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 <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 V1.74.0" /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      messages&#10;    " /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="facets.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>
 The std::messages facet implements message retrieval functionality
 equivalent to Java's java.text.MessageFormat .using either GNU gettext
 or IEEE 1003.1-200 functions.
 </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>
 The std::messages facet is probably the most vaguely defined facet in
 the standard library. It's assumed that this facility was built into
 the standard library in order to convert string literals from one
 locale to the other. For instance, converting the "C" locale's
 <code class="code">const char* c = "please"</code> to a German-localized <code class="code">"bitte"</code>
 during program execution.
 </p><div class="blockquote"><blockquote class="blockquote"><p>
 22.2.7.1 - Template class messages [lib.locale.messages]
 </p></blockquote></div><p>
 This class has three public member functions, which directly
 correspond to three protected virtual member functions.
 </p><p>
 The public member functions are:
 </p><p>
 <code class="code">catalog open(const string&amp;, const locale&amp;) const</code>
 </p><p>
 <code class="code">string_type get(catalog, int, int, const string_type&amp;) const</code>
 </p><p>
 <code class="code">void close(catalog) const</code>
 </p><p>
 While the virtual functions are:
 </p><p>
 <code class="code">catalog do_open(const string&amp;, const locale&amp;) const</code>
 </p><div class="blockquote"><blockquote class="blockquote"><p>
 <span class="emphasis"><em>
 -1- Returns: A value that may be passed to get() to retrieve a
 message, from the message catalog identified by the string name
 according to an implementation-defined mapping. The result can be used
 until it is passed to close().  Returns a value less than 0 if no such
 catalog can be opened.
 </em></span>
 </p></blockquote></div><p>
 <code class="code">string_type do_get(catalog, int, int, const string_type&amp;) const</code>
 </p><div class="blockquote"><blockquote class="blockquote"><p>
 <span class="emphasis"><em>
 -3- Requires: A catalog cat obtained from open() and not yet closed.
 -4- Returns: A message identified by arguments set, msgid, and dfault,
 according to an implementation-defined mapping. If no such message can
 be found, returns dfault.
 </em></span>
 </p></blockquote></div><p>
 <code class="code">void do_close(catalog) const</code>
 </p><div class="blockquote"><blockquote class="blockquote"><p>
 <span class="emphasis"><em>
 -5- Requires: A catalog cat obtained from open() and not yet closed.
 -6- Effects: Releases unspecified resources associated with cat.
 -7- Notes: The limit on such resources, if any, is implementation-defined.
 </em></span>
 </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>
 A couple of notes on the standard.
 </p><p>
 First, why is <code class="code">messages_base::catalog</code> specified as a typedef
 to int? This makes sense for implementations that use
 <code class="code">catopen</code>, but not for others. Fortunately, it's not heavily
 used and so only a minor irritant.
 </p><p>
 Second, by making the member functions <code class="code">const</code>, it is
 impossible to save state in them. Thus, storing away information used
 in the 'open' member function for use in 'get' is impossible. This is
 unfortunate.
 </p><p>
 The 'open' member function in particular seems to be oddly
 designed. The signature seems quite peculiar. Why specify a <code class="code">const
 string&amp; </code> argument, for instance, instead of just <code class="code">const
 char*</code>? Or, why specify a <code class="code">const locale&amp;</code> argument that is
 to be used in the 'get' member function? How, exactly, is this locale
 argument useful? What was the intent? It might make sense if a locale
 argument was associated with a given default message string in the
 'open' member function, for instance. Quite murky and unclear, on
 reflection.
 </p><p>
 Lastly, it seems odd that messages, which explicitly require code
 conversion, don't use the codecvt facet. Because the messages facet
 has only one template parameter, it is assumed that ctype, and not
 codecvt, is to be used to convert between character sets.
 </p><p>
 It is implicitly assumed that the locale for the default message
 string in 'get' is in the "C" locale. Thus, all source code is assumed
 to be written in English, so translations are always from "en_US" to
 other, explicitly named locales.
 </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>
     This is a relatively simple class, on the face of it. The standard
     specifies very little in concrete terms, so generic
     implementations that are conforming yet do very little are the
     norm. Adding functionality that would be useful to programmers and
     comparable to Java's java.text.MessageFormat takes a bit of work,
     and is highly dependent on the capabilities of the underlying
     operating system.
