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   <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
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   <meta name="AUTHOR" content="bkoz@redhat.com (Benjamin Kosnik)" />
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   <meta name="DESCRIPTION" content="Notes on the messages implementation." />
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   <title>Notes on the messages implementation.</title>
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<body>
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<h1>
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Notes on the messages implementation.
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</h1>
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<em>
25
prepared by Benjamin Kosnik (bkoz@redhat.com) on August 8, 2001
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</em>
27
 
28
<h2>
29
1. Abstract
30
</h2>
31
<p>
32
The std::messages facet implements message retrieval functionality
33
equivalent to Java's java.text.MessageFormat .using either GNU gettext
34
or IEEE 1003.1-200 functions.
35
</p>
36
 
37
<h2>
38
2. What the standard says
39
</h2>
40
The std::messages facet is probably the most vaguely defined facet in
41
the standard library. It's assumed that this facility was built into
42
the standard library in order to convert string literals from one
43
locale to the other. For instance, converting the "C" locale's
44
<code>const char* c = "please"</code> to a German-localized <code>"bitte"</code>
45
during program execution.
46
 
47
<blockquote>
48
22.2.7.1 - Template class messages [lib.locale.messages]
49
</blockquote>
50
 
51
This class has three public member functions, which directly
52
correspond to three protected virtual member functions.
53
 
54
The public member functions are:
55
 
56
<p>
57
<code>catalog open(const string&amp;, const locale&amp;) const</code>
58
</p>
59
 
60
<p>
61
<code>string_type get(catalog, int, int, const string_type&amp;) const</code>
62
</p>
63
 
64
<p>
65
<code>void close(catalog) const</code>
66
</p>
67
 
68
<p>
69
While the virtual functions are:
70
</p>
71
 
72
<p>
73
<code>catalog do_open(const string&amp;, const locale&amp;) const</code>
74
</p>
75
<blockquote>
76
<em>
77
-1- Returns: A value that may be passed to get() to retrieve a
78
message, from the message catalog identified by the string name
79
according to an implementation-defined mapping. The result can be used
80
until it is passed to close().  Returns a value less than 0 if no such
81
catalog can be opened.
82
</em>
83
</blockquote>
84
 
85
<p>
86
<code>string_type do_get(catalog, int, int, const string_type&amp;) const</code>
87
</p>
88
<blockquote>
89
<em>
90
-3- Requires: A catalog cat obtained from open() and not yet closed.
91
-4- Returns: A message identified by arguments set, msgid, and dfault,
92
according to an implementation-defined mapping. If no such message can
93
be found, returns dfault.
94
</em>
95
</blockquote>
96
 
97
<p>
98
<code>void do_close(catalog) const</code>
99
</p>
100
<blockquote>
101
<em>
102
-5- Requires: A catalog cat obtained from open() and not yet closed.
103
-6- Effects: Releases unspecified resources associated with cat.
104
-7- Notes: The limit on such resources, if any, is implementation-defined.
105
</em>
106
</blockquote>
107
 
108
 
109
<h2>
110
3. Problems with &quot;C&quot; messages: thread safety,
111
over-specification, and assumptions.
112
</h2>
113
A couple of notes on the standard.
114
 
115
<p>
116
First, why is <code>messages_base::catalog</code> specified as a typedef
117
to int? This makes sense for implementations that use
118
<code>catopen</code>, but not for others. Fortunately, it's not heavily
119
used and so only a minor irritant.
120
</p>
121
 
122
<p>
123
Second, by making the member functions <code>const</code>, it is
124
impossible to save state in them. Thus, storing away information used
125
in the 'open' member function for use in 'get' is impossible. This is
126
unfortunate.
127
</p>
128
 
