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4<title>Logging sources</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.sources"></a><a class="link" href="sources.html" title="Logging sources">Logging sources</a>
21</h3></div></div></div>
22<div class="toc"><dl class="toc">
23<dt><span class="section"><a href="sources.html#log.detailed.sources.basic_logger">Basic loggers</a></span></dt>
24<dt><span class="section"><a href="sources.html#log.detailed.sources.severity_level_logger">Loggers
25        with severity level support</a></span></dt>
26<dt><span class="section"><a href="sources.html#log.detailed.sources.channel_logger">Loggers with channel
27        support</a></span></dt>
28<dt><span class="section"><a href="sources.html#log.detailed.sources.exception_handling">Loggers with
29        exception handling support</a></span></dt>
30<dt><span class="section"><a href="sources.html#log.detailed.sources.mixed_loggers">Loggers with mixed
31        features</a></span></dt>
32<dt><span class="section"><a href="sources.html#log.detailed.sources.global_storage">Global storage
33        for loggers</a></span></dt>
34</dl></div>
35<div class="section">
36<div class="titlepage"><div><div><h4 class="title">
37<a name="log.detailed.sources.basic_logger"></a><a class="link" href="sources.html#log.detailed.sources.basic_logger" title="Basic loggers">Basic loggers</a>
38</h4></div></div></div>
39<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.basic_logger_hpp" title="Header &lt;boost/log/sources/basic_logger.hpp&gt;">boost/log/sources/basic_logger.hpp</a></code><span class="special">&gt;</span>
40</pre>
41<p>
42          The simplest logging sources provided by the library are loggers <code class="computeroutput"><a class="link" href="../../boost/log/sources/logger.html" title="Class logger">logger</a></code> and its thread-safe
43          version, <code class="computeroutput"><a class="link" href="../../boost/log/sources/logger_mt.html" title="Class logger_mt">logger_mt</a></code>
44          (<code class="computeroutput"><a class="link" href="../../boost/log/sources/wlogger.html" title="Class wlogger">wlogger</a></code> and
45          <code class="computeroutput"><a class="link" href="../../boost/log/sources/wlogger_mt.html" title="Class wlogger_mt">wlogger_mt</a></code>
46          for wide-character logging, accordingly). These loggers only provide the
47          ability to store source-specific attributes within themselves and, of course,
48          to form log records. This type of logger should probably be used when there
49          is no need for advanced features like severity level checks. It may well
50          be used as a tool to collect application statistics and register application
51          events, such as notifications and alarms. In such cases the logger is normally
52          used in conjunction with <a class="link" href="attributes.html#log.detailed.attributes.related_components.scoped_attributes" title="Scoped attributes">scoped
53          attributes</a> to attach the needed data to the notification event.
54          Below is an example of usage:
55        </p>
56<p>
57</p>
58<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">network_connection</span>
59<span class="special">{</span>
60    <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger</span> <span class="identifier">m_logger</span><span class="special">;</span>
61    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_set</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">m_remote_addr</span><span class="special">;</span>
62
63<span class="keyword">public</span><span class="special">:</span>
64    <span class="keyword">void</span> <span class="identifier">on_connected</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">remote_addr</span><span class="special">)</span>
65    <span class="special">{</span>
66        <span class="comment">// Put the remote address into the logger to automatically attach it</span>
67        <span class="comment">// to every log record written through the logger</span>
68        <span class="identifier">m_remote_addr</span> <span class="special">=</span> <span class="identifier">m_logger</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"RemoteAddress"</span><span class="special">,</span>
69            <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="identifier">remote_addr</span><span class="special">)).</span><span class="identifier">first</span><span class="special">;</span>
70
71        <span class="comment">// The straightforward way of logging</span>
72        <span class="keyword">if</span> <span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">record</span> <span class="identifier">rec</span> <span class="special">=</span> <span class="identifier">m_logger</span><span class="special">.</span><span class="identifier">open_record</span><span class="special">())</span>
73        <span class="special">{</span>
74            <span class="identifier">rec</span><span class="special">.</span><span class="identifier">attribute_values</span><span class="special">().</span><span class="identifier">insert</span><span class="special">(</span><span class="string">"Message"</span><span class="special">,</span>
75                <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">make_attribute_value</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="string">"Connection established"</span><span class="special">)));</span>
76            <span class="identifier">m_logger</span><span class="special">.</span><span class="identifier">push_record</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">rec</span><span class="special">));</span>
77        <span class="special">}</span>
78    <span class="special">}</span>
79    <span class="keyword">void</span> <span class="identifier">on_disconnected</span><span class="special">()</span>
80    <span class="special">{</span>
81        <span class="comment">// The simpler way of logging: the above "if" condition is wrapped into a neat macro</span>
82        <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 shut down"</span><span class="special">;</span>
83
84        <span class="comment">// Remove the attribute with the remote address</span>
85        <span class="identifier">m_logger</span><span class="special">.</span><span class="identifier">remove_attribute</span><span class="special">(</span><span class="identifier">m_remote_addr</span><span class="special">);</span>
86    <span class="special">}</span>
87    <span class="keyword">void</span> <span class="identifier">on_data_received</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
88    <span class="special">{</span>
89        <span class="comment">// Put the size as an additional attribute</span>
90        <span class="comment">// so it can be collected and accumulated later if needed.</span>
91        <span class="comment">// The attribute will be attached only to this log record.</span>
92        <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="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"ReceivedSize"</span><span class="special">,</span> <span class="identifier">size</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Some data received"</span><span class="special">;</span>
93    <span class="special">}</span>
94    <span class="keyword">void</span> <span class="identifier">on_data_sent</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
95    <span class="special">{</span>
96        <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="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"SentSize"</span><span class="special">,</span> <span class="identifier">size</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Some data sent"</span><span class="special">;</span>
97    <span class="special">}</span>
98<span class="special">};</span>
99</pre>
100<p>
101        </p>
102<p>
103          The class <code class="computeroutput"><span class="identifier">network_connection</span></code>
104          in the code snippet above represents an approach to implementing simple
105          logging and statistical information gathering in a network-related application.
