libstdc++-v3/doc/html/manual/messages.html

   1 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
   2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
   3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>messages</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      messages&#10;    " /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt06ch15.html" title="Chapter 15. Facets aka Categories" /><link rel="prev" href="codecvt.html" title="codecvt" /><link rel="next" href="containers.html" title="Part VII. Containers" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">messages</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="codecvt.html">Prev</a> </td><th width="60%" align="center">Chapter 15. Facets aka Categories</th><td width="20%" align="right"> <a accesskey="n" href="containers.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.localization.facet.messages"></a>messages</h2></div></div></div><p>
   4 The std::messages facet implements message retrieval functionality
   5 equivalent to Java's java.text.MessageFormat .using either GNU gettext
   6 or IEEE 1003.1-200 functions.
   7 </p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.messages.req"></a>Requirements</h3></div></div></div><p>
   8 The std::messages facet is probably the most vaguely defined facet in
   9 the standard library. It's assumed that this facility was built into
  10 the standard library in order to convert string literals from one
  11 locale to the other. For instance, converting the "C" locale's
  12 <code class="code">const char* c = "please"</code> to a German-localized <code class="code">"bitte"</code>
  13 during program execution.
  14 </p><div class="blockquote"><blockquote class="blockquote"><p>
  15 22.2.7.1 - Template class messages [lib.locale.messages]
  16 </p></blockquote></div><p>
  17 This class has three public member functions, which directly
  18 correspond to three protected virtual member functions.
  19 </p><p>
  20 The public member functions are:
  21 </p><p>
  22 <code class="code">catalog open(const string&amp;, const locale&amp;) const</code>
  23 </p><p>
  24 <code class="code">string_type get(catalog, int, int, const string_type&amp;) const</code>
  25 </p><p>
  26 <code class="code">void close(catalog) const</code>
  27 </p><p>
  28 While the virtual functions are:
  29 </p><p>
  30 <code class="code">catalog do_open(const string&amp;, const locale&amp;) const</code>
  31 </p><div class="blockquote"><blockquote class="blockquote"><p>
  32 <span class="emphasis"><em>
  33 -1- Returns: A value that may be passed to get() to retrieve a
  34 message, from the message catalog identified by the string name
  35 according to an implementation-defined mapping. The result can be used
  36 until it is passed to close().  Returns a value less than 0 if no such
  37 catalog can be opened.
  38 </em></span>
  39 </p></blockquote></div><p>
  40 <code class="code">string_type do_get(catalog, int, int, const string_type&amp;) const</code>
  41 </p><div class="blockquote"><blockquote class="blockquote"><p>
  42 <span class="emphasis"><em>
  43 -3- Requires: A catalog cat obtained from open() and not yet closed.
  44 -4- Returns: A message identified by arguments set, msgid, and dfault,
  45 according to an implementation-defined mapping. If no such message can
  46 be found, returns dfault.
  47 </em></span>
  48 </p></blockquote></div><p>
  49 <code class="code">void do_close(catalog) const</code>
  50 </p><div class="blockquote"><blockquote class="blockquote"><p>
  51 <span class="emphasis"><em>
  52 -5- Requires: A catalog cat obtained from open() and not yet closed.
  53 -6- Effects: Releases unspecified resources associated with cat.
  54 -7- Notes: The limit on such resources, if any, is implementation-defined.
  55 </em></span>
  56 </p></blockquote></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.messages.design"></a>Design</h3></div></div></div><p>
  57 A couple of notes on the standard.
  58 </p><p>
  59 First, why is <code class="code">messages_base::catalog</code> specified as a typedef
  60 to int? This makes sense for implementations that use
  61 <code class="code">catopen</code>, but not for others. Fortunately, it's not heavily
  62 used and so only a minor irritant.
  63 </p><p>
  64 Second, by making the member functions <code class="code">const</code>, it is
  65 impossible to save state in them. Thus, storing away information used
  66 in the 'open' member function for use in 'get' is impossible. This is
  67 unfortunate.
