Notes on the messages implementation.

prepared by Benjamin Kosnik (bkoz@redhat.com) on August 8, 2001

1. Abstract

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.

2. What the standard says

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 const char* c = "please" to a German-localized "bitte" during program execution.

22.2.7.1 - Template class messages [lib.locale.messages]

This class has three public member functions, which directly correspond to three protected virtual member functions. The public member functions are:

catalog open(const string&, const locale&) const

string_type get(catalog, int, int, const string_type&) const

void close(catalog) const

While the virtual functions are:

catalog do_open(const string&, const locale&) const

-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.

string_type do_get(catalog, int, int, const string_type&) const

-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.

void do_close(catalog) const

-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.

3. Problems with "C" messages: thread safety, over-specification, and assumptions.

A couple of notes on the standard.

First, why is messages_base::catalog specified as a typedef to int? This makes sense for implementations that use catopen, but not for others. Fortunately, it's not heavily used and so only a minor irritant.

Second, by making the member functions const, 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.

The 'open' member function in particular seems to be oddly designed. The signature seems quite peculiar. Why specify a const string& argument, for instance, instead of just const char*? Or, why specify a const locale& 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.

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.

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.

4. Design and Implementation Details

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.

Three different mechanisms have been provided, selectable via configure flags:

generic
This model does very little, and is what is used by default.
gnu
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 textdomain, bindtextdomain, gettext 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.
ieee_1003.1-200x
This is a complete, though untested, implementation based on the IEEE standard. The functions catopen, catgets, catclose are used to retrieve locale-specific messages given the appropriate message catalogs that have been constructed for their use. Note, the script po2msg.sed that is part of the gettext distribution can convert gettext catalogs into catalogs that catopen can use.

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.

The rest of this document discusses details of the GNU model.

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 LC_MESSAGES mask: LC_CTYPE is also necessary. To avoid any unpleasantness, all bits of the "C" mask (ie LC_ALL) are set before retrieving messages.

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:

Make a source file with the required string literals that need to be translated. See intl/string_literals.cc for an example.
Make initial catalog (see "4 Making the PO Template File" from the gettext docs).
xgettext --c++ --debug string_literals.cc -o libstdc++.pot
Make language and country-specific locale catalogs.
cp libstdc++.pot fr_FR.po

cp libstdc++.pot de_DE.po
Edit localized catalogs in emacs so that strings are translated.
emacs fr_FR.po
Make the binary mo files.
msgfmt fr_FR.po -o fr_FR.mo

msgfmt de_DE.po -o de_DE.mo
Copy the binary files into the correct directory structure.
cp fr_FR.mo (dir)/fr_FR/LC_MESSAGES/libstdc++-v3.mo

cp de_DE.mo (dir)/de_DE/LC_MESSAGES/libstdc++-v3.mo
Use the new message catalogs.
locale loc_de("de_DE");

use_facet<messages<char> >(loc_de).open("libstdc++", locale(), dir);

5. Examples

message converting, simple example using the GNU model.

#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);
}

More information can be found in the following testcases:

testsuite/22_locale/messages.cc
testsuite/22_locale/messages_byname.cc
testsuite/22_locale/messages_char_members.cc

6. Unresolved Issues

Things that are sketchy, or remain unimplemented:
- _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).
- 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.
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.
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.
The following member functions:
catalog open(const basic_string<char>& __s, const locale& __loc) const

catalog open(const basic_string<char>&, const locale&, const char*) const;

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.

7. Acknowledgments

Ulrich Drepper for the character set explanations, gettext details, and patient answering of late-night questions, Tom Tromey for the java details.

8. Bibliography / Referenced Documents

Drepper, Ulrich, GNU libc (glibc) 2.2 manual. In particular, Chapters "7 Locales and Internationalization"

Drepper, Ulrich, Thread-Aware Locale Model, A proposal. This is a draft document describing the design of glibc 2.3 MT locale functionality.

Drepper, Ulrich, Numerous, late-night email correspondence

ISO/IEC 9899:1999 Programming languages - C

ISO/IEC 14882:1998 Programming languages - C++

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

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

GNU gettext tools, version 0.10.38, Native Language Support Library and Tools. http://sources.redhat.com/gettext

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.

Stroustrup, Bjarne, Appendix D, The C++ Programming Language, Special Edition, Addison Wesley, Inc. 2000