   </p><p>
     Three different mechanisms have been provided, selectable via
     configure flags:
   </p><div class="itemizedlist"><ul type="disc"><li><p>
        generic
      </p><p>
        This model does very little, and is what is used by default.
      </p></li><li><p>
        gnu
      </p><p>
        The gnu model is complete and fully tested. It's based on the
        GNU gettext package, which is part of glibc. It uses the
        functions <code class="code">textdomain, bindtextdomain, gettext</code> to
        implement full functionality. Creating message catalogs is a
        relatively straight-forward process and is lightly documented
        below, and fully documented in gettext's distributed
        documentation.
      </p></li><li><p>
        ieee_1003.1-200x
      </p><p>
        This is a complete, though untested, implementation based on
        the IEEE standard. The functions <code class="code">catopen, catgets,
        catclose</code> are used to retrieve locale-specific messages
        given the appropriate message catalogs that have been
        constructed for their use. Note, the script <code class="code">
        po2msg.sed</code> that is part of the gettext distribution can
        convert gettext catalogs into catalogs that
        <code class="code">catopen</code> can use.
    </p></li></ul></div><p>
 A new, standards-conformant non-virtual member function signature was
 added for 'open' so that a directory could be specified with a given
 message catalog. This simplifies calling conventions for the gnu
 model.
 </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>
     The messages facet, because it is retrieving and converting
     between characters sets, depends on the ctype and perhaps the
     codecvt facet in a given locale. In addition, underlying "C"
     library locale support is necessary for more than just the
     <code class="code">LC_MESSAGES</code> mask: <code class="code">LC_CTYPE</code> is also
     necessary. To avoid any unpleasantness, all bits of the "C" mask
     (i.e. <code class="code">LC_ALL</code>) are set before retrieving messages.
   </p><p>
     Making the message catalogs can be initially tricky, but become
     quite simple with practice. For complete info, see the gettext
     documentation. Here's an idea of what is required:
   </p><div class="itemizedlist"><ul type="disc"><li><p>
        Make a source file with the required string literals that need
        to be translated. See <code class="code">intl/string_literals.cc</code> for
        an example.
      </p></li><li><p>
        Make initial catalog (see "4 Making the PO Template File" from
        the gettext docs).</p><p>
    <code class="code"> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
    </p></li><li><p>Make language and country-specific locale catalogs.</p><p>
    <code class="code">cp libstdc++.pot fr_FR.po</code>
    </p><p>
    <code class="code">cp libstdc++.pot de_DE.po</code>
    </p></li><li><p>
        Edit localized catalogs in emacs so that strings are
        translated.
      </p><p>
    <code class="code">emacs fr_FR.po</code>
    </p></li><li><p>Make the binary mo files.</p><p>
    <code class="code">msgfmt fr_FR.po -o fr_FR.mo</code>
    </p><p>
    <code class="code">msgfmt de_DE.po -o de_DE.mo</code>
    </p></li><li><p>Copy the binary files into the correct directory structure.</p><p>
    <code class="code">cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++.mo</code>
    </p><p>
    <code class="code">cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++.mo</code>
    </p></li><li><p>Use the new message catalogs.</p><p>
    <code class="code">locale loc_de("de_DE");</code>
    </p><p>
    <code class="code">
    use_facet&lt;messages&lt;char&gt; &gt;(loc_de).open("libstdc++", locale(), dir);
    </code>
    </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>
    A simple example using the GNU model of message conversion.
  </p><pre class="programlisting">
 #include &lt;iostream&gt;
 #include &lt;locale&gt;
 using namespace std;

 void test01()
 {
   typedef messages&lt;char&gt;::catalog catalog;
   const char* dir =
   "/mnt/egcs/build/i686-pc-linux-gnu/libstdc++/po/share/locale";
   const locale loc_de("de_DE");
   const messages&lt;char&gt;&amp; mssg_de = use_facet&lt;messages&lt;char&gt; &gt;(loc_de);

   catalog cat_de = mssg_de.open("libstdc++", loc_de, dir);
   string s01 = mssg_de.get(cat_de, 0, 0, "please");
   string s02 = mssg_de.get(cat_de, 0, 0, "thank you");
   cout &lt;&lt; "please in german:" &lt;&lt; s01 &lt;&lt; '\n';
   cout &lt;&lt; "thank you in german:" &lt;&lt; s02 &lt;&lt; '\n';
   mssg_de.close(cat_de);
 }
 </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>
     Things that are sketchy, or remain unimplemented:
   </p><div class="itemizedlist"><ul type="circle"><li><p>
 	  _M_convert_from_char, _M_convert_to_char are in flux,
 	  depending on how the library ends up doing character set
 	  conversions. It might not be possible to do a real character
 	  set based conversion, due to the fact that the template
 	  parameter for messages is not enough to instantiate the
 	  codecvt facet (1 supplied, need at least 2 but would prefer
 	  3).