129
<p>
130
The 'open' member function in particular seems to be oddly
131
designed. The signature seems quite peculiar. Why specify a <code>const
132
string&amp; </code> argument, for instance, instead of just <code>const
133
char*</code>? Or, why specify a <code>const locale&amp;</code> argument that is
134
to be used in the 'get' member function? How, exactly, is this locale
135
argument useful? What was the intent? It might make sense if a locale
136
argument was associated with a given default message string in the
137
'open' member function, for instance. Quite murky and unclear, on
138
reflection.
139
</p>
140
 
141
<p>
142
Lastly, it seems odd that messages, which explicitly require code
143
conversion, don't use the codecvt facet. Because the messages facet
144
has only one template parameter, it is assumed that ctype, and not
145
codecvt, is to be used to convert between character sets.
146
</p>
147
 
148
<p>
149
It is implicitly assumed that the locale for the default message
150
string in 'get' is in the "C" locale. Thus, all source code is assumed
151
to be written in English, so translations are always from "en_US" to
152
other, explicitly named locales.
153
</p>
154
 
155
<h2>
156
4. Design and Implementation Details
157
</h2>
158
This is a relatively simple class, on the face of it. The standard
159
specifies very little in concrete terms, so generic implementations
160
that are conforming yet do very little are the norm. Adding
161
functionality that would be useful to programmers and comparable to
162
Java's java.text.MessageFormat takes a bit of work, and is highly
163
dependent on the capabilities of the underlying operating system.
164
 
165
<p>
166
Three different mechanisms have been provided, selectable via
167
configure flags:
168
</p>
169
 
170
<ul>
171
   <li> generic
172
   <p>
173
   This model does very little, and is what is used by default.
174
   </p>
175
   </li>
176
 
177
   <li> gnu
178
   <p>
179
   The gnu model is complete and fully tested. It's based on the
180
   GNU gettext package, which is part of glibc. It uses the functions
181
   <code>textdomain, bindtextdomain, gettext</code>
182
   to implement full functionality. Creating message
183
   catalogs is a relatively straight-forward process and is
184
   lightly documented below, and fully documented in gettext's
185
   distributed documentation.
186
   </p>
187
   </li>
188
 
189
   <li> ieee_1003.1-200x
190
   <p>
191
   This is a complete, though untested, implementation based on
192
   the IEEE standard. The functions
193
   <code>catopen, catgets, catclose</code>
194
   are used to retrieve locale-specific messages given the
195
   appropriate message catalogs that have been constructed for
196
   their use. Note, the script <code> po2msg.sed</code> that is part
197
   of the gettext distribution can convert gettext catalogs into
198
   catalogs that <code>catopen</code> can use.
199
   </p>
200
   </li>
201
</ul>
202
 
203
<p>
204
A new, standards-conformant non-virtual member function signature was
205
added for 'open' so that a directory could be specified with a given
206
message catalog. This simplifies calling conventions for the gnu
207
model.
208
</p>
209
 
210
<p>
211
The rest of this document discusses details of the GNU model.
212
</p>
213
 
214
<p>
215
The messages facet, because it is retrieving and converting between
216
characters sets, depends on the ctype and perhaps the codecvt facet in
217
a given locale. In addition, underlying "C" library locale support is
218
necessary for more than just the <code>LC_MESSAGES</code> mask:
219
<code>LC_CTYPE</code> is also necessary. To avoid any unpleasantness, all
220
bits of the "C" mask (ie <code>LC_ALL</code>) are set before retrieving
221
messages.
222
</p>
223
 
224
<p>
225
Making the message catalogs can be initially tricky, but become quite
226
simple with practice. For complete info, see the gettext
227
documentation. Here's an idea of what is required:
228
</p>
229
 
230
<ul>
231
   <li> Make a source file with the required string literals
232
   that need to be translated. See
233
   <code>intl/string_literals.cc</code> for an example.
234
   </li>
235
 
236
   <li> Make initial catalog (see "4 Making the PO Template File"
237
   from the gettext docs).
238
   <p>
239
   <code> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
240
   </p>
241
   </li>
242
 
