Apply our visitor</span>
863    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">visitation_result</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">visit</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">,</span> <span class="identifier">print_visitor</span><span class="special">());</span>
864
865    <span class="comment">// Check the result</span>
866    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">result</span><span class="special">)</span>
867        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visitation succeeded"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
868    <span class="keyword">else</span>
869        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visitation failed"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
870<span class="special">}</span>
871</pre>
872<p>
873            </p>
874<p>
875              <a href="../../../../../../libs/log/example/doc/attr_value_visitation.cpp" target="_top">See
876              the complete code</a>.
877            </p>
878<p>
879              In this example we print the stored attribute value in our <code class="computeroutput"><span class="identifier">print_visitor</span></code>. We expect the attribute
880              value to have either <code class="computeroutput"><span class="keyword">int</span></code>
881              or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> stored type; only in this
882              case the visitor will be invoked and the visitation result will be
883              positive. In case of failure the <code class="computeroutput"><a class="link" href="../../boost/log/visitation_result.html" title="Class visitation_result">visitation_result</a></code>
884              class provides additional information on the failure reason. The class
885              has the method named <code class="computeroutput"><span class="identifier">code</span></code>
886              which returns visitation error code. The following error codes are
887              possible:
888            </p>
889<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
890<li class="listitem">
891                  <code class="computeroutput"><span class="identifier">ok</span></code> - visitation
892                  succeeded, the visitor has been invoked; visitation result is positive
893                  when this code is used
894                </li>
895<li class="listitem">
896                  <code class="computeroutput"><span class="identifier">value_not_found</span></code>
897                  - visitation failed because the requested value was not found;
898                  this code is used when visitation is applied to a log record or
899                  a set of attribute values rather than a single value
900                </li>
901<li class="listitem">
902                  <code class="computeroutput"><span class="identifier">value_has_invalid_type</span></code>
903                  - visitation failed because the value has type differing from any
904                  of the expected types
905                </li>
906</ul></div>
907<p>
908              By default the visitor function result is ignored but it is possible
909              to obtain it. To do this one should use a special <code class="computeroutput"><a class="link" href="../../boost/log/save_result.html" title="Function template save_result">save_result</a></code> wrapper
910              for the visitor; the wrapper will save the visitor resulting value
911              into an external variable captured by reference. The visitor result
912              is initialized when the returned <code class="computeroutput"><a class="link" href="../../boost/log/visitation_result.html" title="Class visitation_result">visitation_result</a></code>
913              is positive. See the following example where we compute the hash value
914              on the stored value.
915            </p>
916<p>
917</p>
918<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">hash_visitor</span>
919<span class="special">{</span>
920    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">result_type</span><span class="special">;</span>
921
922    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">val</span><span class="special">)</span> <span class="keyword">const</span>
923    <span class="special">{</span>
924        <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">h</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">;</span>
925        <span class="identifier">h</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">15</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">h</span><span class="special">;</span>
926        <span class="identifier">h</span> <span class="special">^=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&gt;&gt;</span> <span class="number">6</span><span class="special">)</span> <span class="special">+</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">7</span><span class="special">);</span>
927        <span class="keyword">return</span> <span class="identifier">h</span><span class="special">;</span>
928    <span class="special">}</span>
929
930    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">val</span><span class="special">)</span> <span class="keyword">const</span>
931    <span class="special">{</span>
932        <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">h</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
933        <span class="keyword">for</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">const_iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">end</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">end</span><span class="special">();</span> <span class="identifier">it</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">;</span> <span class="special">++</span><span class="identifier">it</span><span class="special">)</span>
934            <span class="identifier">h</span> <span class="special">+=</span> <span class="special">*</span><span class="identifier">it</span><span class="special">;</span>
935
936        <span class="identifier">h</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">15</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">h</span><span class="special">;</span>
937        <span class="identifier">h</span> <span class="special">^=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&gt;&gt;</span> <span class="number">6</span><span class="special">)</span> <span class="special">+</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">7</span><span class="special">);</span>
938        <span class="keyword">return</span> <span class="identifier">h</span><span class="special">;</span>
939    <span class="special">}</span>
940<span class="special">};</span>
941
942<span class="keyword">void</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_value</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</span>
943<span class="special">{</span>
944    <span class="comment">// Define the set of expected types of the stored value</span>
945    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
946
947    <span class="comment">// Apply our visitor</span>
948    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">h</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
949    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">visitation_result</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">visit</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">,</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">save_result</span><span class="special">(</span><span class="identifier">hash_visitor</span><span class="special">(),</span> <span class="identifier">h</span><span class="special">));</span>
950
951    <span class="comment">// Check the result</span>
952    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">result</span><span class="special">)</span>
953        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visitation succeeded, hash value: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
954    <span class="keyword">else</span>
955        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visitation failed"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
956<span class="special">}</span>
957</pre>
958<p>
959            </p>
960<p>
961              <a href="../../../../../../libs/log/example/doc/attr_value_visitation.cpp" target="_top">See
962              the complete code</a>.