 	</p></li><li><p>
 	  There are issues with gettext needing the global locale set
 	  to extract a message. This dependence on the global locale
 	  makes the current "gnu" model non MT-safe. Future versions
 	  of glibc, i.e. glibc 2.3.x will fix this, and the C++ library
 	  bits are already in place.
 	</p></li></ul></div></li><li><p>
     Development versions of the GNU "C" library, glibc 2.3 will allow
     a more efficient, MT implementation of std::messages, and will
     allow the removal of the _M_name_messages data member. If this is
     done, it will change the library ABI. The C++ parts to support
     glibc 2.3 have already been coded, but are not in use: once this
     version of the "C" library is released, the marked parts of the
     messages implementation can be switched over to the new "C"
     library functionality.
   </p></li><li><p>
     At some point in the near future, std::numpunct will probably use
     std::messages facilities to implement truename/falsename
     correctly. This is currently not done, but entries in
     libstdc++.pot have already been made for "true" and "false" string
     literals, so all that remains is the std::numpunct coding and the
     configure/make hassles to make the installed library search its
     own catalog. Currently the libstdc++.mo catalog is only searched
     for the testsuite cases involving messages members.
   </p></li><li><p> The following member functions:</p><p>
    <code class="code">
         catalog
         open(const basic_string&lt;char&gt;&amp; __s, const locale&amp; __loc) const
    </code>
    </p><p>
    <code class="code">
    catalog
    open(const basic_string&lt;char&gt;&amp;, const locale&amp;, const char*) const;
    </code>
    </p><p>
    Don't actually return a "value less than 0 if no such catalog
    can be opened" as required by the standard in the "gnu"
    model. As of this writing, it is unknown how to query to see
    if a specified message catalog exists using the gettext
    package.
    </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="id427517"></a><p><span class="title"><i>
       The GNU C Library
     </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
     . </span></p></div><div class="biblioentry"><a id="id391329"></a><p><span class="title"><i>
       Correspondence
     </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="id391358"></a><p><span class="title"><i>
       ISO/IEC 14882:1998 Programming languages - C++
     </i>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id514997"></a><p><span class="title"><i>
       ISO/IEC 9899:1999 Programming languages - C
     </i>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry"><a id="id515015"></a><p><span class="title"><i>
       System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x)
     </i>. </span><span class="copyright">Copyright © 1999
       The Open Group/The Institute of Electrical and Electronics Engineers, Inc.. </span><span class="biblioid">
       <a class="ulink" href="http://www.opennc.org/austin/docreg.html" target="_top">
       </a>
     . </span></p></div><div class="biblioentry"><a id="id427733"></a><p><span class="title"><i>
       The C++ Programming Language, Special Edition
     </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">
 	Addison Wesley
       . </span></span></p></div><div class="biblioentry"><a id="id427774"></a><p><span class="title"><i>
       Standard C++ IOStreams and Locales
     </i>. </span><span class="subtitle">
       Advanced Programmer's Guide and Reference
     . </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">
 	Addison Wesley Longman
       . </span></span></p></div><div class="biblioentry"><a id="id402618"></a><p><span class="title"><i>
       Java 2 Platform, Standard Edition, v 1.3.1 API Specification
     </i>. </span><span class="pagenums">java.util.Properties, java.text.MessageFormat,
 java.util.Locale, java.util.ResourceBundle. </span><span class="biblioid">
       <a class="ulink" href="http://java.sun.com/j2se/1.3/docs/api" target="_top">
       </a>
     . </span></p></div><div class="biblioentry"><a id="id404627"></a><p><span class="title"><i>
        GNU gettext tools, version 0.10.38, Native Language Support
 Library and Tools.
