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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.
+ Containers
+
+</td></tr></table></div></body></html>
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