963            </p>
964<div class="tip"><table border="0" summary="Tip">
965<tr>
966<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
967<th align="left">Tip</th>
968</tr>
969<tr><td align="left" valign="top"><p>
970                When there is no default state for the visitor result it is convenient
971                to use <a href="http://www.boost.org/doc/libs/release/libs/optional/index.html" target="_top">Boost.Optional</a>
972                to wrap the returned value. The <code class="computeroutput"><span class="identifier">optional</span></code>
973                will be initialized with the visitor result if visitation succeeded.
974                In case if visitor is polymorphic (i.e. it has different result types
975                depending on its argument type) <a href="http://www.boost.org/doc/libs/release/doc/html/variant.html" target="_top">Boost.Variant</a>
976                can be used to receive the resulting value. It is also worthwhile
977                to use an empty type, such as <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">blank</span></code>,
978                to indicate the uninitialized state of the <code class="computeroutput"><span class="identifier">variant</span></code>.
979              </p></td></tr>
980</table></div>
981<p>
982              As it has been mentioned, visitation can also be applied to log records
983              and attribute value sets. The syntax is the same, except that the attribute
984              name also has to be specified. The <code class="computeroutput"><a class="link" href="../../boost/log/visit_idm46079578815792.html" title="Function template visit">visit</a></code> algorithm will
985              try to find the attribute value by name and then apply the visitor
986              to the found element.
987            </p>
988<p>
989</p>
990<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">record_view</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rec</span><span class="special">,</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_name</span> <span class="identifier">name</span><span class="special">)</span>
991<span class="special">{</span>
992    <span class="comment">// Define the set of expected types of the stored value</span>
993    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
994
995    <span class="comment">// Apply our visitor</span>
996    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">h</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
997    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">visitation_result</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">visit</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;(</span><span class="identifier">name</span><span class="special">,</span> <span class="identifier">rec</span><span class="special">,</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">save_result</span><span class="special">(</span><span class="identifier">hash_visitor</span><span class="special">(),</span> <span class="identifier">h</span><span class="special">));</span>
998
999    <span class="comment">// Check the result</span>
1000    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">result</span><span class="special">)</span>
1001        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visitation succeeded, hash value: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1002    <span class="keyword">else</span>
1003        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Visitation failed"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1004<span class="special">}</span>
1005</pre>
1006<p>
1007            </p>
1008<p>
1009              Also, for convenience <code class="computeroutput"><a class="link" href="../../boost/log/attribute_value.html" title="Class attribute_value">attribute_value</a></code>
1010              has the method named <code class="computeroutput"><span class="identifier">visit</span></code>
1011              with the same meaning as the free function applied to the attribute
1012              value.
1013            </p>
1014</div>
1015<div class="section">
1016<div class="titlepage"><div><div><h6 class="title">
1017<a name="log.detailed.attributes.related_components.value_processing.extraction"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components.value_processing.extraction" title="Value extraction">Value
1018            extraction</a>
1019</h6></div></div></div>
1020<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.value_extraction_fwd_hpp" title="Header &lt;boost/log/attributes/value_extraction_fwd.hpp&gt;">boost/log/attributes/value_extraction_fwd.hpp</a></code><span class="special">&gt;</span>
1021<span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><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></code><span class="special">&gt;</span>
1022</pre>
1023<p>
1024              Attribute value extraction API allows to acquire a reference to the
1025              stored value. It does not require a visitor function object, but the
1026              user still has to provide the expected type or a set of types the stored
1027              value may have.