  68 </p><p>
  69 The 'open' member function in particular seems to be oddly
  70 designed. The signature seems quite peculiar. Why specify a <code class="code">const
  71 string&amp; </code> argument, for instance, instead of just <code class="code">const
  72 char*</code>? Or, why specify a <code class="code">const locale&amp;</code> argument that is
  73 to be used in the 'get' member function? How, exactly, is this locale
  74 argument useful? What was the intent? It might make sense if a locale
  75 argument was associated with a given default message string in the
  76 'open' member function, for instance. Quite murky and unclear, on
  77 reflection.
  78 </p><p>
  79 Lastly, it seems odd that messages, which explicitly require code
  80 conversion, don't use the codecvt facet. Because the messages facet
  81 has only one template parameter, it is assumed that ctype, and not
  82 codecvt, is to be used to convert between character sets.
  83 </p><p>
  84 It is implicitly assumed that the locale for the default message
  85 string in 'get' is in the "C" locale. Thus, all source code is assumed
  86 to be written in English, so translations are always from "en_US" to
  87 other, explicitly named locales.
  88 </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.messages.impl"></a>Implementation</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="messages.impl.models"></a>Models</h4></div></div></div><p>
  89     This is a relatively simple class, on the face of it. The standard
  90     specifies very little in concrete terms, so generic
  91     implementations that are conforming yet do very little are the
  92     norm. Adding functionality that would be useful to programmers and
  93     comparable to Java's java.text.MessageFormat takes a bit of work,
  94     and is highly dependent on the capabilities of the underlying
  95     operating system.
  96   </p><p>
  97     Three different mechanisms have been provided, selectable via
  98     configure flags:
  99   </p><div class="itemizedlist"><ul type="disc"><li><p>
 100        generic
 101      </p><p>
 102        This model does very little, and is what is used by default.
 103      </p></li><li><p>
 104        gnu
 105      </p><p>
 106        The gnu model is complete and fully tested. It's based on the
 107        GNU gettext package, which is part of glibc. It uses the
 108        functions <code class="code">textdomain, bindtextdomain, gettext</code> to
 109        implement full functionality. Creating message catalogs is a
 110        relatively straight-forward process and is lightly documented
 111        below, and fully documented in gettext's distributed
 112        documentation.
 113      </p></li><li><p>
 114        ieee_1003.1-200x
 115      </p><p>
 116        This is a complete, though untested, implementation based on
 117        the IEEE standard. The functions <code class="code">catopen, catgets,
 118        catclose</code> are used to retrieve locale-specific messages
 119        given the appropriate message catalogs that have been
 120        constructed for their use. Note, the script <code class="code">
 121        po2msg.sed</code> that is part of the gettext distribution can
 122        convert gettext catalogs into catalogs that
 123        <code class="code">catopen</code> can use.
 124    </p></li></ul></div><p>
 125 A new, standards-conformant non-virtual member function signature was
 126 added for 'open' so that a directory could be specified with a given
 127 message catalog. This simplifies calling conventions for the gnu
 128 model.
 129 </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="messages.impl.gnu"></a>The GNU Model</h4></div></div></div><p>
 130     The messages facet, because it is retrieving and converting
 131     between characters sets, depends on the ctype and perhaps the
 132     codecvt facet in a given locale. In addition, underlying "C"
 133     library locale support is necessary for more than just the
 134     <code class="code">LC_MESSAGES</code> mask: <code class="code">LC_CTYPE</code> is also
 135     necessary. To avoid any unpleasantness, all bits of the "C" mask
 136     (i.e. <code class="code">LC_ALL</code>) are set before retrieving messages.
 137   </p><p>
 138     Making the message catalogs can be initially tricky, but become
 139     quite simple with practice. For complete info, see the gettext
 140     documentation. Here's an idea of what is required:
 141   </p><div class="itemizedlist"><ul type="disc"><li><p>
 142        Make a source file with the required string literals that need
 143        to be translated. See <code class="code">intl/string_literals.cc</code> for
 144        an example.