     </i>. </span><span class="biblioid">
       <a class="ulink" href="http://sources.redhat.com/gettext" target="_top">
       </a>
     . </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="facets.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.
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	<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 V1.74.0" /><meta name="keywords" content=" ISO C++ , messages " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="facets.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>
	The std::messages facet implements message retrieval functionality
	equivalent to Java's java.text.MessageFormat .using either GNU gettext
	or IEEE 1003.1-200 functions.
	</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>
	The std::messages facet is probably the most vaguely defined facet in
	the standard library. It's assumed that this facility was built into
	the standard library in order to convert string literals from one
	locale to the other. For instance, converting the "C" locale's
	<code class="code">const char* c = "please"</code> to a German-localized <code class="code">"bitte"</code>
	during program execution.
	</p><div class="blockquote"><blockquote class="blockquote"><p>
	22.2.7.1 - Template class messages [lib.locale.messages]
	</p></blockquote></div><p>
	This class has three public member functions, which directly
	correspond to three protected virtual member functions.
	</p><p>
	The public member functions are:
	</p><p>
	<code class="code">catalog open(const string&, const locale&) const</code>
	</p><p>
	<code class="code">string_type get(catalog, int, int, const string_type&) const</code>
	</p><p>
	<code class="code">void close(catalog) const</code>
	</p><p>
	While the virtual functions are:
	</p><p>
	<code class="code">catalog do_open(const string&, const locale&) const</code>
	</p><div class="blockquote"><blockquote class="blockquote"><p>
	<span class="emphasis"><em>
	-1- Returns: A value that may be passed to get() to retrieve a
	message, from the message catalog identified by the string name
	according to an implementation-defined mapping. The result can be used
	until it is passed to close(). Returns a value less than 0 if no such
	catalog can be opened.
	</em></span>
	</p></blockquote></div><p>
	<code class="code">string_type do_get(catalog, int, int, const string_type&) const</code>
	</p><div class="blockquote"><blockquote class="blockquote"><p>
	<span class="emphasis"><em>
	-3- Requires: A catalog cat obtained from open() and not yet closed.
	-4- Returns: A message identified by arguments set, msgid, and dfault,
	according to an implementation-defined mapping. If no such message can
	be found, returns dfault.
	</em></span>
	</p></blockquote></div><p>
	<code class="code">void do_close(catalog) const</code>
	</p><div class="blockquote"><blockquote class="blockquote"><p>
	<span class="emphasis"><em>
	-5- Requires: A catalog cat obtained from open() and not yet closed.
	-6- Effects: Releases unspecified resources associated with cat.
	-7- Notes: The limit on such resources, if any, is implementation-defined.
	</em></span>
	</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>
	A couple of notes on the standard.
	</p><p>
	First, why is <code class="code">messages_base::catalog</code> specified as a typedef
	to int? This makes sense for implementations that use
	<code class="code">catopen</code>, but not for others. Fortunately, it's not heavily
	used and so only a minor irritant.
	</p><p>
	Second, by making the member functions <code class="code">const</code>, it is
	impossible to save state in them. Thus, storing away information used
	in the 'open' member function for use in 'get' is impossible. This is
	unfortunate.
	</p><p>
	The 'open' member function in particular seems to be oddly
	designed. The signature seems quite peculiar. Why specify a <code class="code">const
	string& </code> argument, for instance, instead of just <code class="code">const
	char*</code>? Or, why specify a <code class="code">const locale&</code> argument that is
	to be used in the 'get' member function? How, exactly, is this locale
	argument useful? What was the intent? It might make sense if a locale
	argument was associated with a given default message string in the
	'open' member function, for instance. Quite murky and unclear, on
	reflection.
	</p><p>
	Lastly, it seems odd that messages, which explicitly require code
	conversion, don't use the codecvt facet. Because the messages facet
	has only one template parameter, it is assumed that ctype, and not
	codecvt, is to be used to convert between character sets.
	</p><p>
	It is implicitly assumed that the locale for the default message
	string in 'get' is in the "C" locale. Thus, all source code is assumed
	to be written in English, so translations are always from "en_US" to
	other, explicitly named locales.
	</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>
	This is a relatively simple class, on the face of it. The standard
	specifies very little in concrete terms, so generic
	implementations that are conforming yet do very little are the
	norm. Adding functionality that would be useful to programmers and
	comparable to Java's java.text.MessageFormat takes a bit of work,
	and is highly dependent on the capabilities of the underlying
	operating system.
