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+<sect1 id="manual.localization.facet.messages" xreflabel="messages">
+<?dbhtml filename="messages.html"?>
+
+<sect1info>
+ <keywordset>
+ <keyword>
+ ISO C++
+ </keyword>
+ <keyword>
+ messages
+ </keyword>
+ </keywordset>
+</sect1info>
+
+<title>messages</title>
+
+<para>
+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.
+</para>
+
+<sect2 id="facet.messages.req" xreflabel="facet.messages.req">
+<title>Requirements</title>
+
+<para>
+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>const char* c = "please"</code> to a German-localized <code>"bitte"</code>
+during program execution.
+</para>
+
+<blockquote>
+<para>
+22.2.7.1 - Template class messages [lib.locale.messages]
+</para>
+</blockquote>
+
+<para>
+This class has three public member functions, which directly
+correspond to three protected virtual member functions.
+</para>
+
+<para>
+The public member functions are:
+</para>
+
+<para>
+<code>catalog open(const string&, const locale&) const</code>
+</para>
+
+<para>
+<code>string_type get(catalog, int, int, const string_type&) const</code>
+</para>
+
+<para>
+<code>void close(catalog) const</code>
+</para>
+
+<para>
+While the virtual functions are:
+</para>
+
+<para>
+<code>catalog do_open(const string&, const locale&) const</code>
+</para>
+<blockquote>
+<para>
+<emphasis>
+-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.
+</emphasis>
+</para>
+</blockquote>
+
+<para>
+<code>string_type do_get(catalog, int, int, const string_type&) const</code>
+</para>
+<blockquote>
+<para>
+<emphasis>
+-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.
+</emphasis>
+</para>
+</blockquote>
+
+<para>
+<code>void do_close(catalog) const</code>
+</para>
+<blockquote>
+<para>
+<emphasis>
+-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.
+</emphasis>
+</para>
+</blockquote>
+
+
+</sect2>
+
+<sect2 id="facet.messages.design" xreflabel="facet.messages.design">
+<title>Design</title>
+
+<para>
+A couple of notes on the standard.
+</para>
+
+<para>
+First, why is <code>messages_base::catalog</code> specified as a typedef
+to int? This makes sense for implementations that use
+<code>catopen</code>, but not for others. Fortunately, it's not heavily
+used and so only a minor irritant.
+</para>
+
+<para>
+Second, by making the member functions <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.
+</para>
+
+<para>
+The 'open' member function in particular seems to be oddly
+designed. The signature seems quite peculiar. Why specify a <code>const
+string& </code> argument, for instance, instead of just <code>const
+char*</code>? Or, why specify a <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.
+</para>
+
+<para>
+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.
+</para>
+
+<para>
+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.
+</para>
+
+</sect2>
+
+<sect2 id="facet.messages.impl" xreflabel="facet.messages.impl">
+<title>Implementation</title>
+
+ <sect3 id="messages.impl.models" xreflabel="messages.impl.models">
+ <title>Models</title>
+ <para>
+ 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.
+ </para>
+
+ <para>
+ Three different mechanisms have been provided, selectable via
+ configure flags:
+ </para>
+
+<itemizedlist>
+ <listitem>
+ <para>
+ generic
+ </para>
+ <para>
+ This model does very little, and is what is used by default.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ gnu
+ </para>
+ <para>
+ 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>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.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ ieee_1003.1-200x
+ </para>
+ <para>
+ This is a complete, though untested, implementation based on
+ the IEEE standard. The functions <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>
+ po2msg.sed</code> that is part of the gettext distribution can
+ convert gettext catalogs into catalogs that
+ <code>catopen</code> can use.
+ </para>
+ </listitem>
+</itemizedlist>
+
+<para>
+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.
+</para>
+
+ </sect3>
+
+ <sect3 id="messages.impl.gnu" xreflabel="messages.impl.gnu">
+ <title>The GNU Model</title>
+
+ <para>
+ 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>LC_MESSAGES</code> mask: <code>LC_CTYPE</code> is also
+ necessary. To avoid any unpleasantness, all bits of the "C" mask
+ (i.e. <code>LC_ALL</code>) are set before retrieving messages.
+ </para>
+
+ <para>
+ 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:
+ </para>
+
+<itemizedlist>
+ <listitem>
+ <para>
+ Make a source file with the required string literals that need
+ to be translated. See <code>intl/string_literals.cc</code> for
+ an example.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Make initial catalog (see "4 Making the PO Template File" from
+ the gettext docs).</para>
+ <para>
+ <code> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>Make language and country-specific locale catalogs.</para>
+ <para>
+ <code>cp libstdc++.pot fr_FR.po</code>
+ </para>
+ <para>
+ <code>cp libstdc++.pot de_DE.po</code>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Edit localized catalogs in emacs so that strings are
+ translated.
+ </para>
+ <para>
+ <code>emacs fr_FR.po</code>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>Make the binary mo files.</para>
+ <para>
+ <code>msgfmt fr_FR.po -o fr_FR.mo</code>
+ </para>
+ <para>
+ <code>msgfmt de_DE.po -o de_DE.mo</code>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>Copy the binary files into the correct directory structure.</para>
+ <para>
+ <code>cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++.mo</code>
+ </para>
+ <para>
+ <code>cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++.mo</code>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>Use the new message catalogs.</para>
+ <para>
+ <code>locale loc_de("de_DE");</code>
+ </para>
+ <para>
+ <code>
+ use_facet<messages<char> >(loc_de).open("libstdc++", locale(), dir);
+ </code>
+ </para>
+ </listitem>
+</itemizedlist>
+
+ </sect3>
+</sect2>
+
+<sect2 id="facet.messages.use" xreflabel="facet.messages.use">
+<title>Use</title>
+ <para>
+ A simple example using the GNU model of message conversion.