1028            </p>
1029<p>
1030</p>
1031<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">print_value</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_value</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</span>
1032<span class="special">{</span>
1033    <span class="comment">// Extract a reference to the stored value</span>
1034    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">value_ref</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">extract</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">);</span>
1035
1036    <span class="comment">// Check the result</span>
1037    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">val</span><span class="special">)</span>
1038        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction succeeded: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1039    <span class="keyword">else</span>
1040        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction failed"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1041<span class="special">}</span>
1042</pre>
1043<p>
1044            </p>
1045<p>
1046              <a href="../../../../../../libs/log/example/doc/attr_value_extraction.cpp" target="_top">See
1047              the complete code</a>.
1048            </p>
1049<p>
1050              In this example we expect the attribute value to have the stored type
1051              <code class="computeroutput"><span class="keyword">int</span></code>. The <code class="computeroutput"><a class="link" href="../../boost/log/extract_idm46079579055520.html" title="Function template extract">extract</a></code>
1052              function attempts to extract a reference to the stored value and returns
1053              the filled <a class="link" href="utilities.html#log.detailed.utilities.value_ref" title="Value reference wrapper"><code class="computeroutput"><span class="identifier">value_ref</span></code></a> object if succeeded.
1054            </p>
1055<p>
1056              Value extraction can also be used with a set of expected stored types.
1057              The following code snippet demonstrates this:
1058            </p>
1059<p>
1060</p>
1061<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">print_value_multiple_types</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_value</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</span>
1062<span class="special">{</span>
1063    <span class="comment">// Define the set of expected types of the stored value</span>
1064    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
1065
1066    <span class="comment">// Extract a reference to the stored value</span>
1067    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">value_ref</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">extract</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">);</span>
1068
1069    <span class="comment">// Check the result</span>
1070    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">val</span><span class="special">)</span>
1071    <span class="special">{</span>
1072        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction succeeded"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1073        <span class="keyword">switch</span> <span class="special">(</span><span class="identifier">val</span><span class="special">.</span><span class="identifier">which</span><span class="special">())</span>
1074        <span class="special">{</span>
1075        <span class="keyword">case</span> <span class="number">0</span><span class="special">:</span>
1076            <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"int: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">get</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1077            <span class="keyword">break</span><span class="special">;</span>
1078
1079        <span class="keyword">case</span> <span class="number">1</span><span class="special">:</span>
1080            <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"string: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">get</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1081            <span class="keyword">break</span><span class="special">;</span>
1082        <span class="special">}</span>
1083    <span class="special">}</span>
1084    <span class="keyword">else</span>
1085        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction failed"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1086<span class="special">}</span>
1087</pre>
1088<p>
1089            </p>
1090<p>
1091              Notice that we used <code class="computeroutput"><span class="identifier">which</span></code>
1092              method of the returned reference to dispatch between possible types.
1093              The method returns the index of the type in the <code class="computeroutput"><span class="identifier">types</span></code>
1094              sequence. Also note that the <code class="computeroutput"><span class="identifier">get</span></code>
1095              method now accepts an explicit template parameter to select the reference
1096              type to acquire; naturally, this type must correspond to the actual
1097              referred type, which is warranted by the switch/case statement in our
1098              case.
