1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Logging sources</title>
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
<link rel="home" href="../../index.html" title="Chapter 1. Boost.Log v2">
<link rel="up" href="../detailed.html" title="Detailed features description">
<link rel="prev" href="../detailed.html" title="Detailed features description">
<link rel="next" href="sink_frontends.html" title="Sink frontends">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td></tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../detailed.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../detailed.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="sink_frontends.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="log.detailed.sources"></a><a class="link" href="sources.html" title="Logging sources">Logging sources</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="sources.html#log.detailed.sources.basic_logger">Basic loggers</a></span></dt>
<dt><span class="section"><a href="sources.html#log.detailed.sources.severity_level_logger">Loggers
with severity level support</a></span></dt>
<dt><span class="section"><a href="sources.html#log.detailed.sources.channel_logger">Loggers with channel
support</a></span></dt>
<dt><span class="section"><a href="sources.html#log.detailed.sources.exception_handling">Loggers with
exception handling support</a></span></dt>
<dt><span class="section"><a href="sources.html#log.detailed.sources.mixed_loggers">Loggers with mixed
features</a></span></dt>
<dt><span class="section"><a href="sources.html#log.detailed.sources.global_storage">Global storage
for loggers</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.basic_logger_hpp" title="Header <boost/log/sources/basic_logger.hpp>">boost/log/sources/basic_logger.hpp</a></code><span class="special">></span>
</pre>
<p>
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
version, <code class="computeroutput"><a class="link" href="../../boost/log/sources/logger_mt.html" title="Class logger_mt">logger_mt</a></code>
(<code class="computeroutput"><a class="link" href="../../boost/log/sources/wlogger.html" title="Class wlogger">wlogger</a></code> and
<code class="computeroutput"><a class="link" href="../../boost/log/sources/wlogger_mt.html" title="Class wlogger_mt">wlogger_mt</a></code>
for wide-character logging, accordingly). These loggers only provide the
ability to store source-specific attributes within themselves and, of course,
to form log records. This type of logger should probably be used when there
is no need for advanced features like severity level checks. It may well
be used as a tool to collect application statistics and register application
events, such as notifications and alarms. In such cases the logger is normally
used in conjunction with <a class="link" href="attributes.html#log.detailed.attributes.related_components.scoped_attributes" title="Scoped attributes">scoped
attributes</a> to attach the needed data to the notification event.
Below is an example of usage:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">network_connection</span>
<span class="special">{</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">logger</span> <span class="identifier">m_logger</span><span class="special">;</span>
<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>
<span class="keyword">public</span><span class="special">:</span>
<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">&</span> <span class="identifier">remote_addr</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// Put the remote address into the logger to automatically attach it</span>
<span class="comment">// to every log record written through the logger</span>
<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>
<span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</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">remote_addr</span><span class="special">)).</span><span class="identifier">first</span><span class="special">;</span>
<span class="comment">// The straightforward way of logging</span>
<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>
<span class="special">{</span>
<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>
<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>
<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>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">on_disconnected</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// The simpler way of logging: the above "if" condition is wrapped into a neat macro</span>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_logger</span><span class="special">)</span> <span class="special"><<</span> <span class="string">"Connection shut down"</span><span class="special">;</span>
<span class="comment">// Remove the attribute with the remote address</span>
<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>
<span class="special">}</span>
<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>
<span class="special">{</span>
<span class="comment">// Put the size as an additional attribute</span>
<span class="comment">// so it can be collected and accumulated later if needed.</span>
<span class="comment">// The attribute will be attached only to this log record.</span>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_logger</span><span class="special">)</span> <span class="special"><<</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"><<</span> <span class="string">"Some data received"</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_logger</span><span class="special">)</span> <span class="special"><<</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"><<</span> <span class="string">"Some data sent"</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
The class <code class="computeroutput"><span class="identifier">network_connection</span></code>
in the code snippet above represents an approach to implementing simple
logging and statistical information gathering in a network-related application.