243
   <li> Make language and country-specific locale catalogs.
244
   <p>
245
   <code>cp libstdc++.pot fr_FR.po</code>
246
   </p>
247
   <p>
248
   <code>cp libstdc++.pot de_DE.po</code>
249
   </p>
250
   </li>
251
 
252
   <li> Edit localized catalogs in emacs so that strings are
253
   translated.
254
   <p>
255
   <code>emacs fr_FR.po</code>
256
   </p>
257
   </li>
258
 
259
   <li> Make the binary mo files.
260
   <p>
261
   <code>msgfmt fr_FR.po -o fr_FR.mo</code>
262
   </p>
263
   <p>
264
   <code>msgfmt de_DE.po -o de_DE.mo</code>
265
   </p>
266
   </li>
267
 
268
   <li> Copy the binary files into the correct directory structure.
269
   <p>
270
   <code>cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++-v3.mo</code>
271
   </p>
272
   <p>
273
   <code>cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++-v3.mo</code>
274
   </p>
275
   </li>
276
 
277
   <li> Use the new message catalogs.
278
   <p>
279
   <code>locale loc_de("de_DE");</code>
280
   </p>
281
   <p>
282
   <code>
283
   use_facet&lt;messages&lt;char&gt; &gt;(loc_de).open("libstdc++", locale(), dir);
284
   </code>
285
   </p>
286
   </li>
287
</ul>
288
 
289
<h2>
290
5.  Examples
291
</h2>
292
 
293
<ul>
294
   <li> message converting, simple example using the GNU model.
295
 
296
<pre>
297
#include &lt;iostream&gt;
298
#include &lt;locale&gt;
299
using namespace std;
300
 
301
void test01()
302
{
303
  typedef messages&lt;char&gt;::catalog catalog;
304
  const char* dir =
305
  "/mnt/egcs/build/i686-pc-linux-gnu/libstdc++-v3/po/share/locale";
306
  const locale loc_de("de_DE");
307
  const messages&lt;char&gt;&amp; mssg_de = use_facet&lt;messages&lt;char&gt; &gt;(loc_de);
308
 
309
  catalog cat_de = mssg_de.open("libstdc++", loc_de, dir);
310
  string s01 = mssg_de.get(cat_de, 0, 0, "please");
311
  string s02 = mssg_de.get(cat_de, 0, 0, "thank you");
312
  cout &lt;&lt; "please in german:" &lt;&lt; s01 &lt;&lt; '\n';
313
  cout &lt;&lt; "thank you in german:" &lt;&lt; s02 &lt;&lt; '\n';
314
  mssg_de.close(cat_de);
315
}
316
</pre>
317
   </li>
318
</ul>
319
 
320
More information can be found in the following testcases:
321
<ul>
322
<li> testsuite/22_locale/messages.cc              </li>
323
<li> testsuite/22_locale/messages_byname.cc       </li>
324
<li> testsuite/22_locale/messages_char_members.cc </li>
325
</ul>
326
 
327
<h2>
328
6.  Unresolved Issues
329
</h2>
330
<ul>
331
<li>  Things that are sketchy, or remain unimplemented:
332
   <ul>
333
      <li>_M_convert_from_char, _M_convert_to_char are in
334
      flux, depending on how the library ends up doing
335
      character set conversions. It might not be possible to
336
      do a real character set based conversion, due to the
337
      fact that the template parameter for messages is not
338
      enough to instantiate the codecvt facet (1 supplied,
339
      need at least 2 but would prefer 3).
340
      </li>
341
 
342
      <li> There are issues with gettext needing the global
343
      locale set to extract a message. This dependence on
344
      the global locale makes the current "gnu" model non
345
      MT-safe. Future versions of glibc, ie glibc 2.3.x will
346
      fix this, and the C++ library bits are already in
347
      place.
348
      </li>
349
   </ul>
350
</li>
351
 