+ </para>
+
+<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);
+}
+</programlisting>
+
+</sect2>
+
+<sect2 id="facet.messages.future" xreflabel="facet.messages.future">
+<title>Future</title>
+
+<itemizedlist>
+<listitem>
+ <para>
+ Things that are sketchy, or remain unimplemented:
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>
+ _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).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ 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.
+ </para>
+ </listitem>
+ </itemizedlist>
+</listitem>
+
+<listitem>
+ <para>
+ 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.
+ </para>
+</listitem>
+<listitem>
+ <para>
+ 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.
+ </para>
+</listitem>
+
+<listitem>
+ <para> The following member functions:</para>
+
+ <para>
+ <code>
+ catalog
+ open(const basic_string<char>& __s, const locale& __loc) const
+ </code>
+ </para>
+
+ <para>
+ <code>
+ catalog
+ open(const basic_string<char>&, const locale&, const char*) const;
+ </code>
+ </para>
+
+ <para>
+ 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.
+ </para>
+</listitem>
+</itemizedlist>
+
+</sect2>
+
+<bibliography id="facet.messages.biblio" xreflabel="facet.messages.biblio">
+<title>Bibliography</title>
+
+ <biblioentry>
+ <title>
+ The GNU C Library
+ </title>
+
+ <author>
+ <surname>McGrath</surname>
+ <firstname>Roland</firstname>
+ </author>
+ <author>
+ <surname>Drepper</surname>
+ <firstname>Ulrich</firstname>
+ </author>
+
+ <copyright>
+ <year>2007</year>
+ <holder>FSF</holder>
+ </copyright>
+ <pagenums>Chapters 6 Character Set Handling, and 7 Locales and Internationalization
+ </pagenums>
+
+ </biblioentry>
+
+ <biblioentry>
+ <title>
+ Correspondence
+ </title>
+
+ <author>
+ <surname>Drepper</surname>
+ <firstname>Ulrich</firstname>
+ </author>
+
+ <copyright>
+ <year>2002</year>
+ <holder></holder>
+ </copyright>
+ </biblioentry>
+
+ <biblioentry>
+ <title>
+ ISO/IEC 14882:1998 Programming languages - C++
+ </title>
+
+ <copyright>
+ <year>1998</year>
+ <holder>ISO</holder>
+ </copyright>
+ </biblioentry>
+
+ <biblioentry>
+ <title>
+ ISO/IEC 9899:1999 Programming languages - C
+ </title>
+
+ <copyright>
+ <year>1999</year>
+ <holder>ISO</holder>
+ </copyright>
+ </biblioentry>
+
+ <biblioentry>
+ <title>
+ System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x)
+ </title>
+
+ <copyright>
+ <year>1999</year>
+ <holder>
+ The Open Group/The Institute of Electrical and Electronics Engineers, Inc.</holder>
+ </copyright>
+
+ <biblioid>
+ <ulink url="http://www.opennc.org/austin/docreg.html">
+ </ulink>
+ </biblioid>
+
+ </biblioentry>
+
+ <biblioentry>
+ <title>
+ The C++ Programming Language, Special Edition
+ </title>
+
+ <author>
+ <surname>Stroustrup</surname>
+ <firstname>Bjarne</firstname>
+ </author>
+
+ <copyright>
+ <year>2000</year>
+ <holder>Addison Wesley, Inc.</holder>
+ </copyright>
+ <pagenums>Appendix D</pagenums>
+
+ <publisher>
+ <publishername>
+ Addison Wesley
+ </publishername>
+ </publisher>
+
+ </biblioentry>
+
+
+ <biblioentry>
+ <title>
+ Standard C++ IOStreams and Locales
+ </title>
+ <subtitle>
+ Advanced Programmer's Guide and Reference
+ </subtitle>
+
+ <author>
+ <surname>Langer</surname>
+ <firstname>Angelika</firstname>
+ </author>
+
+ <author>
+ <surname>Kreft</surname>
+ <firstname>Klaus</firstname>
+ </author>
+
+ <copyright>
+ <year>2000</year>
+ <holder>Addison Wesley Longman, Inc.</holder>
+ </copyright>
+
+ <publisher>
+ <publishername>
+ Addison Wesley Longman
+ </publishername>
+ </publisher>
+
+ </biblioentry>
+
+ <biblioentry>
+ <title>
+ Java 2 Platform, Standard Edition, v 1.3.1 API Specification
+ </title>
+ <pagenums>java.util.Properties, java.text.MessageFormat,
+java.util.Locale, java.util.ResourceBundle</pagenums>
+ <biblioid>
+ <ulink url="http://java.sun.com/j2se/1.3/docs/api">
+ </ulink>
+ </biblioid>
+ </biblioentry>
+
+ <biblioentry>
+ <title>
+ GNU gettext tools, version 0.10.38, Native Language Support
+Library and Tools.
+ </title>
+ <biblioid>
+ <ulink url="http://sources.redhat.com/gettext">
+ </ulink>
+ </biblioid>
+ </biblioentry>
+
+</bibliography>
+
+</sect1>
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