Each of the presented methods of the class effectively marks a corresponding
event that can be tracked and collected on the sinks level. Furthermore,
other methods of the class, that are not shown here for simplicity, are
able to write logs too. Note that every log record ever made in the connected
state of the <code class="computeroutput"><span class="identifier">network_connection</span></code>
object will be implicitly marked up with the address of the remote site.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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
with severity level support</a>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.severity_feature_hpp" title="Header <boost/log/sources/severity_feature.hpp>">boost/log/sources/severity_feature.hpp</a></code><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.severity_logger_hpp" title="Header <boost/log/sources/severity_logger.hpp>">boost/log/sources/severity_logger.hpp</a></code><span class="special">></span>
</pre>
<p>
The ability to distinguish some log records from others based on some kind
of level of severity or importance is one of the most frequently requested
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>
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>
(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>
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>
wide-character counterparts) provide this functionality.
</p>
<p>
The loggers automatically register a special source-specific attribute
"Severity", which can be set for every record in a compact and
efficient manner, with a named argument <code class="computeroutput"><span class="identifier">severity</span></code>
that can be passed to the constructor and/or the <code class="computeroutput"><span class="identifier">open_record</span></code>
method. If passed to the logger constructor, the <code class="computeroutput"><span class="identifier">severity</span></code>
argument sets the default value of the severity level that will be used
if none is provided in the <code class="computeroutput"><span class="identifier">open_record</span></code>
arguments. The <code class="computeroutput"><span class="identifier">severity</span></code>
argument passed to the <code class="computeroutput"><span class="identifier">open_record</span></code>
method sets the level of the particular log record being made. The type
of the severity level can be provided as a template argument for the logger
class template. The default type is <code class="computeroutput"><span class="keyword">int</span></code>.
</p>
<p>
The actual values of this attribute and their meaning are entirely user-defined.
However, it is recommended to use the level of value equivalent to zero
as a base point for other values. This is because the default-constructed
logger object sets its default severity level to zero. It is also recommended
to define the same levels of severity for the entire application in order
to avoid confusion in the written logs later. The following code snippet
shows the usage of <code class="computeroutput"><span class="identifier">severity_logger</span></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// We define our own severity levels</span>
<span class="keyword">enum</span> <span class="identifier">severity_level</span>
<span class="special">{</span>
<span class="identifier">normal</span><span class="special">,</span>
<span class="identifier">notification</span><span class="special">,</span>
<span class="identifier">warning</span><span class="special">,</span>
<span class="identifier">error</span><span class="special">,</span>
<span class="identifier">critical</span>
<span class="special">};</span>
<span class="keyword">void</span> <span class="identifier">logging_function</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// The logger implicitly adds a source-specific attribute 'Severity'</span>
<span class="comment">// of type 'severity_level' on construction</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger</span><span class="special"><</span> <span class="identifier">severity_level</span> <span class="special">></span> <span class="identifier">slg</span><span class="special">;</span>
<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"><<</span> <span class="string">"A regular message"</span><span class="special">;</span>
<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"><<</span> <span class="string">"Something bad is going on but I can handle it"</span><span class="special">;</span>
<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"><<</span> <span class="string">"Everything crumbles, shoot me now!"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">default_severity</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// The default severity can be specified in constructor.</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger</span><span class="special"><</span> <span class="identifier">severity_level</span> <span class="special">></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>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">error_lg</span><span class="special">)</span> <span class="special"><<</span> <span class="string">"An error level log record (by default)"</span><span class="special">;</span>
<span class="comment">// The explicitly specified level overrides the default</span>
<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"><<</span> <span class="string">"A warning level log record (overrode the default)"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Or, if you prefer logging without macros:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">manual_logging</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger</span><span class="special"><</span> <span class="identifier">severity_level</span> <span class="special">></span> <span class="identifier">slg</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">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>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">rec</span><span class="special">)</span>
<span class="special">{</span>
<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>
<span class="identifier">strm</span> <span class="special"><<</span> <span class="string">"A regular message"</span><span class="special">;</span>
<span class="identifier">strm</span><span class="special">.</span><span class="identifier">flush</span><span class="special">();</span>
<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>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
And, of course, severity loggers also provide the same functionality the
<a class="link" href="sources.html#log.detailed.sources.basic_logger" title="Basic loggers">basic loggers</a>
do.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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
support</a>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.channel_feature_hpp" title="Header <boost/log/sources/channel_feature.hpp>">boost/log/sources/channel_feature.hpp</a></code><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.channel_logger_hpp" title="Header <boost/log/sources/channel_logger.hpp>">boost/log/sources/channel_logger.hpp</a></code><span class="special">></span>
</pre>
<p>
Sometimes it is important to associate log records with some application
component, such as the module or class name, the relation of the logged
information to some specific domain of application functionality (e.g.
network or file system related messages) or some arbitrary tag that can
be used later to route these records to a specific sink. This feature is
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>,
<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>
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>,
<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>.