1099            </p>
1100<p>
1101              Value visitation is also supported by the <a class="link" href="utilities.html#log.detailed.utilities.value_ref" title="Value reference wrapper"><code class="computeroutput"><span class="identifier">value_ref</span></code></a> object. Here is
1102              how we compute a hash value from the extracted value:
1103            </p>
1104<p>
1105</p>
1106<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">hash_visitor</span>
1107<span class="special">{</span>
1108    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">result_type</span><span class="special">;</span>
1109
1110    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">val</span><span class="special">)</span> <span class="keyword">const</span>
1111    <span class="special">{</span>
1112        <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">h</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">;</span>
1113        <span class="identifier">h</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">15</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">h</span><span class="special">;</span>
1114        <span class="identifier">h</span> <span class="special">^=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&gt;&gt;</span> <span class="number">6</span><span class="special">)</span> <span class="special">+</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">7</span><span class="special">);</span>
1115        <span class="keyword">return</span> <span class="identifier">h</span><span class="special">;</span>
1116    <span class="special">}</span>
1117
1118    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">val</span><span class="special">)</span> <span class="keyword">const</span>
1119    <span class="special">{</span>
1120        <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">h</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
1121        <span class="keyword">for</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">const_iterator</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">end</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">end</span><span class="special">();</span> <span class="identifier">it</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">;</span> <span class="special">++</span><span class="identifier">it</span><span class="special">)</span>
1122            <span class="identifier">h</span> <span class="special">+=</span> <span class="special">*</span><span class="identifier">it</span><span class="special">;</span>
1123
1124        <span class="identifier">h</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">15</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">h</span><span class="special">;</span>
1125        <span class="identifier">h</span> <span class="special">^=</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&gt;&gt;</span> <span class="number">6</span><span class="special">)</span> <span class="special">+</span> <span class="special">(</span><span class="identifier">h</span> <span class="special">&lt;&lt;</span> <span class="number">7</span><span class="special">);</span>
1126        <span class="keyword">return</span> <span class="identifier">h</span><span class="special">;</span>
1127    <span class="special">}</span>
1128<span class="special">};</span>
1129
1130<span class="keyword">void</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_value</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</span>
1131<span class="special">{</span>
1132    <span class="comment">// Define the set of expected types of the stored value</span>
1133    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
1134
1135    <span class="comment">// Extract the stored value</span>
1136    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">value_ref</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">extract</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">);</span>
1137
1138    <span class="comment">// Check the result</span>
1139    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">val</span><span class="special">)</span>
1140        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction succeeded, hash value: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">apply_visitor</span><span class="special">(</span><span class="identifier">hash_visitor</span><span class="special">())</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1141    <span class="keyword">else</span>
1142        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction failed"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1143<span class="special">}</span>
1144</pre>
1145<p>
1146            </p>
1147<p>
1148              Lastly, like with value visitation, value extraction can also be applied
1149              to log records and attribute value sets.
1150            </p>
1151<p>
1152</p>
1153<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">record_view</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rec</span><span class="special">,</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_name</span> <span class="identifier">name</span><span class="special">)</span>
1154<span class="special">{</span>
1155    <span class="comment">// Define the set of expected types of the stored value</span>
1156    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
1157
1158    <span class="comment">// Extract the stored value</span>
1159    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">value_ref</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">extract</span><span class="special">&lt;</span> <span class="identifier">types</span> <span class="special">&gt;(</span><span class="identifier">name</span><span class="special">,</span> <span class="identifier">rec</span><span class="special">);</span>
1160
1161    <span class="comment">// Check the result</span>
1162    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">val</span><span class="special">)</span>
1163        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction succeeded, hash value: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">.</span><span class="identifier">apply_visitor</span><span class="special">(</span><span class="identifier">hash_visitor</span><span class="special">())</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1164    <span class="keyword">else</span>
1165        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Extraction failed"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
1166<span class="special">}</span>
1167</pre>
1168<p>
1169            </p>
1170<p>
1171              In addition the library provides two special variants of the <code class="computeroutput"><a class="link" href="../../boost/log/extract_idm46079579055520.html" title="Function template extract">extract</a></code>
1172              function: <code class="computeroutput"><a class="link" href="../../boost/log/extract__idm46079579026960.html" title="Function template extract_or_throw">extract_or_throw</a></code> and
1173              <code class="computeroutput"><a class="link" href="../../boost/log/extract__idm46079578995456.html" title="Function template extract_or_default">extract_or_default</a></code>.
1174              As the naming implies, the functions provide different behavior in
1175              case if the attribute value cannot be extracted. The former one throws
1176              an exception if the value cannot be extracted and the latter one returns
1177              the default value.