 145      </p></li><li><p>
 146        Make initial catalog (see "4 Making the PO Template File" from
 147        the gettext docs).</p><p>
 148    <code class="code"> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
 149    </p></li><li><p>Make language and country-specific locale catalogs.</p><p>
 150    <code class="code">cp libstdc++.pot fr_FR.po</code>
 151    </p><p>
 152    <code class="code">cp libstdc++.pot de_DE.po</code>
 153    </p></li><li><p>
 154        Edit localized catalogs in emacs so that strings are
 155        translated.
 156      </p><p>
 157    <code class="code">emacs fr_FR.po</code>
 158    </p></li><li><p>Make the binary mo files.</p><p>
 159    <code class="code">msgfmt fr_FR.po -o fr_FR.mo</code>
 160    </p><p>
 161    <code class="code">msgfmt de_DE.po -o de_DE.mo</code>
 162    </p></li><li><p>Copy the binary files into the correct directory structure.</p><p>
 163    <code class="code">cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++.mo</code>
 164    </p><p>
 165    <code class="code">cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++.mo</code>
 166    </p></li><li><p>Use the new message catalogs.</p><p>
 167    <code class="code">locale loc_de("de_DE");</code>
 168    </p><p>
 169    <code class="code">
 170    use_facet&lt;messages&lt;char&gt; &gt;(loc_de).open("libstdc++", locale(), dir);
 171    </code>
 172    </p></li></ul></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.messages.use"></a>Use</h3></div></div></div><p>
 173    A simple example using the GNU model of message conversion.
 174  </p><pre class="programlisting">
 175 #include &lt;iostream&gt;
 176 #include &lt;locale&gt;
 177 using namespace std;
 178
 179 void test01()
 180 {
 181   typedef messages&lt;char&gt;::catalog catalog;
 182   const char* dir =
 183   "/mnt/egcs/build/i686-pc-linux-gnu/libstdc++/po/share/locale";
 184   const locale loc_de("de_DE");
 185   const messages&lt;char&gt;&amp; mssg_de = use_facet&lt;messages&lt;char&gt; &gt;(loc_de);
 186
 187   catalog cat_de = mssg_de.open("libstdc++", loc_de, dir);
 188   string s01 = mssg_de.get(cat_de, 0, 0, "please");
 189   string s02 = mssg_de.get(cat_de, 0, 0, "thank you");
 190   cout &lt;&lt; "please in german:" &lt;&lt; s01 &lt;&lt; '\n';
 191   cout &lt;&lt; "thank you in german:" &lt;&lt; s02 &lt;&lt; '\n';
 192   mssg_de.close(cat_de);
 193 }
 194 </pre></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="facet.messages.future"></a>Future</h3></div></div></div><div class="itemizedlist"><ul type="disc"><li><p>
 195     Things that are sketchy, or remain unimplemented:
 196   </p><div class="itemizedlist"><ul type="circle"><li><p>
 197           _M_convert_from_char, _M_convert_to_char are in flux,
 198           depending on how the library ends up doing character set
 199           conversions. It might not be possible to do a real character
 200           set based conversion, due to the fact that the template
 201           parameter for messages is not enough to instantiate the
 202           codecvt facet (1 supplied, need at least 2 but would prefer
 203           3).
 204         </p></li><li><p>
 205           There are issues with gettext needing the global locale set
 206           to extract a message. This dependence on the global locale
 207           makes the current "gnu" model non MT-safe. Future versions
 208           of glibc, i.e. glibc 2.3.x will fix this, and the C++ library
 209           bits are already in place.
 210         </p></li></ul></div></li><li><p>
 211     Development versions of the GNU "C" library, glibc 2.3 will allow
 212     a more efficient, MT implementation of std::messages, and will
 213     allow the removal of the _M_name_messages data member. If this is
 214     done, it will change the library ABI. The C++ parts to support
 215     glibc 2.3 have already been coded, but are not in use: once this
 216     version of the "C" library is released, the marked parts of the
 217     messages implementation can be switched over to the new "C"
 218     library functionality.