352
<li>  Development versions of the GNU "C" library, glibc 2.3 will allow
353
   a more efficient, MT implementation of std::messages, and will
354
   allow the removal of the _M_name_messages data member. If this
355
   is done, it will change the library ABI. The C++ parts to
356
   support glibc 2.3 have already been coded, but are not in use:
357
   once this version of the "C" library is released, the marked
358
   parts of the messages implementation can be switched over to
359
   the new "C" library functionality.
360
</li>
361
<li>    At some point in the near future, std::numpunct will probably use
362
   std::messages facilities to implement truename/falename
363
   correctly. This is currently not done, but entries in
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   libstdc++.pot have already been made for "true" and "false"
365
   string literals, so all that remains is the std::numpunct
366
   coding and the configure/make hassles to make the installed
367
   library search its own catalog. Currently the libstdc++.mo
368
   catalog is only searched for the testsuite cases involving
369
   messages members.
370
</li>
371
 
372
<li>  The following member functions:
373
 
374
   <p>
375
   <code>
376
        catalog
377
        open(const basic_string&lt;char&gt;&amp; __s, const locale&amp; __loc) const
378
   </code>
379
   </p>
380
 
381
   <p>
382
   <code>
383
   catalog
384
   open(const basic_string&lt;char&gt;&amp;, const locale&amp;, const char*) const;
385
   </code>
386
   </p>
387
 
388
   <p>
389
   Don't actually return a "value less than 0 if no such catalog
390
   can be opened" as required by the standard in the "gnu"
391
   model. As of this writing, it is unknown how to query to see
392
   if a specified message catalog exists using the gettext
393
   package.
394
   </p>
395
</li>
396
</ul>
397
 
398
<h2>
399
7. Acknowledgments
400
</h2>
401
Ulrich Drepper for the character set explanations, gettext details,
402
and patient answering of late-night questions, Tom Tromey for the java details.
403
 
404
 
405
<h2>
406
8. Bibliography / Referenced Documents
407
</h2>
408
 
409
Drepper, Ulrich, GNU libc (glibc) 2.2 manual. In particular, Chapters
410
&quot;7 Locales and Internationalization&quot;
411
 
412
<p>
413
Drepper, Ulrich, Thread-Aware Locale Model, A proposal. This is a
414
draft document describing the design of glibc 2.3 MT locale
415
functionality.
416
</p>
417
 
418
<p>
419
Drepper, Ulrich, Numerous, late-night email correspondence
420
</p>
421
 
422
<p>
423
ISO/IEC 9899:1999 Programming languages - C
424
</p>
425
 
426
<p>
427
ISO/IEC 14882:1998 Programming languages - C++
428
</p>
429
 
430
<p>
431
Java 2 Platform, Standard Edition, v 1.3.1 API Specification. In
432
particular, java.util.Properties, java.text.MessageFormat,
433
java.util.Locale, java.util.ResourceBundle.
434
http://java.sun.com/j2se/1.3/docs/api
435
</p>
436
 
437
<p>
438
System Interface Definitions, Issue 7 (IEEE Std. 1003.1-200x)
439
The Open Group/The Institute of Electrical and Electronics Engineers, Inc.
440
In particular see lines 5268-5427.
441
http://www.opennc.org/austin/docreg.html
442
</p>
443
 
444
<p> GNU gettext tools, version 0.10.38, Native Language Support
445
Library and Tools.
446
http://sources.redhat.com/gettext
447
</p>
448
 
449
<p>
450
Langer, Angelika and Klaus Kreft, Standard C++ IOStreams and Locales,
451
Advanced Programmer's Guide and Reference, Addison Wesley Longman,
452
Inc. 2000. See page 725, Internationalized Messages.
453
</p>
454
 
455
<p>
456
Stroustrup, Bjarne, Appendix D, The C++ Programming Language, Special Edition, Addison Wesley, Inc. 2000
457
</p>
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</body>
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</html>
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