1178            </p>
1179<div class="warning"><table border="0" summary="Warning">
1180<tr>
1181<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
1182<th align="left">Warning</th>
1183</tr>
1184<tr><td align="left" valign="top"><p>
1185                Care must be taken with the <code class="computeroutput"><a class="link" href="../../boost/log/extract__idm46079578995456.html" title="Function template extract_or_default">extract_or_default</a></code>
1186                function. The function accepts the default value is accepted by constant
1187                reference, and this reference can eventually be returned from <code class="computeroutput"><a class="link" href="../../boost/log/extract__idm46079578995456.html" title="Function template extract_or_default">extract_or_default</a></code>.
1188                If a temporary object as used for the default value, user must ensure
1189                that the result of <code class="computeroutput"><a class="link" href="../../boost/log/extract__idm46079578995456.html" title="Function template extract_or_default">extract_or_default</a></code>
1190                is saved by value and not by reference. Otherwise the saved reference
1191                may become dangling when the temporary is destroyed.
1192              </p></td></tr>
1193</table></div>
1194<p>
1195              Similarly to <code class="computeroutput"><span class="identifier">visit</span></code>,
1196              the <code class="computeroutput"><a class="link" href="../../boost/log/attribute_value.html" title="Class attribute_value">attribute_value</a></code>
1197              class has methods named <code class="computeroutput"><span class="identifier">extract</span></code>,
1198              <code class="computeroutput"><span class="identifier">extract_or_throw</span></code> and
1199              <code class="computeroutput"><span class="identifier">extract_or_default</span></code>
1200              with the same meaning as the corresponding free functions applied to
1201              the attribute value.
1202            </p>
1203</div>
1204</div>
1205<div class="section">
1206<div class="titlepage"><div><div><h5 class="title">
1207<a name="log.detailed.attributes.related_components.scoped_attributes"></a><a class="link" href="attributes.html#log.detailed.attributes.related_components.scoped_attributes" title="Scoped attributes">Scoped
1208          attributes</a>
1209</h5></div></div></div>
1210<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../attributes.html#header.boost.log.attributes.scoped_attribute_hpp" title="Header &lt;boost/log/attributes/scoped_attribute.hpp&gt;">boost/log/attributes/scoped_attribute.hpp</a></code><span class="special">&gt;</span>
1211</pre>
1212<p>
1213            Scoped attributes are a powerful mechanism of tagging log records that
1214            can be used for different purposes. As the naming implies, scoped attributes
1215            are registered in the beginning of a scope and unregistered on the end
1216            of the scope. The mechanism includes the following macros:
1217          </p>
1218<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079579098656.html" title="Macro BOOST_LOG_SCOPED_LOGGER_ATTR">BOOST_LOG_SCOPED_LOGGER_ATTR</a></code><span class="special">(</span><span class="identifier">logger</span><span class="special">,</span> <span class="identifier">attr_name</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span>
1219<code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079579093888.html" title="Macro BOOST_LOG_SCOPED_THREAD_ATTR">BOOST_LOG_SCOPED_THREAD_ATTR</a></code><span class="special">(</span><span class="identifier">attr_name</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span>
1220</pre>
1221<p>
1222            The first macro registers a source-specific attribute in the <code class="computeroutput"><span class="identifier">logger</span></code> logger object. The attribute
1223            name and the attribute itself are given in the <code class="computeroutput"><span class="identifier">attr_name</span></code>
1224            and <code class="computeroutput"><span class="identifier">attr</span></code> arguments. The
1225            second macro does exactly the same but the attribute is registered for
1226            the current thread in the logging core (which does not require a logger).
1227          </p>
1228<div class="note"><table border="0" summary="Note">
1229<tr>
1230<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
1231<th align="left">Note</th>
1232</tr>
1233<tr><td align="left" valign="top"><p>
1234              If an attribute with the same name is already registered in the logger/logging
1235              core, the macros won't override the existing attribute and will eventually
1236              have no effect. See <a class="link" href="../rationale/why_weak_scoped_attributes.html" title="Why scoped attributes don't override existing attributes?">Rationale</a>
1237              for a more detailed explanation of the reasons for such behavior.