 219   </p></li><li><p>
 220     At some point in the near future, std::numpunct will probably use
 221     std::messages facilities to implement truename/falsename
 222     correctly. This is currently not done, but entries in
 223     libstdc++.pot have already been made for "true" and "false" string
 224     literals, so all that remains is the std::numpunct coding and the
 225     configure/make hassles to make the installed library search its
 226     own catalog. Currently the libstdc++.mo catalog is only searched
 227     for the testsuite cases involving messages members.
 228   </p></li><li><p> The following member functions:</p><p>
 229    <code class="code">
 230         catalog
 231         open(const basic_string&lt;char&gt;&amp; __s, const locale&amp; __loc) const
 232    </code>
 233    </p><p>
 234    <code class="code">
 235    catalog
 236    open(const basic_string&lt;char&gt;&amp;, const locale&amp;, const char*) const;
 237    </code>
 238    </p><p>
 239    Don't actually return a "value less than 0 if no such catalog
 240    can be opened" as required by the standard in the "gnu"
 241    model. As of this writing, it is unknown how to query to see
 242    if a specified message catalog exists using the gettext
 243    package.
 244    </p></li></ul></div></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="facet.messages.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id413448"></a><p><span class="title"><i>
 245       The GNU C Library
 246     </i>. </span><span class="author"><span class="firstname">Roland</span> <span class="surname">McGrath</span>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2007 FSF. </span><span class="pagenums">Chapters 6 Character Set Handling, and 7 Locales and Internationalization
 247     . </span></p></div><div class="biblioentry"><a id="id460385"></a><p><span class="title"><i>
 248       Correspondence
 249     </i>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="copyright">Copyright © 2002 . </span></p></div><div class="biblioentry"><a id="id434210"></a><p><span class="title"><i>
 250       ISO/IEC 14882:1998 Programming languages - C++
 251     </i>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id434229"></a><p><span class="title"><i>
 252       ISO/IEC 9899:1999 Programming languages - C
 253     </i>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry"><a id="id480692"></a><p><span class="title"><i>
 254       System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x)
 255     </i>. </span><span class="copyright">Copyright © 1999
 256       The Open Group/The Institute of Electrical and Electronics Engineers, Inc.. </span><span class="biblioid">
 257       <a class="ulink" href="http://www.opennc.org/austin/docreg.html" target="_top">
 258       </a>
 259     . </span></p></div><div class="biblioentry"><a id="id480719"></a><p><span class="title"><i>
 260       The C++ Programming Language, Special Edition
 261     </i>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley, Inc.. </span><span class="pagenums">Appendix D. </span><span class="publisher"><span class="publishername">
 262         Addison Wesley
 263       . </span></span></p></div><div class="biblioentry"><a id="id410108"></a><p><span class="title"><i>
 264       Standard C++ IOStreams and Locales
 265     </i>. </span><span class="subtitle">
 266       Advanced Programmer's Guide and Reference
 267     . </span><span class="author"><span class="firstname">Angelika</span> <span class="surname">Langer</span>. </span><span class="author"><span class="firstname">Klaus</span> <span class="surname">Kreft</span>. </span><span class="copyright">Copyright © 2000 Addison Wesley Longman, Inc.. </span><span class="publisher"><span class="publishername">
 268         Addison Wesley Longman
 269       . </span></span></p></div><div class="biblioentry"><a id="id405725"></a><p><span class="title"><i>
 270       Java 2 Platform, Standard Edition, v 1.3.1 API Specification
 271     </i>. </span><span class="pagenums">java.util.Properties, java.text.MessageFormat,
 272 java.util.Locale, java.util.ResourceBundle. </span><span class="biblioid">
 273       <a class="ulink" href="http://java.sun.com/j2se/1.3/docs/api" target="_top">
 274       </a>
 275     . </span></p></div><div class="biblioentry"><a id="id421361"></a><p><span class="title"><i>
 276        GNU gettext tools, version 0.10.38, Native Language Support
 277 Library and Tools.
 278     </i>. </span><span class="biblioid">
 279       <a class="ulink" href="http://sources.redhat.com/gettext" target="_top">
 280       </a>
 281     . </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="codecvt.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt06ch15.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="containers.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">codecvt </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Part VII. Containers</td></tr></table></div></body></html>