106          Each of the presented methods of the class effectively marks a corresponding
107          event that can be tracked and collected on the sinks level. Furthermore,
108          other methods of the class, that are not shown here for simplicity, are
109          able to write logs too. Note that every log record ever made in the connected
110          state of the <code class="computeroutput"><span class="identifier">network_connection</span></code>
111          object will be implicitly marked up with the address of the remote site.
112        </p>
113</div>
114<div class="section">
115<div class="titlepage"><div><div><h4 class="title">
116<a name="log.detailed.sources.severity_level_logger"></a><a class="link" href="sources.html#log.detailed.sources.severity_level_logger" title="Loggers with severity level support">Loggers
117        with severity level support</a>
118</h4></div></div></div>
119<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.severity_feature_hpp" title="Header &lt;boost/log/sources/severity_feature.hpp&gt;">boost/log/sources/severity_feature.hpp</a></code><span class="special">&gt;</span>
120<span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.severity_logger_hpp" title="Header &lt;boost/log/sources/severity_logger.hpp&gt;">boost/log/sources/severity_logger.hpp</a></code><span class="special">&gt;</span>
121</pre>
122<p>
123          The ability to distinguish some log records from others based on some kind
124          of level of severity or importance is one of the most frequently requested
125          features. The class templates <code class="computeroutput"><a class="link" href="../../boost/log/sources/severity_logger.html" title="Class template severity_logger">severity_logger</a></code>
126          and <code class="computeroutput"><a class="link" href="../../boost/log/sources/severity_logger_mt.html" title="Class template severity_logger_mt">severity_logger_mt</a></code>
127          (along with their <code class="computeroutput"><a class="link" href="../../boost/log/sources/wseverity_logger.html" title="Class template wseverity_logger">wseverity_logger</a></code>
128          and <code class="computeroutput"><a class="link" href="../../boost/log/sources/wseverity_logger_mt.html" title="Class template wseverity_logger_mt">wseverity_logger_mt</a></code>
129          wide-character counterparts) provide this functionality.
130        </p>
131<p>
132          The loggers automatically register a special source-specific attribute
133          "Severity", which can be set for every record in a compact and
134          efficient manner, with a named argument <code class="computeroutput"><span class="identifier">severity</span></code>
135          that can be passed to the constructor and/or the <code class="computeroutput"><span class="identifier">open_record</span></code>
136          method. If passed to the logger constructor, the <code class="computeroutput"><span class="identifier">severity</span></code>
137          argument sets the default value of the severity level that will be used
138          if none is provided in the <code class="computeroutput"><span class="identifier">open_record</span></code>
139          arguments. The <code class="computeroutput"><span class="identifier">severity</span></code>
140          argument passed to the <code class="computeroutput"><span class="identifier">open_record</span></code>
141          method sets the level of the particular log record being made. The type
142          of the severity level can be provided as a template argument for the logger
143          class template. The default type is <code class="computeroutput"><span class="keyword">int</span></code>.
144        </p>
145<p>
146          The actual values of this attribute and their meaning are entirely user-defined.
147          However, it is recommended to use the level of value equivalent to zero
148          as a base point for other values. This is because the default-constructed
149          logger object sets its default severity level to zero. It is also recommended
150          to define the same levels of severity for the entire application in order
151          to avoid confusion in the written logs later. The following code snippet
152          shows the usage of <code class="computeroutput"><span class="identifier">severity_logger</span></code>.