1238            </p></td></tr>
1239</table></div>
1240<p>
1241            Usage example follows:
1242          </p>
1243<pre class="programlisting"><span class="identifier">BOOST_LOG_DECLARE_GLOBAL_LOGGER</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">,</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">)</span>
1244
1245<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
1246<span class="special">{</span>
1247    <span class="comment">// This log record will also be marked with the "Tag" attribute,</span>
1248    <span class="comment">// whenever it is called from the A::bar function.</span>
1249    <span class="comment">// It will not be marked when called from other places.</span>
1250    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special">&lt;&lt;</span> <span class="string">"A log message from foo"</span><span class="special">;</span>
1251<span class="special">}</span>
1252
1253<span class="keyword">struct</span> <span class="identifier">A</span>
1254<span class="special">{</span>
1255    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger</span> <span class="identifier">m_Logger</span><span class="special">;</span>
1256
1257    <span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
1258    <span class="special">{</span>
1259        <span class="comment">// Set a thread-wide markup tag.</span>
1260        <span class="comment">// Note the additional parentheses to form a Boost.PP sequence.</span>
1261        <span class="identifier">BOOST_LOG_SCOPED_THREAD_ATTR</span><span class="special">(</span><span class="string">"Tag"</span><span class="special">,</span>
1262            <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;(</span><span class="string">"Called from A::bar"</span><span class="special">));</span>
1263
1264        <span class="comment">// This log record will be marked</span>
1265        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_Logger</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"A log message from A::bar"</span><span class="special">;</span>
1266
1267        <span class="identifier">foo</span><span class="special">();</span>
1268    <span class="special">}</span>
1269<span class="special">};</span>
1270
1271<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*[])</span>
1272<span class="special">{</span>
1273    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger</span> <span class="identifier">lg</span><span class="special">;</span>
1274
1275    <span class="comment">// Let's measure our application run time</span>
1276    <span class="identifier">BOOST_LOG_SCOPED_LOGGER_ATTR</span><span class="special">(</span><span class="identifier">lg</span><span class="special">,</span> <span class="string">"RunTime"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span><span class="special">());</span>
1277
1278    <span class="comment">// Mark application start.</span>
1279    <span class="comment">// The "RunTime" attribute should be nearly 0 at this point.</span>
1280    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Application started"</span><span class="special">;</span>
1281
1282    <span class="comment">// Note that no other log records are affected by the "RunTime" attribute.</span>
1283    <span class="identifier">foo</span><span class="special">();</span>
1284
1285    <span class="identifier">A</span> <span class="identifier">a</span><span class="special">;</span>
1286    <span class="identifier">a</span><span class="special">.</span><span class="identifier">bar</span><span class="special">();</span>
1287
1288    <span class="comment">// Mark application ending.</span>
1289    <span class="comment">// The "RunTime" attribute will show the execution time elapsed.</span>
1290    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Application ended"</span><span class="special">;</span>
1291
1292    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
1293<span class="special">}</span>
1294</pre>
1295<p>
1296            It is quite often convenient to mark a group of log records with a constant
1297            value in order to be able to filter the records later. The library provides
1298            two convenience macros just for this purpose:
1299          </p>
1300<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079579096272.html" title="Macro BOOST_LOG_SCOPED_LOGGER_TAG">BOOST_LOG_SCOPED_LOGGER_TAG</a></code><span class="special">(</span><span class="identifier">logger</span><span class="special">,</span> <span class="identifier">tag_name</span><span class="special">,</span> <span class="identifier">tag_value</span><span class="special">);</span>
1301<code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079579091904.html" title="Macro BOOST_LOG_SCOPED_THREAD_TAG">BOOST_LOG_SCOPED_THREAD_TAG</a></code><span class="special">(</span><span class="identifier">tag_name</span><span class="special">,</span> <span class="identifier">tag_value</span><span class="special">);</span>
1302</pre>
1303<p>
1304            The macros are effectively wrappers around <code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079579098656.html" title="Macro BOOST_LOG_SCOPED_LOGGER_ATTR">BOOST_LOG_SCOPED_LOGGER_ATTR</a></code>
1305            and <code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079579093888.html" title="Macro BOOST_LOG_SCOPED_THREAD_ATTR">BOOST_LOG_SCOPED_THREAD_ATTR</a></code>,