These loggers automatically register an attribute named "Channel".
The default channel name can be set in the logger constructor with a named
argument <code class="computeroutput"><span class="identifier">channel</span></code>. The type
of the channel attribute value can be specified as a template argument
for the logger, with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
(<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">wstring</span></code> in case of wide character loggers)
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
loggers</a>:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">network_connection</span>
<span class="special">{</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">channel_logger</span><span class="special"><</span> <span class="special">></span> <span class="identifier">m_net</span><span class="special">,</span> <span class="identifier">m_stat</span><span class="special">;</span>
<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>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">network_connection</span><span class="special">()</span> <span class="special">:</span>
<span class="comment">// We can dump network-related messages through this logger</span>
<span class="comment">// and be able to filter them later</span>
<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>
<span class="comment">// We also can separate statistic records in a different channel</span>
<span class="comment">// in order to route them to a different sink</span>
<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>
<span class="special">{</span>
<span class="special">}</span>
<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">&</span> <span class="identifier">remote_addr</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// Add the remote address to both channels</span>
<span class="identifier">attrs</span><span class="special">::</span><span class="identifier">constant</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">addr</span><span class="special">(</span><span class="identifier">remote_addr</span><span class="special">);</span>
<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>
<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>
<span class="comment">// Put message to the "net" channel</span>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_net</span><span class="special">)</span> <span class="special"><<</span> <span class="string">"Connection established"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">on_disconnected</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// Put message to the "net" channel</span>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_net</span><span class="special">)</span> <span class="special"><<</span> <span class="string">"Connection shut down"</span><span class="special">;</span>
<span class="comment">// Remove the attribute with the remote address</span>
<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>
<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>
<span class="special">}</span>
<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>
<span class="special">{</span>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_stat</span><span class="special">)</span> <span class="special"><<</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"><<</span> <span class="string">"Some data received"</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">m_stat</span><span class="special">)</span> <span class="special"><<</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"><<</span> <span class="string">"Some data sent"</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
It is also possible to set the channel name of individual log records.
This can be useful when a <a class="link" href="sources.html#log.detailed.sources.global_storage" title="Global storage for loggers">global
logger</a> is used instead of an object-specific one. The channel name
can be set by calling the <code class="computeroutput"><span class="identifier">channel</span></code>
modifier on the logger or by using a special macro for logging. For example:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// Define a global logger</span>
<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"><</span> <span class="special">>,</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>
<span class="keyword">class</span> <span class="identifier">network_connection</span>
<span class="special">{</span>
<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>
<span class="keyword">public</span><span class="special">:</span>
<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">&</span> <span class="identifier">remote_addr</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">m_remote_addr</span> <span class="special">=</span> <span class="identifier">remote_addr</span><span class="special">;</span>
<span class="comment">// Put message to the "net" channel</span>
<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>
<span class="special"><<</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>
<span class="special"><<</span> <span class="string">"Connection established"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">on_disconnected</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// Put message to the "net" channel</span>
<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>
<span class="special"><<</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>
<span class="special"><<</span> <span class="string">"Connection shut down"</span><span class="special">;</span>
<span class="identifier">m_remote_addr</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<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>
<span class="special"><<</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>
<span class="special"><<</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"><<</span> <span class="string">"Some data received"</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<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>
<span class="special"><<</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>
<span class="special"><<</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"><<</span> <span class="string">"Some data sent"</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Note that changing the channel name is persistent, so unless the channel
name is reset, the subsequent records will also belong to the new channel.
</p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
For performance reasons it is advised to avoid dynamically setting the
channel name individually for every log record, when possible. Changing
the channel name involves dynamic memory allocation. Using distinct loggers
for different channels allows to avoid this overhead.