153        </p>
154<p>
155</p>
156<pre class="programlisting"><span class="comment">// We define our own severity levels</span>
157<span class="keyword">enum</span> <span class="identifier">severity_level</span>
158<span class="special">{</span>
159    <span class="identifier">normal</span><span class="special">,</span>
160    <span class="identifier">notification</span><span class="special">,</span>
161    <span class="identifier">warning</span><span class="special">,</span>
162    <span class="identifier">error</span><span class="special">,</span>
163    <span class="identifier">critical</span>
164<span class="special">};</span>
165
166<span class="keyword">void</span> <span class="identifier">logging_function</span><span class="special">()</span>
167<span class="special">{</span>
168    <span class="comment">// The logger implicitly adds a source-specific attribute 'Severity'</span>
169    <span class="comment">// of type 'severity_level' on construction</span>
170    <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger</span><span class="special">&lt;</span> <span class="identifier">severity_level</span> <span class="special">&gt;</span> <span class="identifier">slg</span><span class="special">;</span>
171
172    <span class="identifier">BOOST_LOG_SEV</span><span class="special">(</span><span class="identifier">slg</span><span class="special">,</span> <span class="identifier">normal</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"A regular message"</span><span class="special">;</span>
173    <span class="identifier">BOOST_LOG_SEV</span><span class="special">(</span><span class="identifier">slg</span><span class="special">,</span> <span class="identifier">warning</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Something bad is going on but I can handle it"</span><span class="special">;</span>
174    <span class="identifier">BOOST_LOG_SEV</span><span class="special">(</span><span class="identifier">slg</span><span class="special">,</span> <span class="identifier">critical</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Everything crumbles, shoot me now!"</span><span class="special">;</span>
175<span class="special">}</span>
176</pre>
177<p>
178        </p>
179<p>
180</p>
181<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">default_severity</span><span class="special">()</span>
182<span class="special">{</span>
183    <span class="comment">// The default severity can be specified in constructor.</span>
184    <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger</span><span class="special">&lt;</span> <span class="identifier">severity_level</span> <span class="special">&gt;</span> <span class="identifier">error_lg</span><span class="special">(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">severity</span> <span class="special">=</span> <span class="identifier">error</span><span class="special">);</span>
185
186    <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">error_lg</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"An error level log record (by default)"</span><span class="special">;</span>
187
188    <span class="comment">// The explicitly specified level overrides the default</span>
189    <span class="identifier">BOOST_LOG_SEV</span><span class="special">(</span><span class="identifier">error_lg</span><span class="special">,</span> <span class="identifier">warning</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"A warning level log record (overrode the default)"</span><span class="special">;</span>
190<span class="special">}</span>
191</pre>
192<p>
193        </p>
194<p>
195          Or, if you prefer logging without macros:
196        </p>
197<p>
198</p>
199<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">manual_logging</span><span class="special">()</span>
200<span class="special">{</span>
201    <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger</span><span class="special">&lt;</span> <span class="identifier">severity_level</span> <span class="special">&gt;</span> <span class="identifier">slg</span><span class="special">;</span>
202
203    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">record</span> <span class="identifier">rec</span> <span class="special">=</span> <span class="identifier">slg</span><span class="special">.</span><span class="identifier">open_record</span><span class="special">(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">severity</span> <span class="special">=</span> <span class="identifier">normal</span><span class="special">);</span>
204    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">rec</span><span class="special">)</span>
205    <span class="special">{</span>
206        <span class="identifier">logging</span><span class="special">::</span><span class="identifier">record_ostream</span> <span class="identifier">strm</span><span class="special">(</span><span class="identifier">rec</span><span class="special">);</span>
207        <span class="identifier">strm</span> <span class="special">&lt;&lt;</span> <span class="string">"A regular message"</span><span class="special">;</span>
208        <span class="identifier">strm</span><span class="special">.</span><span class="identifier">flush</span><span class="special">();</span>
209        <span class="identifier">slg</span><span class="special">.</span><span class="identifier">push_record</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">rec</span><span class="special">));</span>
210    <span class="special">}</span>
211<span class="special">}</span>
212</pre>
213<p>
214        </p>
215<p>
216          And, of course, severity loggers also provide the same functionality the
217          <a class="link" href="sources.html#log.detailed.sources.basic_logger" title="Basic loggers">basic loggers</a>
218          do.
219        </p>
220</div>
221<div class="section">
222<div class="titlepage"><div><div><h4 class="title">
223<a name="log.detailed.sources.channel_logger"></a><a class="link" href="sources.html#log.detailed.sources.channel_logger" title="Loggers with channel support">Loggers with channel
224        support</a>
225</h4></div></div></div>
226<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.channel_feature_hpp" title="Header &lt;boost/log/sources/channel_feature.hpp&gt;">boost/log/sources/channel_feature.hpp</a></code><span class="special">&gt;</span>
227<span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.channel_logger_hpp" title="Header &lt;boost/log/sources/channel_logger.hpp&gt;">boost/log/sources/channel_logger.hpp</a></code><span class="special">&gt;</span>
228</pre>
229<p>
230          Sometimes it is important to associate log records with some application
231          component, such as the module or class name, the relation of the logged
232          information to some specific domain of application functionality (e.g.
233          network or file system related messages) or some arbitrary tag that can
234          be used later to route these records to a specific sink. This feature is
235          fulfilled with loggers <code class="computeroutput"><a class="link" href="../../boost/log/sources/channel_logger.html" title="Class template channel_logger">channel_logger</a></code>,
236          <code class="computeroutput"><a class="link" href="../../boost/log/sources/channel_logger_mt.html" title="Class template channel_logger_mt">channel_logger_mt</a></code>
237          and their wide-char counterparts <code class="computeroutput"><a class="link" href="../../boost/log/sources/wchannel_logger.html" title="Class template wchannel_logger">wchannel_logger</a></code>,
238          <code class="computeroutput"><a class="link" href="../../boost/log/sources/wchannel_logger_mt.html" title="Class template wchannel_logger_mt">wchannel_logger_mt</a></code>.