	</p><p>
	Three different mechanisms have been provided, selectable via
	configure flags:
	</p><div class="itemizedlist"><ul type="disc"><li><p>
	generic
	</p><p>
	This model does very little, and is what is used by default.
	</p></li><li><p>
	gnu
	</p><p>
	The gnu model is complete and fully tested. It's based on the
	GNU gettext package, which is part of glibc. It uses the
	functions <code class="code">textdomain, bindtextdomain, gettext</code> to
	implement full functionality. Creating message catalogs is a
	relatively straight-forward process and is lightly documented
	below, and fully documented in gettext's distributed
	documentation.
	</p></li><li><p>
	ieee_1003.1-200x
	</p><p>
	This is a complete, though untested, implementation based on
	the IEEE standard. The functions <code class="code">catopen, catgets,
	catclose</code> are used to retrieve locale-specific messages
	given the appropriate message catalogs that have been
	constructed for their use. Note, the script <code class="code">
	po2msg.sed</code> that is part of the gettext distribution can
	convert gettext catalogs into catalogs that
	<code class="code">catopen</code> can use.
	</p></li></ul></div><p>
	A new, standards-conformant non-virtual member function signature was
	added for 'open' so that a directory could be specified with a given
	message catalog. This simplifies calling conventions for the gnu
	model.
	</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>
	The messages facet, because it is retrieving and converting
	between characters sets, depends on the ctype and perhaps the
	codecvt facet in a given locale. In addition, underlying "C"
	library locale support is necessary for more than just the
	<code class="code">LC_MESSAGES</code> mask: <code class="code">LC_CTYPE</code> is also
	necessary. To avoid any unpleasantness, all bits of the "C" mask
	(i.e. <code class="code">LC_ALL</code>) are set before retrieving messages.
	</p><p>
	Making the message catalogs can be initially tricky, but become
	quite simple with practice. For complete info, see the gettext
	documentation. Here's an idea of what is required:
	</p><div class="itemizedlist"><ul type="disc"><li><p>
	Make a source file with the required string literals that need
	to be translated. See <code class="code">intl/string_literals.cc</code> for
	an example.
	</p></li><li><p>
	Make initial catalog (see "4 Making the PO Template File" from
	the gettext docs).</p><p>
	<code class="code"> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
	</p></li><li><p>Make language and country-specific locale catalogs.</p><p>
	<code class="code">cp libstdc++.pot fr_FR.po</code>
	</p><p>
	<code class="code">cp libstdc++.pot de_DE.po</code>
	</p></li><li><p>
	Edit localized catalogs in emacs so that strings are
	translated.
	</p><p>
	<code class="code">emacs fr_FR.po</code>
	</p></li><li><p>Make the binary mo files.</p><p>
	<code class="code">msgfmt fr_FR.po -o fr_FR.mo</code>
	</p><p>
	<code class="code">msgfmt de_DE.po -o de_DE.mo</code>
	</p></li><li><p>Copy the binary files into the correct directory structure.</p><p>
	<code class="code">cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++.mo</code>
	</p><p>
	<code class="code">cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++.mo</code>
	</p></li><li><p>Use the new message catalogs.</p><p>
	<code class="code">locale loc_de("de_DE");</code>
	</p><p>
	<code class="code">
	use_facet<messages<char> >(loc_de).open("libstdc++", locale(), dir);
	</code>
	</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>
	A simple example using the GNU model of message conversion.
	</p><pre class="programlisting">
	#include <iostream>
	#include <locale>
	using namespace std;

	void test01()
	{
	typedef messages<char>::catalog catalog;
	const char* dir =
	"/mnt/egcs/build/i686-pc-linux-gnu/libstdc++/po/share/locale";
	const locale loc_de("de_DE");
	const messages<char>& mssg_de = use_facet<messages<char> >(loc_de);

	catalog cat_de = mssg_de.open("libstdc++", loc_de, dir);
	string s01 = mssg_de.get(cat_de, 0, 0, "please");
	string s02 = mssg_de.get(cat_de, 0, 0, "thank you");
	cout << "please in german:" << s01 << '\n';
	cout << "thank you in german:" << s02 << '\n';
	mssg_de.close(cat_de);
	}
	</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>
	Things that are sketchy, or remain unimplemented:
	</p><div class="itemizedlist"><ul type="circle"><li><p>
	_M_convert_from_char, _M_convert_to_char are in flux,
	depending on how the library ends up doing character set
	conversions. It might not be possible to do a real character
	set based conversion, due to the fact that the template
	parameter for messages is not enough to instantiate the
	codecvt facet (1 supplied, need at least 2 but would prefer
	3).