</p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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
exception handling support</a>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.exception_handler_feature_hpp" title="Header <boost/log/sources/exception_handler_feature.hpp>">boost/log/sources/exception_handler_feature.hpp</a></code><span class="special">></span>
</pre>
<p>
The library provides a logger feature that enables the user to handle and/or
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>
feature adds a <code class="computeroutput"><span class="identifier">set_exception_handler</span></code>
method to the logger that allows setting a function object to be called
if an exception is thrown from the logging core during the filtering or
processing of log records. One can use the <a class="link" href="utilities.html#log.detailed.utilities.exception_handlers" title="Exception handlers">library-provided
adapters</a> to simplify implementing exception handlers. Usage example
is as follows:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">enum</span> <span class="identifier">severity_level</span>
<span class="special">{</span>
<span class="identifier">normal</span><span class="special">,</span>
<span class="identifier">warning</span><span class="special">,</span>
<span class="identifier">error</span>
<span class="special">};</span>
<span class="comment">// A logger class that allows to intercept exceptions and supports severity level</span>
<span class="keyword">class</span> <span class="identifier">my_logger_mt</span> <span class="special">:</span>
<span class="keyword">public</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">basic_composite_logger</span><span class="special"><</span>
<span class="keyword">char</span><span class="special">,</span>
<span class="identifier">my_logger_mt</span><span class="special">,</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">multi_thread_model</span><span class="special"><</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_mutex</span> <span class="special">>,</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">features</span><span class="special"><</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">severity</span><span class="special"><</span> <span class="identifier">severity_level</span> <span class="special">>,</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">exception_handler</span>
<span class="special">></span>
<span class="special">></span>
<span class="special">{</span>
<span class="identifier">BOOST_LOG_FORWARD_LOGGER_MEMBERS</span><span class="special">(</span><span class="identifier">my_logger_mt</span><span class="special">)</span>
<span class="special">};</span>
<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>
<span class="special">{</span>
<span class="identifier">my_logger_mt</span> <span class="identifier">lg</span><span class="special">;</span>
<span class="comment">// Set up exception handler: all exceptions that occur while</span>
<span class="comment">// logging through this logger, will be suppressed</span>
<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>
<span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">logging_function</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// This will not throw</span>
<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"><<</span> <span class="string">"Hello, world"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
<a class="link" href="../detailed.html#log.detailed.core.core.exception_handling" title="Exception handling">Logging core</a>
and <a class="link" href="sink_frontends.html#log.detailed.sink_frontends.basic_services.exception_handling" title="Exception handling">sink
frontends</a> also support installing exception handlers.
</p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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
features</a>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.severity_channel_logger_hpp" title="Header <boost/log/sources/severity_channel_logger.hpp>">boost/log/sources/severity_channel_logger.hpp</a></code><span class="special">></span>
</pre>
<p>
If you wonder whether you can use a mixed set of several logger features
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>
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>
(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>
and <code class="computeroutput"><a class="link" href="../../boost/log/sources/wseverity_chan_idp37022656.html" title="Class template wseverity_channel_logger_mt">wseverity_channel_logger_mt</a></code>)
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
level</a> and <a class="link" href="sources.html#log.detailed.sources.channel_logger" title="Loggers with channel support">channels</a>
support. The composite loggers are templates, too, which allows you to
specify severity level and channel types. You can also design your own
logger features and combine them with the ones provided by the library,
as described in the <a class="link" href="../extension/sources.html" title="Writing your own sources">Extending the
library</a> section.