239          These loggers automatically register an attribute named "Channel".
240          The default channel name can be set in the logger constructor with a named
241          argument <code class="computeroutput"><span class="identifier">channel</span></code>. The type
242          of the channel attribute value can be specified as a template argument
243          for the logger, with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
244          (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">wstring</span></code> in case of wide character loggers)
245          as a default. Aside from that, the usage is similar to the <a class="link" href="sources.html#log.detailed.sources.basic_logger" title="Basic loggers">basic
246          loggers</a>:
247        </p>
248<p>
249</p>
250<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">network_connection</span>
251<span class="special">{</span>
252    <span class="identifier">src</span><span class="special">::</span><span class="identifier">channel_logger</span><span class="special">&lt;</span> <span class="special">&gt;</span> <span class="identifier">m_net</span><span class="special">,</span> <span class="identifier">m_stat</span><span class="special">;</span>
253    <span class="identifier">logging</span><span class="special">::</span><span class="identifier">attribute_set</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">m_net_remote_addr</span><span class="special">,</span> <span class="identifier">m_stat_remote_addr</span><span class="special">;</span>
254
255<span class="keyword">public</span><span class="special">:</span>
256    <span class="identifier">network_connection</span><span class="special">()</span> <span class="special">:</span>
257        <span class="comment">// We can dump network-related messages through this logger</span>
258        <span class="comment">// and be able to filter them later</span>
259        <span class="identifier">m_net</span><span class="special">(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">channel</span> <span class="special">=</span> <span class="string">"net"</span><span class="special">),</span>
260        <span class="comment">// We also can separate statistic records in a different channel</span>
261        <span class="comment">// in order to route them to a different sink</span>
262        <span class="identifier">m_stat</span><span class="special">(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">channel</span> <span class="special">=</span> <span class="string">"stat"</span><span class="special">)</span>
263    <span class="special">{</span>
264    <span class="special">}</span>
265
266    <span class="keyword">void</span> <span class="identifier">on_connected</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">remote_addr</span><span class="special">)</span>
267    <span class="special">{</span>
268        <span class="comment">// Add the remote address to both channels</span>
269        <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="identifier">addr</span><span class="special">(</span><span class="identifier">remote_addr</span><span class="special">);</span>
270        <span class="identifier">m_net_remote_addr</span> <span class="special">=</span> <span class="identifier">m_net</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"RemoteAddress"</span><span class="special">,</span> <span class="identifier">addr</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
271        <span class="identifier">m_stat_remote_addr</span> <span class="special">=</span> <span class="identifier">m_stat</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"RemoteAddress"</span><span class="special">,</span> <span class="identifier">addr</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
272
273        <span class="comment">// Put message to the "net" channel</span>
274        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_net</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Connection established"</span><span class="special">;</span>
275    <span class="special">}</span>
276
277    <span class="keyword">void</span> <span class="identifier">on_disconnected</span><span class="special">()</span>
278    <span class="special">{</span>
279        <span class="comment">// Put message to the "net" channel</span>
280        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_net</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Connection shut down"</span><span class="special">;</span>
281
282        <span class="comment">// Remove the attribute with the remote address</span>
283        <span class="identifier">m_net</span><span class="special">.</span><span class="identifier">remove_attribute</span><span class="special">(</span><span class="identifier">m_net_remote_addr</span><span class="special">);</span>
284        <span class="identifier">m_stat</span><span class="special">.</span><span class="identifier">remove_attribute</span><span class="special">(</span><span class="identifier">m_stat_remote_addr</span><span class="special">);</span>
285    <span class="special">}</span>
286
287    <span class="keyword">void</span> <span class="identifier">on_data_received</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
288    <span class="special">{</span>
289        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_stat</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"ReceivedSize"</span><span class="special">,</span> <span class="identifier">size</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Some data received"</span><span class="special">;</span>
290    <span class="special">}</span>
291
292    <span class="keyword">void</span> <span class="identifier">on_data_sent</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
293    <span class="special">{</span>
294        <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_stat</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"SentSize"</span><span class="special">,</span> <span class="identifier">size</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Some data sent"</span><span class="special">;</span>
295    <span class="special">}</span>
296<span class="special">};</span>
297</pre>
298<p>
299        </p>
300<p>
301          It is also possible to set the channel name of individual log records.
302          This can be useful when a <a class="link" href="sources.html#log.detailed.sources.global_storage" title="Global storage for loggers">global
303          logger</a> is used instead of an object-specific one. The channel name
304          can be set by calling the <code class="computeroutput"><span class="identifier">channel</span></code>
305          modifier on the logger or by using a special macro for logging. For example:
306        </p>
307<p>
308</p>
309<pre class="programlisting"><span class="comment">// Define a global logger</span>
310<span class="identifier">BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS</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">channel_logger_mt</span><span class="special">&lt;</span> <span class="special">&gt;,</span> <span class="special">(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">channel</span> <span class="special">=</span> <span class="string">"general"</span><span class="special">))</span>
311
312<span class="keyword">class</span> <span class="identifier">network_connection</span>
313<span class="special">{</span>
314    <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">m_remote_addr</span><span class="special">;</span>
315
316<span class="keyword">public</span><span class="special">:</span>
317    <span class="keyword">void</span> <span class="identifier">on_connected</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">remote_addr</span><span class="special">)</span>
318    <span class="special">{</span>
319        <span class="identifier">m_remote_addr</span> <span class="special">=</span> <span class="identifier">remote_addr</span><span class="special">;</span>
320
321        <span class="comment">// Put message to the "net" channel</span>
322        <span class="identifier">BOOST_LOG_CHANNEL</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">::</span><span class="identifier">get</span><span class="special">(),</span> <span class="string">"net"</span><span class="special">)</span>
323            <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"RemoteAddress"</span><span class="special">,</span> <span class="identifier">m_remote_addr</span><span class="special">)</span>
324            <span class="special">&lt;&lt;</span> <span class="string">"Connection established"</span><span class="special">;</span>
325    <span class="special">}</span>
326
327    <span class="keyword">void</span> <span class="identifier">on_disconnected</span><span class="special">()</span>
328    <span class="special">{</span>
329        <span class="comment">// Put message to the "net" channel</span>
