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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." />
- <title>Notes on the messages implementation.</title>
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-<h1>
-Notes on the messages implementation.
-</h1>
-<em>
-prepared by Benjamin Kosnik (bkoz@redhat.com) on August 8, 2001
-</em>
-
-<h2>
-1. Abstract
-</h2>
-<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>
-
-<h2>
-2. What the standard says
-</h2>
-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.
-
-<blockquote>
-22.2.7.1 - Template class messages [lib.locale.messages]
-</blockquote>
-
-This class has three public member functions, which directly
-correspond to three protected virtual member functions.
-
-The public member functions are:
-
-<p>
-<code>catalog open(const string&, const locale&) const</code>
-</p>
-
-<p>
-<code>string_type get(catalog, int, int, const string_type&) const</code>
-</p>
-
-<p>
-<code>void close(catalog) const</code>
-</p>
-
-<p>
-While the virtual functions are:
-</p>
-
-<p>
-<code>catalog do_open(const string&, const locale&) const</code>
-</p>
-<blockquote>
-<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>
-</blockquote>
-
-<p>
-<code>string_type do_get(catalog, int, int, const string_type&) const</code>
-</p>
-<blockquote>
-<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>
-</blockquote>
-
-<p>
-<code>void do_close(catalog) const</code>
-</p>
-<blockquote>
-<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>
-</blockquote>
-
-
-<h2>
-3. Problems with "C" messages: thread safety,
-over-specification, and assumptions.
-</h2>
-A couple of notes on the standard.
-
-<p>
-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.
-</p>
-
-<p>
-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.
-</p>
-
-<p>
-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.
-</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>
-
-<h2>
-4. Design and Implementation Details
-</h2>
-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>
-Three different mechanisms have been provided, selectable via
-configure flags:
-</p>
-
-<ul>
- <li> generic
- <p>
- This model does very little, and is what is used by default.
- </p>
- </li>
-
- <li> gnu
- <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>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> ieee_1003.1-200x
- <p>
- 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.
- </p>
- </li>
-</ul>
-
-<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>
-
-<p>
-The rest of this document discusses details of the GNU model.
-</p>
-
-<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>LC_MESSAGES</code> mask:
-<code>LC_CTYPE</code> is also necessary. To avoid any unpleasantness, all
-bits of the "C" mask (ie <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>
-
-<ul>
- <li> Make a source file with the required string literals
- that need to be translated. See
- <code>intl/string_literals.cc</code> for an example.
- </li>
-
- <li> Make initial catalog (see "4 Making the PO Template File"
- from the gettext docs).
- <p>
- <code> xgettext --c++ --debug string_literals.cc -o libstdc++.pot </code>
- </p>
- </li>
-
- <li> Make language and country-specific locale catalogs.
- <p>
- <code>cp libstdc++.pot fr_FR.po</code>
- </p>
- <p>
- <code>cp libstdc++.pot de_DE.po</code>
- </p>
- </li>
-
- <li> Edit localized catalogs in emacs so that strings are
- translated.
- <p>
- <code>emacs fr_FR.po</code>
- </p>
- </li>
-
- <li> Make the binary mo files.
- <p>
- <code>msgfmt fr_FR.po -o fr_FR.mo</code>
- </p>
- <p>
- <code>msgfmt de_DE.po -o de_DE.mo</code>
- </p>
- </li>
-
- <li> Copy the binary files into the correct directory structure.
- <p>
- <code>cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++-v3.mo</code>
- </p>
- <p>
- <code>cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++-v3.mo</code>
- </p>
- </li>
-
- <li> Use the new message catalogs.
- <p>
- <code>locale loc_de("de_DE");</code>
- </p>
- <p>
- <code>
- use_facet<messages<char> >(loc_de).open("libstdc++", locale(), dir);
- </code>
- </p>
- </li>
-</ul>
-
-<h2>
-5. Examples
-</h2>
-
-<ul>
- <li> message converting, simple example using the GNU model.
-
-<pre>
-#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++-v3/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>
- </li>
-</ul>
-
-More information can be found in the following testcases:
-<ul>
-<li> testsuite/22_locale/messages.cc </li>
-<li> testsuite/22_locale/messages_byname.cc </li>
-<li> testsuite/22_locale/messages_char_members.cc </li>
-</ul>
-
-<h2>
-6. Unresolved Issues
-</h2>
-<ul>
-<li> Things that are sketchy, or remain unimplemented:
- <ul>
- <li>_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).
- </li>
-
- <li> 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, ie glibc 2.3.x will
- fix this, and the C++ library bits are already in
- place.
- </li>
- </ul>
-</li>
-
-<li> 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.
-</li>
-<li> At some point in the near future, std::numpunct will probably use
- std::messages facilities to implement truename/falename
- 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.
-</li>
-
-<li> The following member functions:
-
- <p>
- <code>
- catalog
- open(const basic_string<char>& __s, const locale& __loc) const
- </code>
- </p>
-
- <p>
- <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>
-
-<h2>
-7. Acknowledgments
-</h2>
-Ulrich Drepper for the character set explanations, gettext details,
-and patient answering of late-night questions, Tom Tromey for the java details.
-
-
-<h2>
-8. Bibliography / Referenced Documents
-</h2>
-
-Drepper, Ulrich, GNU libc (glibc) 2.2 manual. In particular, Chapters
-"7 Locales and Internationalization"
-
-<p>
-Drepper, Ulrich, Thread-Aware Locale Model, A proposal. This is a
-draft document describing the design of glibc 2.3 MT locale
-functionality.
-</p>
-
-<p>
-Drepper, Ulrich, Numerous, late-night email correspondence
-</p>
-
-<p>
-ISO/IEC 9899:1999 Programming languages - C
-</p>
-
-<p>
-ISO/IEC 14882:1998 Programming languages - C++
-</p>
-
-<p>
-Java 2 Platform, Standard Edition, v 1.3.1 API Specification. In
-particular, java.util.Properties, java.text.MessageFormat,
-java.util.Locale, java.util.ResourceBundle.
-http://java.sun.com/j2se/1.3/docs/api
-</p>
-
-<p>
-System Interface Definitions, Issue 7 (IEEE Std. 1003.1-200x)
-The Open Group/The Institute of Electrical and Electronics Engineers, Inc.
-In particular see lines 5268-5427.
-http://www.opennc.org/austin/docreg.html
-</p>
-
-<p> GNU gettext tools, version 0.10.38, Native Language Support
-Library and Tools.
-http://sources.redhat.com/gettext
-</p>
-
-<p>
-Langer, Angelika and Klaus Kreft, Standard C++ IOStreams and Locales,
-Advanced Programmer's Guide and Reference, Addison Wesley Longman,
-Inc. 2000. See page 725, Internationalized Messages.
-</p>
-
-<p>
-Stroustrup, Bjarne, Appendix D, The C++ Programming Language, Special Edition, Addison Wesley, Inc. 2000
-</p>
-
-</body>
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