</p>
<p>
The usage of the loggers with several features does not conceptually differ
from the usage of the single-featured loggers. For instance, here is how
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>
could be used:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">enum</span> <span class="identifier">severity_level</span>
<span class="special">{</span>
<span class="identifier">normal</span><span class="special">,</span>
<span class="identifier">notification</span><span class="special">,</span>
<span class="identifier">warning</span><span class="special">,</span>
<span class="identifier">error</span><span class="special">,</span>
<span class="identifier">critical</span>
<span class="special">};</span>
<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"><</span>
<span class="identifier">severity_level</span><span class="special">,</span> <span class="comment">// the type of the severity level</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="comment">// the type of the channel name</span>
<span class="special">></span> <span class="identifier">my_logger_mt</span><span class="special">;</span>
<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>
<span class="special">{</span>
<span class="comment">// Specify the channel name on construction, similarly as with the channel_logger</span>
<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>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">logging_function</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// Do logging with the severity level. The record will have both</span>
<span class="comment">// the severity level and the channel name attached.</span>
<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"><<</span> <span class="string">"Hello, world!"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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
for loggers</a>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><code class="computeroutput"><a class="link" href="../../logging_sources.html#header.boost.log.sources.global_logger_storage_hpp" title="Header <boost/log/sources/global_logger_storage.hpp>">boost/log/sources/global_logger_storage.hpp</a></code><span class="special">></span>
</pre>
<p>
Sometimes it is inconvenient to have a logger object to be able to write
logs. This issue is often present in functional-style code with no obvious
places where a logger could be stored. Another domain where the problem
persists is generic libraries that would benefit from logging. In such
cases it would be more convenient to have one or several global loggers
in order to easily access them in every place when needed. In this regard
<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.
</p>
<p>
The library provides a way to declare global loggers that can be accessed
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
with any logger, including user-defined ones. Having declared a global
logger, one can be sure to have a thread-safe access to this logger instance
from any place of the application code. The library also guarantees that
a global logger instance will be unique even across module boundaries.
This allows employing logging even in header-only components that may get
compiled into different modules.
</p>
<p>
One may wonder why there is a need for something special in order to create
global loggers. Why not just declare a logger variable at namespace scope
and use it wherever you need? While technically this is possible, declaring
and using global logger variables is complicated for the following reasons:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Order of initialization of namespace scope variables is not specified
by the C++ Standard. This means that generally you cannot use the logger
during this stage of initialization (i.e. before <code class="computeroutput"><span class="identifier">main</span></code>).
</li>
<li class="listitem">
Initialization of namespace scope variables is not thread-safe. You
may end up initializing the same logger twice or using an uninitialized
logger.
</li>
<li class="listitem">
Using namespace scope variables in a header-only library is quite complicated.
One either has to declare a variable with external linkage and define
it only in a single translation unit (that is, in a separate .cpp file,
which defeats the "header-only" thesis), or define a variable
with internal linkage, or as a special case in an anonymous namespace
(this will most likely break ODR and give unexpected results when the
header is used in different translation units). There are other compiler-specific
and standard tricks to tackle the problem, but they are not quite trivial
and portable.
</li>
<li class="listitem">
On most platforms namespace scope variables are local to the module
where they were compiled in. That is, if variable <code class="computeroutput"><span class="identifier">a</span></code>
has external linkage and was compiled into modules X and Y, each of
these modules has its own copy of variable <code class="computeroutput"><span class="identifier">a</span></code>.
To make things worse, on other platforms this variable can be shared
between the modules.
</li>
</ul></div>
<p>
Global logger storage is intended to eliminate all these problems.
</p>
<p>
The easiest way to declare a global logger is to use the following macro:
</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT.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"><</span> <span class="special">>)</span>
</pre>
<p>
The <code class="computeroutput"><span class="identifier">my_logger</span></code> argument
gives the logger a name that may be used to acquire the logger instance.
This name acts as a tag of the declared logger. The second parameter denotes
the logger type. In multithreaded applications, when the logger can be
accessed from different threads, users will normally want to use the thread-safe
versions of loggers.
</p>
<p>
If passing arguments to the logger constructor is needed, there is another
macro:
</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS">BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS</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_channel_logger</span><span class="special"><</span> <span class="special">>,</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><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>
</pre>
<p>
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>
sequence of arguments passed to the logger constructor. Be careful, however,
when using non-constant expressions and references to objects as constructor
arguments, since the arguments are evaluated only once and it is often
difficult to tell the exact moment when it is done. The logger is constructed
on the first request from whichever part of the application that has the
knowledge of the logger declaration. It is up to user to make sure that
all arguments have valid states at that point.
</p>
<p>
The third macro of this section provides maximum initialization flexibility,
allowing the user to actually define the logic of creating the logger.