330        <span class="identifier">BOOST_LOG_CHANNEL</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">::</span><span class="identifier">get</span><span class="special">(),</span> <span class="string">"net"</span><span class="special">)</span>
331            <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"RemoteAddress"</span><span class="special">,</span> <span class="identifier">m_remote_addr</span><span class="special">)</span>
332            <span class="special">&lt;&lt;</span> <span class="string">"Connection shut down"</span><span class="special">;</span>
333
334        <span class="identifier">m_remote_addr</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
335    <span class="special">}</span>
336
337    <span class="keyword">void</span> <span class="identifier">on_data_received</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
338    <span class="special">{</span>
339        <span class="identifier">BOOST_LOG_CHANNEL</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">::</span><span class="identifier">get</span><span class="special">(),</span> <span class="string">"stat"</span><span class="special">)</span>
340            <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"RemoteAddress"</span><span class="special">,</span> <span class="identifier">m_remote_addr</span><span class="special">)</span>
341            <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"ReceivedSize"</span><span class="special">,</span> <span class="identifier">size</span><span class="special">)</span>
342            <span class="special">&lt;&lt;</span> <span class="string">"Some data received"</span><span class="special">;</span>
343    <span class="special">}</span>
344
345    <span class="keyword">void</span> <span class="identifier">on_data_sent</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
346    <span class="special">{</span>
347        <span class="identifier">BOOST_LOG_CHANNEL</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">::</span><span class="identifier">get</span><span class="special">(),</span> <span class="string">"stat"</span><span class="special">)</span>
348            <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"RemoteAddress"</span><span class="special">,</span> <span class="identifier">m_remote_addr</span><span class="special">)</span>
349            <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"SentSize"</span><span class="special">,</span> <span class="identifier">size</span><span class="special">)</span>
350            <span class="special">&lt;&lt;</span> <span class="string">"Some data sent"</span><span class="special">;</span>
351    <span class="special">}</span>
352<span class="special">};</span>
353</pre>
354<p>
355        </p>
356<p>
357          Note that changing the channel name is persistent, so unless the channel
358          name is reset, the subsequent records will also belong to the new channel.
359        </p>
360<div class="tip"><table border="0" summary="Tip">
361<tr>
362<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
363<th align="left">Tip</th>
364</tr>
365<tr><td align="left" valign="top"><p>
366            For performance reasons it is advised to avoid dynamically setting the
367            channel name individually for every log record, when possible. Changing
368            the channel name involves dynamic memory allocation. Using distinct loggers
369            for different channels allows to avoid this overhead.
370          </p></td></tr>
371</table></div>
372</div>
373<div class="section">
374<div class="titlepage"><div><div><h4 class="title">
375<a name="log.detailed.sources.exception_handling"></a><a class="link" href="sources.html#log.detailed.sources.exception_handling" title="Loggers with exception handling support">Loggers with
376        exception handling support</a>
377</h4></div></div></div>
378<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.exception_handler_feature_hpp" title="Header &lt;boost/log/sources/exception_handler_feature.hpp&gt;">boost/log/sources/exception_handler_feature.hpp</a></code><span class="special">&gt;</span>
379</pre>
380<p>
381          The library provides a logger feature that enables the user to handle and/or
382          suppress exceptions at the logger level. The <code class="computeroutput"><a class="link" href="../../boost/log/sources/exception_handler.html" title="Struct exception_handler">exception_handler</a></code>
383          feature adds a <code class="computeroutput"><span class="identifier">set_exception_handler</span></code>
384          method to the logger that allows setting a function object to be called
385          if an exception is thrown from the logging core during the filtering or
386          processing of log records. One can use the <a class="link" href="utilities.html#log.detailed.utilities.exception_handlers" title="Exception handlers">library-provided
387          adapters</a> to simplify implementing exception handlers. Usage example
388          is as follows:
389        </p>
390<p>
391</p>
392<pre class="programlisting"><span class="keyword">enum</span> <span class="identifier">severity_level</span>
393<span class="special">{</span>
394    <span class="identifier">normal</span><span class="special">,</span>
395    <span class="identifier">warning</span><span class="special">,</span>
396    <span class="identifier">error</span>
397<span class="special">};</span>
398
399<span class="comment">// A logger class that allows to intercept exceptions and supports severity level</span>
400<span class="keyword">class</span> <span class="identifier">my_logger_mt</span> <span class="special">:</span>
401    <span class="keyword">public</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">basic_composite_logger</span><span class="special">&lt;</span>
402        <span class="keyword">char</span><span class="special">,</span>
403        <span class="identifier">my_logger_mt</span><span class="special">,</span>
404        <span class="identifier">src</span><span class="special">::</span><span class="identifier">multi_thread_model</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>
405        <span class="identifier">src</span><span class="special">::</span><span class="identifier">features</span><span class="special">&lt;</span>
406            <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity</span><span class="special">&lt;</span> <span class="identifier">severity_level</span> <span class="special">&gt;,</span>
407            <span class="identifier">src</span><span class="special">::</span><span class="identifier">exception_handler</span>
408        <span class="special">&gt;</span>
409    <span class="special">&gt;</span>
410<span class="special">{</span>
411    <span class="identifier">BOOST_LOG_FORWARD_LOGGER_MEMBERS</span><span class="special">(</span><span class="identifier">my_logger_mt</span><span class="special">)</span>
412<span class="special">};</span>
413
414<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">my_logger_mt</span><span class="special">)</span>
415<span class="special">{</span>
416    <span class="identifier">my_logger_mt</span> <span class="identifier">lg</span><span class="special">;</span>
417
418    <span class="comment">// Set up exception handler: all exceptions that occur while</span>
419    <span class="comment">// logging through this logger, will be suppressed</span>
420    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">set_exception_handler</span><span class="special">(</span><span class="identifier">logging</span><span class="special">::</span><span class="identifier">make_exception_suppressor</span><span class="special">());</span>
421
422    <span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
423<span class="special">}</span>
424
425<span class="keyword">void</span> <span class="identifier">logging_function</span><span class="special">()</span>
426<span class="special">{</span>
427    <span class="comment">// This will not throw</span>
428    <span class="identifier">BOOST_LOG_SEV</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">::</span><span class="identifier">get</span><span class="special">(),</span> <span class="identifier">normal</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Hello, world"</span><span class="special">;</span>
429<span class="special">}</span>
430</pre>
431<p>
432        </p>
433<div class="tip"><table border="0" summary="Tip">
434<tr>
435<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
436<th align="left">Tip</th>
437</tr>
438<tr><td align="left" valign="top"><p>
439            <a class="link" href="../detailed.html#log.detailed.core.core.exception_handling" title="Exception handling">Logging core</a>
440            and <a class="link" href="sink_frontends.html#log.detailed.sink_frontends.basic_services.exception_handling" title="Exception handling">sink
441            frontends</a> also support installing exception handlers.