	</p></li><li><p>
	There are issues with gettext needing the global locale set
	to extract a message. This dependence on the global locale
	makes the current "gnu" model non MT-safe. Future versions
	of glibc, i.e. glibc 2.3.x will fix this, and the C++ library
	bits are already in place.
	</p></li></ul></div></li><li><p>
	Development versions of the GNU "C" library, glibc 2.3 will allow
	a more efficient, MT implementation of std::messages, and will
	allow the removal of the _M_name_messages data member. If this is
	done, it will change the library ABI. The C++ parts to support
	glibc 2.3 have already been coded, but are not in use: once this
	version of the "C" library is released, the marked parts of the
	messages implementation can be switched over to the new "C"
	library functionality.
	</p></li><li><p>
	At some point in the near future, std::numpunct will probably use
	std::messages facilities to implement truename/falsename
	correctly. This is currently not done, but entries in
	libstdc++.pot have already been made for "true" and "false" string
	literals, so all that remains is the std::numpunct coding and the
	configure/make hassles to make the installed library search its
	own catalog. Currently the libstdc++.mo catalog is only searched
	for the testsuite cases involving messages members.
	</p></li><li><p> The following member functions:</p><p>
	<code class="code">
	catalog
	open(const basic_string<char>& __s, const locale& __loc) const
	</code>
	</p><p>
	<code class="code">
	catalog
	open(const basic_string<char>&, const locale&, const char*) const;
	</code>
	</p><p>
	Don't actually return a "value less than 0 if no such catalog
	can be opened" as required by the standard in the "gnu"
	model. As of this writing, it is unknown how to query to see
	if a specified message catalog exists using the gettext
	package.
	</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="id427517"></a><p><span class="title"><i>
	The GNU C Library
	</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
	. </span></p></div><div class="biblioentry"><a id="id391329"></a><p><span class="title"><i>
	Correspondence
	</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="id391358"></a><p><span class="title"><i>
	ISO/IEC 14882:1998 Programming languages - C++
	</i>. </span><span class="copyright">Copyright © 1998 ISO. </span></p></div><div class="biblioentry"><a id="id514997"></a><p><span class="title"><i>
	ISO/IEC 9899:1999 Programming languages - C
	</i>. </span><span class="copyright">Copyright © 1999 ISO. </span></p></div><div class="biblioentry"><a id="id515015"></a><p><span class="title"><i>
	System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x)
	</i>. </span><span class="copyright">Copyright © 1999
	The Open Group/The Institute of Electrical and Electronics Engineers, Inc.. </span><span class="biblioid">
	<a class="ulink" href="http://www.opennc.org/austin/docreg.html" target="_top">
	</a>
	. </span></p></div><div class="biblioentry"><a id="id427733"></a><p><span class="title"><i>
	The C++ Programming Language, Special Edition
	</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">
	Addison Wesley
	. </span></span></p></div><div class="biblioentry"><a id="id427774"></a><p><span class="title"><i>
	Standard C++ IOStreams and Locales
	</i>. </span><span class="subtitle">
	Advanced Programmer's Guide and Reference
	. </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">
	Addison Wesley Longman
	. </span></span></p></div><div class="biblioentry"><a id="id402618"></a><p><span class="title"><i>
	Java 2 Platform, Standard Edition, v 1.3.1 API Specification
	</i>. </span><span class="pagenums">java.util.Properties, java.text.MessageFormat,
	java.util.Locale, java.util.ResourceBundle. </span><span class="biblioid">
	<a class="ulink" href="http://java.sun.com/j2se/1.3/docs/api" target="_top">
	</a>
	. </span></p></div><div class="biblioentry"><a id="id404627"></a><p><span class="title"><i>
	GNU gettext tools, version 0.10.38, Native Language Support
	Library and Tools.
	</i>. </span><span class="biblioid">
	<a class="ulink" href="http://sources.redhat.com/gettext" target="_top">
	</a>
	. </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="facets.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.
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