</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../../BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT.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>
<span class="special">{</span>
<span class="comment">// Do something that needs to be done on logger initialization,</span>
<span class="comment">// e.g. add a stop watch attribute.</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger_mt</span><span class="special"><</span> <span class="special">></span> <span class="identifier">lg</span><span class="special">;</span>
<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">boost</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special"><</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span> <span class="special">>());</span>
<span class="comment">// The initializing routine must return the logger instance</span>
<span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
Like the <code class="computeroutput"><a class="link" href="../../BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS">BOOST_LOG_INLINE_GLOBAL_LOGGER_CTOR_ARGS</a></code>
macro, the initialization code is called only once, on the first request
of the logger.
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
Beware of One Definition Rule (ODR) issues. Regardless of the way of
logger declaration you choose, you should ensure that <span class="underline">the
logger is declared in exactly the same way at all occurrences</span>
and <span class="underline">all symbol names involved in the declaration
resolve to the same entities</span>. The latter includes the names
used within the initialization routine of the <code class="computeroutput"><a class="link" href="../../BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT.html" title="Macro BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT">BOOST_LOG_INLINE_GLOBAL_LOGGER_INIT</a></code>
macro, such as references to external variables, functions and types.
The library tries to protect itself from ODR violations to a certain
degree, but in general the behavior is undefined if the rule is violated.
</p></td></tr>
</table></div>
<p>
In order to alleviate ODR problems, it is possible to separate the logger
declaration and its initialization routine. The library provides the following
macros to achieve this:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput"><a class="link" href="../../BOOST_LOG_GLOBAL_LOGGER.html" title="Macro BOOST_LOG_GLOBAL_LOGGER">BOOST_LOG_GLOBAL_LOGGER</a></code>
provides the logger declaration. It can be used in a header, similarly
to the <code class="computeroutput"><span class="identifier">BOOST_LOG_INLINE_GLOBAL_LOGGER</span><span class="special">*</span></code> macros described above.
</li>
<li class="listitem">
<code class="computeroutput"><a class="link" href="../../BOOST_LOG_GLOBAL_LOGGER_INIT.html" title="Macro BOOST_LOG_GLOBAL_LOGGER_INIT">BOOST_LOG_GLOBAL_LOGGER_INIT</a></code>,
<code class="computeroutput"><a class="link" href="../../BOOST_LOG_GLOBAL_LOGGER_DEFAULT.html" title="Macro BOOST_LOG_GLOBAL_LOGGER_DEFAULT">BOOST_LOG_GLOBAL_LOGGER_DEFAULT</a></code>
and <code class="computeroutput"><a class="link" href="../../BOOST_LOG_GLOBAL_LOGGER_CTOR_ARGS.html" title="Macro BOOST_LOG_GLOBAL_LOGGER_CTOR_ARGS">BOOST_LOG_GLOBAL_LOGGER_CTOR_ARGS</a></code>
define the logger initialization routine. Their semantics and usage
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
should be used in a single .cpp file.
</li>
</ul></div>
<p>
For example:
</p>
<pre class="programlisting"><span class="comment">// my_logger.h</span>
<span class="comment">// ===========</span>
<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>
<span class="comment">// my_logger.cpp</span>
<span class="comment">// ===========</span>
<span class="preprocessor">#include</span> <span class="string">"my_logger.h"</span>
<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>
<span class="special">{</span>
<span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger_mt</span><span class="special"><</span> <span class="special">></span> <span class="identifier">lg</span><span class="special">;</span>
<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">boost</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special"><</span> <span class="identifier">attrs</span><span class="special">::</span><span class="identifier">timer</span> <span class="special">>());</span>
<span class="keyword">return</span> <span class="identifier">lg</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
Regardless of the macro you used to declare the logger, you can acquire
the logger instance with the static <code class="computeroutput"><span class="identifier">get</span></code>
function of the logger tag:
</p>
<pre class="programlisting"><span class="identifier">src</span><span class="special">::</span><span class="identifier">severity_logger_mt</span><span class="special"><</span> <span class="special">>&</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>
</pre>
<p>
Further usage of the logger is the same as if it was a regular logger object
of the corresponding type.
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
It should be noted that it is not advised to use global loggers during
the deinitialization stage of the application. Like any other global
object in your application, the global logger may get destroyed before
you try to use it. In such cases it's better to have a dedicated logger
object that is guaranteed to be available as long as needed.
</p></td></tr>
</table></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2007-2014 Andrey Semashev<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>).
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../detailed.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../detailed.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="sink_frontends.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
|