442          </p></td></tr>
443</table></div>
444</div>
445<div class="section">
446<div class="titlepage"><div><div><h4 class="title">
447<a name="log.detailed.sources.mixed_loggers"></a><a class="link" href="sources.html#log.detailed.sources.mixed_loggers" title="Loggers with mixed features">Loggers with mixed
448        features</a>
449</h4></div></div></div>
450<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.severity_channel_logger_hpp" title="Header &lt;boost/log/sources/severity_channel_logger.hpp&gt;">boost/log/sources/severity_channel_logger.hpp</a></code><span class="special">&gt;</span>
451</pre>
452<p>
453          If you wonder whether you can use a mixed set of several logger features
454          in one logger, then yes, you certainly can. The library provides <code class="computeroutput"><a class="link" href="../../boost/log/sources/severity_channel_logger.html" title="Class template severity_channel_logger">severity_channel_logger</a></code>
455          and <code class="computeroutput"><a class="link" href="../../boost/log/sources/severity_channel_logger_mt.html" title="Class template severity_channel_logger_mt">severity_channel_logger_mt</a></code>
456          (with their wide-char analogues <code class="computeroutput"><a class="link" href="../../boost/log/sources/wseverity_channel_logger.html" title="Class template wseverity_channel_logger">wseverity_channel_logger</a></code>
457          and <code class="computeroutput"><a class="link" href="../../boost/log/sources/wseverit_idm46079577509680.html" title="Class template wseverity_channel_logger_mt">wseverity_channel_logger_mt</a></code>)
458          which combine features of the described loggers with <a class="link" href="sources.html#log.detailed.sources.severity_level_logger" title="Loggers with severity level support">severity
459          level</a> and <a class="link" href="sources.html#log.detailed.sources.channel_logger" title="Loggers with channel support">channels</a>
460          support. The composite loggers are templates, too, which allows you to
461          specify severity level and channel types. You can also design your own
462          logger features and combine them with the ones provided by the library,
463          as described in the <a class="link" href="../extension/sources.html" title="Writing your own sources">Extending the
464          library</a> section.
465        </p>
466<p>
467          The usage of the loggers with several features does not conceptually differ
468          from the usage of the single-featured loggers. For instance, here is how
469          a <code class="computeroutput"><a class="link" href="../../boost/log/sources/severity_channel_logger_mt.html" title="Class template severity_channel_logger_mt">severity_channel_logger_mt</a></code>
470          could be used:
471        </p>
472<p>
473</p>
474<pre class="programlisting"><span class="keyword">enum</span> <span class="identifier">severity_level</span>
475<span class="special">{</span>
476    <span class="identifier">normal</span><span class="special">,</span>
477    <span class="identifier">notification</span><span class="special">,</span>
478    <span class="identifier">warning</span><span class="special">,</span>
479    <span class="identifier">error</span><span class="special">,</span>
480    <span class="identifier">critical</span>
481<span class="special">};</span>
482
483<span class="keyword">typedef</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_channel_logger_mt</span><span class="special">&lt;</span>
484    <span class="identifier">severity_level</span><span class="special">,</span>     <span class="comment">// the type of the severity level</span>
485    <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span>         <span class="comment">// the type of the channel name</span>
486<span class="special">&gt;</span> <span class="identifier">my_logger_mt</span><span class="special">;</span>
487
488
489<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">my_logger_mt</span><span class="special">)</span>
490<span class="special">{</span>
491    <span class="comment">// Specify the channel name on construction, similarly as with the channel_logger</span>
492    <span class="keyword">return</span> <span class="identifier">my_logger_mt</span><span class="special">(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">channel</span> <span class="special">=</span> <span class="string">"my_logger"</span><span class="special">);</span>
493<span class="special">}</span>
494
495<span class="keyword">void</span> <span class="identifier">logging_function</span><span class="special">()</span>
496<span class="special">{</span>
497    <span class="comment">// Do logging with the severity level. The record will have both</span>
498    <span class="comment">// the severity level and the channel name attached.</span>
499    <span class="identifier">BOOST_LOG_SEV</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">::</span><span class="identifier">get</span><span class="special">(),</span> <span class="identifier">normal</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"Hello, world!"</span><span class="special">;</span>
500<span class="special">}</span>
501</pre>
502<p>
503        </p>
504</div>
505<div class="section">
506<div class="titlepage"><div><div><h4 class="title">
507<a name="log.detailed.sources.global_storage"></a><a class="link" href="sources.html#log.detailed.sources.global_storage" title="Global storage for loggers">Global storage
508        for loggers</a>
509</h4></div></div></div>
510<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.global_logger_storage_hpp" title="Header &lt;boost/log/sources/global_logger_storage.hpp&gt;">boost/log/sources/global_logger_storage.hpp</a></code><span class="special">&gt;</span>
511</pre>
512<p>
513          Sometimes it is inconvenient to have a logger object to be able to write
514          logs. This issue is often present in functional-style code with no obvious
515          places where a logger could be stored. Another domain where the problem
516          persists is generic libraries that would benefit from logging. In such
517          cases it would be more convenient to have one or several global loggers
518          in order to easily access them in every place when needed. In this regard
519          <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code> is a good example of such a logger.