1306            respectively. For example, the "Tag" scoped attribute from
1307            the example above can be registered like this:
1308          </p>
1309<pre class="programlisting"><span class="identifier">BOOST_LOG_SCOPED_THREAD_TAG</span><span class="special">(</span><span class="string">"Tag"</span><span class="special">,</span> <span class="string">"Called from A::bar"</span><span class="special">);</span>
1310</pre>
1311<div class="warning"><table border="0" summary="Warning">
1312<tr>
1313<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
1314<th align="left">Warning</th>
1315</tr>
1316<tr><td align="left" valign="top"><p>
1317              When using scoped attributes, make sure that the scoped attribute is
1318              not altered in the attribute set in which it was registered. For example,
1319              one should not clear or reinstall the attribute set of the logger if
1320              there are logger-specific scoped attributes registered in it. Otherwise
1321              the program will likely crash. This issue is especially critical in
1322              multithreaded application, when one thread may not know whether there
1323              are scoped attributes in the logger or there are not. Future releases
1324              may solve this limitation but currently the scoped attribute must remain
1325              intact until unregistered on leaving the scope.
1326            </p></td></tr>
1327</table></div>
1328<p>
1329            Although the described macros are intended to be the primary interface
1330            for the functionality, there is also a C++ interface available. It may
1331            be useful if the user decides to develop his own macros that cannot be
1332            based on the existing ones.
1333          </p>
1334<p>
1335            Any scoped attribute is attached to a generic sentry object of type
1336            <code class="computeroutput"><span class="identifier">scoped_attribute</span></code>. As
1337            long as the sentry exists, the attribute is registered. There are several
1338            functions that create sentries for source or thread-specific attributes:
1339          </p>
1340<pre class="programlisting"><span class="comment">// Source-specific scoped attribute registration</span>
1341<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">LoggerT</span> <span class="special">&gt;</span>
1342<span class="special">[</span><span class="identifier">unspecified</span><span class="special">]</span> <span class="identifier">add_scoped_logger_attribute</span><span class="special">(</span>
1343    <span class="identifier">LoggerT</span><span class="special">&amp;</span> <span class="identifier">l</span><span class="special">,</span>
1344    <span class="identifier">attribute_name</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">name</span><span class="special">,</span>
1345    <span class="identifier">attribute</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">);</span>
1346
1347<span class="comment">// Thread-specific scoped attribute registration</span>
1348<span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">CharT</span> <span class="special">&gt;</span>
1349<span class="special">[</span><span class="identifier">unspecified</span><span class="special">]</span> <span class="identifier">add_scoped_thread_attribute</span><span class="special">(</span>
1350    <span class="identifier">attribute_name</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">name</span><span class="special">,</span>
1351    <span class="identifier">attribute</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">);</span>
1352</pre>
1353<p>
1354            An object of the <code class="computeroutput"><span class="identifier">scoped_attribute</span></code>
1355            type is able to attach results of each of these functions on its construction.
1356            For example, <code class="computeroutput"><span class="identifier">BOOST_LOG_SCOPED_LOGGER_ATTR</span><span class="special">(</span><span class="identifier">lg</span><span class="special">,</span> <span class="string">"RunTime"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span><span class="special">())</span></code> can roughly be expanded to this:
1357          </p>
1358<pre class="programlisting"><span class="identifier">attrs</span><span class="special">::</span><span class="identifier">scoped_attribute</span> <span class="identifier">sentry</span> <span class="special">=</span>
1359    <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">add_scoped_logger_attribute</span><span class="special">(</span><span class="identifier">lg</span><span class="special">,</span> <span class="string">"RunTime"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span><span class="special">());</span>
1360</pre>
1361</div>
1362</div>
1363</div>
1364<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
1365<td align="left"></td>
1366<td align="right"><div class="copyright-footer">Copyright © 2007-2019 Andrey Semashev<p>
1367        Distributed under the Boost Software License, Version 1.0. (See accompanying
1368        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>).
1369      </p>
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