520        </p>
521<p>
522          The library provides a way to declare global loggers that can be accessed
523          pretty much like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>. In fact, this feature can be used
524          with any logger, including user-defined ones. Having declared a global
525          logger, one can be sure to have a thread-safe access to this logger instance
526          from any place of the application code. The library also guarantees that
527          a global logger instance will be unique even across module boundaries.
528          This allows employing logging even in header-only components that may get
529          compiled into different modules.
530        </p>
531<p>
532          One may wonder why there is a need for something special in order to create
533          global loggers. Why not just declare a logger variable at namespace scope
534          and use it wherever you need? While technically this is possible, declaring
535          and using global logger variables is complicated for the following reasons:
536        </p>
537<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
538<li class="listitem">
539              Order of initialization of namespace scope variables is not specified
540              by the C++ Standard. This means that generally you cannot use the logger
541              during this stage of initialization (i.e. before <code class="computeroutput"><span class="identifier">main</span></code>).
542            </li>
543<li class="listitem">
544              Initialization of namespace scope variables is not thread-safe. You
545              may end up initializing the same logger twice or using an uninitialized
546              logger.
547            </li>
548<li class="listitem">
549              Using namespace scope variables in a header-only library is quite complicated.
550              One either has to declare a variable with external linkage and define
551              it only in a single translation unit (that is, in a separate .cpp file,
552              which defeats the "header-only" thesis), or define a variable
553              with internal linkage, or as a special case in an anonymous namespace
554              (this will most likely break ODR and give unexpected results when the
555              header is used in different translation units). There are other compiler-specific
556              and standard tricks to tackle the problem, but they are not quite trivial
557              and portable.
558            </li>
559<li class="listitem">
560              On most platforms namespace scope variables are local to the module
561              where they were compiled in. That is, if variable <code class="computeroutput"><span class="identifier">a</span></code>
562              has external linkage and was compiled into modules X and Y, each of
563              these modules has its own copy of variable <code class="computeroutput"><span class="identifier">a</span></code>.
564              To make things worse, on other platforms this variable can be shared
565              between the modules.
566            </li>
567</ul></div>
568<p>
569          Global logger storage is intended to eliminate all these problems.
570        </p>
571<p>
572          The easiest way to declare a global logger is to use the following macro:
573        </p>
574<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577699824.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT">BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT</a></code><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">severity_logger_mt</span><span class="special">&lt;</span> <span class="special">&gt;)</span>
575</pre>
576<p>
577          The <code class="computeroutput"><span class="identifier">my_logger</span></code> argument
578          gives the logger a name that may be used to acquire the logger instance.
579          This name acts as a tag of the declared logger. The second parameter denotes
580          the logger type. In multithreaded applications, when the logger can be
581          accessed from different threads, users will normally want to use the thread-safe
582          versions of loggers.
583        </p>
584<p>
585          If passing arguments to the logger constructor is needed, there is another
586          macro:
587        </p>
588<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577697840.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS">BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS</a></code><span class="special">(</span>
589    <span class="identifier">my_logger</span><span class="special">,</span>
590    <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_channel_logger</span><span class="special">&lt;</span> <span class="special">&gt;,</span>
591    <span class="special">(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">severity</span> <span class="special">=</span> <span class="identifier">error</span><span class="special">)(</span><span class="identifier">keywords</span><span class="special">::</span><span class="identifier">channel</span> <span class="special">=</span> <span class="string">"my_channel"</span><span class="special">))</span>
592</pre>
593<p>
594          The last macro argument is a <a href="http://www.boost.org/doc/libs/release/libs/preprocessor/doc/index.html" target="_top">Boost.Preprocessor</a>
595          sequence of arguments passed to the logger constructor. Be careful, however,
596          when using non-constant expressions and references to objects as constructor
597          arguments, since the arguments are evaluated only once and it is often
598          difficult to tell the exact moment when it is done. The logger is constructed
599          on the first request from whichever part of the application that has the
600          knowledge of the logger declaration. It is up to user to make sure that
601          all arguments have valid states at that point.
602        </p>
603<p>
604          The third macro of this section provides maximum initialization flexibility,
605          allowing the user to actually define the logic of creating the logger.
606        </p>
607<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577701808.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT">BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT</a></code><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">severity_logger_mt</span><span class="special">)</span>
608<span class="special">{</span>
609    <span class="comment">// Do something that needs to be done on logger initialization,</span>
610    <span class="comment">// e.g. add a stop watch attribute.</span>
611    <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger_mt</span><span class="special">&lt;</span> <span class="special">&gt;</span> <span class="identifier">lg</span><span class="special">;</span>
612    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"StopWatch"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span><span class="special">());</span>
613    <span class="comment">// The initializing routine must return the logger instance</span>
614    <span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
615<span class="special">}</span>
616</pre>
617<p>
618          Like the <code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577697840.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS">BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS</a></code>
619          macro, the initialization code is called only once, on the first request
620          of the logger.
621        </p>
622<div class="important"><table border="0" summary="Important">
623<tr>
624<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td>
625<th align="left">Important</th>
626</tr>
627<tr><td align="left" valign="top"><p>
628            Beware of One Definition Rule (ODR) issues. Regardless of the way of
629            logger declaration you choose, you should ensure that <span class="underline">the
630            logger is declared in exactly the same way at all occurrences</span>
631            and <span class="underline">all symbol names involved in the declaration
632            resolve to the same entities</span>. The latter includes the names
633            used within the initialization routine of the <code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577701808.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT">BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT</a></code>
634            macro, such as references to external variables, functions and types.
635            The library tries to protect itself from ODR violations to a certain
636            degree, but in general the behavior is undefined if the rule is violated.
637          </p></td></tr>
638</table></div>
639<p>
640          In order to alleviate ODR problems, it is possible to separate the logger
641          declaration and its initialization routine. The library provides the following
642          macros to achieve this:
643        </p>
644<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
645<li class="listitem">
646              <code class="computeroutput"><a class="link" href="../../BOOST_LOG_GLOBAL_LOGGER.html" title="Macro BOOST_LOG_GLOBAL_LOGGER">BOOST_LOG_GLOBAL_LOGGER</a></code>
647              provides the logger declaration. It can be used in a header, similarly
648              to the <code class="computeroutput"><span class="identifier">BOOST_LOG_INLINE_GLOBAL_LOGGER</span><span class="special">*</span></code> macros described above.
649            </li>
650<li class="listitem">
651              <code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577708224.html" title="Macro BOOST_LOG_GLOBAL_LOGGER_INIT">BOOST_LOG_GLOBAL_LOGGER_INIT</a></code>,
652              <code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577706240.html" title="Macro BOOST_LOG_GLOBAL_LOGGER_DEFAULT">BOOST_LOG_GLOBAL_LOGGER_DEFAULT</a></code>
653              and <code class="computeroutput"><a class="link" href="../../BOOST_LO_idm46079577704240.html" title="Macro BOOST_LOG_GLOBAL_LOGGER_CTOR_ARGS">BOOST_LOG_GLOBAL_LOGGER_CTOR_ARGS</a></code>
654              define the logger initialization routine. Their semantics and usage
655              is similar to the corresponding <code class="computeroutput"><span class="identifier">BOOST_LOG_INLINE_GLOBAL_LOGGER</span><span class="special">*</span></code> macros, for one exception: these macros
656              should be used in a single .cpp file.
657            </li>
658</ul></div>
659<p>
660          For example:
661        </p>
662<pre class="programlisting"><span class="comment">// my_logger.h</span>
663<span class="comment">// ===========</span>
664
665<span class="identifier">BOOST_LOG_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">severity_logger_mt</span><span class="special">)</span>
666
667
668<span class="comment">// my_logger.cpp</span>
669<span class="comment">// =============</span>
670
671<span class="preprocessor">#include</span> <span class="string">"my_logger.h"</span>
672
673<span class="identifier">BOOST_LOG_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">severity_logger_mt</span><span class="special">)</span>
674<span class="special">{</span>
675    <span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger_mt</span><span class="special">&lt;</span> <span class="special">&gt;</span> <span class="identifier">lg</span><span class="special">;</span>
676    <span class="identifier">lg</span><span class="special">.</span><span class="identifier">add_attribute</span><span class="special">(</span><span class="string">"StopWatch"</span><span class="special">,</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span><span class="special">());</span>
677    <span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
678<span class="special">}</span>
679</pre>
680<p>
681          Regardless of the macro you used to declare the logger, you can acquire
682          the logger instance with the static <code class="computeroutput"><span class="identifier">get</span></code>
683          function of the logger tag:
684        </p>
685<pre class="programlisting"><span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger_mt</span><span class="special">&lt;</span> <span class="special">&gt;&amp;</span> <span class="identifier">lg</span> <span class="special">=</span> <span class="identifier">my_logger</span><span class="special">::</span><span class="identifier">get</span><span class="special">();</span>
686</pre>
687<p>
688          Further usage of the logger is the same as if it was a regular logger object
689          of the corresponding type.
690        </p>
691<div class="warning"><table border="0" summary="Warning">
692<tr>
693<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
694<th align="left">Warning</th>
695</tr>
696<tr><td align="left" valign="top"><p>
697            It should be noted that it is not advised to use global loggers during
698            the deinitialization stage of the application. Like any other global
699            object in your application, the global logger may get destroyed before
700            you try to use it. In such cases it's better to have a dedicated logger
701            object that is guaranteed to be available as long as needed.
702          </p></td></tr>
703</table></div>
704</div>
705</div>
706<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
707<td align="left"></td>
708<td align="right"><div class="copyright-footer">Copyright © 2007-2019 Andrey Semashev<p>
709        Distributed under the Boost Software License, Version 1.0. (See accompanying
710        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>).
711      </p>
712</div></td>
713</tr></table>
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