path: root/doc/i18n.doc

/****************************************************************************
**
** Explanation of moc and the meta object system
**
** Copyright (C) 1992-2008 Trolltech ASA.  All rights reserved.
**
** This file is part of the Qt GUI Toolkit.
**
** This file may be used under the terms of the GNU General
** Public License versions 2.0 or 3.0 as published by the Free
** Software Foundation and appearing in the files LICENSE.GPL2
** and LICENSE.GPL3 included in the packaging of this file.
** Alternatively you may (at your option) use any later version
** of the GNU General Public License if such license has been
** publicly approved by Trolltech ASA (or its successors, if any)
** and the KDE Free Qt Foundation.
**
** Please review the following information to ensure GNU General
** Public Licensing requirements will be met:
** http://trolltech.com/products/qt/licenses/licensing/opensource/.
** If you are unsure which license is appropriate for your use, please
** review the following information:
** http://trolltech.com/products/qt/licenses/licensing/licensingoverview
** or contact the sales department at [email protected].
**
** This file may be used under the terms of the Q Public License as
** defined by Trolltech ASA and appearing in the file LICENSE.QPL
** included in the packaging of this file.  Licensees holding valid Qt
** Commercial licenses may use this file in accordance with the Qt
** Commercial License Agreement provided with the Software.
**
** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
** herein.
**
**********************************************************************/

/*! \defgroup i18n

\title Internationalization with Qt

\keyword internationalization
\keyword i18n

The internationalization of an application is the process of making
the application usable by people in countries other than one's own. 

\tableofcontents

In some cases internationalization is simple, for example, making a US
application accessible to Australian or British users may require
little more than a few spelling corrections. But to make a US
application usable by Japanese users, or a Korean application usable
by German users, will require that the software operate not only in
different languages, but use different input techniques, character
encodings and presentation conventions.

Qt tries to make internationalization as painless as possible for
developers. All input widgets and text drawing methods in Qt offer
built-in support for all supported languages. The built-in font engine
is capable of correctly and attractively rendering text that contains
characters from a variety of different writing systems at the same
time.

Qt supports most languages in use today, in particular:
\list
\i All East Asian languages (Chinese, Japanese and Korean)
\i All Western languages (using Latin script)
\i Arabic
\i Cyrillic languages (Russian)
\i Greek
\i Hebrew
\i Thai and Lao
\i All scripts in Unicode 3.2 that do not require special processing
\endlist

On Windows NT/2000/XP and Unix/X11 with Xft (client side font support)
the following languages are also supported:
\list
\i Bengali
\i Devanagari
\i Dhivehi (Thaana)
\i Gujarati
\i Gurmukhi
\i Kannada
\i Khmer
\i Malayalam (X11 only)
\i Myanmar (X11 only)
\i Syriac
\i Tamil
\i Telugu
\i Tibetan (X11 only)
\endlist

Many of these writing systems exhibit special features:

\list

\i <b>Special line breaking behavior.</b> Some of the Asian languages are
written without spaces between words. Line breaking can occur either
after every character (with exceptions) as in Chinese, Japanese and
Korean, or after logical word boundaries as in Thai.

\i <b>Bidirectional writing.</b> Arabic and Hebrew are written from right to
left, except for numbers and embedded English text which is written
left to right. The exact behavior is defined in the \link
http://www.unicode.org/unicode/reports/tr9/ Unicode Technical Report
#9 \endlink.

\i <b>Non spacing or diacritical marks</b> (accents or umlauts in European
languages). Some languages such as Vietnamese make extensive use of
these marks and some characters can have more than one mark at the
same time to clarify pronunciation.

\i <b>Ligatures.</b> In special contexts, some pairs of characters get
replaced by a combined glyph forming a ligature. Common examples are
the fl and fi ligatures used in typesetting US and European books.

\endlist

Qt tries to take care of all the special features listed above. You
usually don't have to worry about these features so long as you use
Qt's input widgets (e.g. QLineEdit, QTextEdit, and derived classes)
and Qt's display widgets (e.g. QLabel).

Support for these writing systems is transparent to the programmer
and completely encapsulated in Qt's text engine. This means that you
don't need to have any knowledge about the writing system used in a
particular language, except for the following small points:
\list

\i QPainter::drawText( int x, int y, const QString &str ) will always
draw the string with it's left edge at the position specified with
the x, y parameters. This will usually give you left aligned strings.
Arabic and Hebrew application strings are usually right
aligned, so for these languages use the version of drawText() that
takes a QRect since this will align in accordance with the language.

\i When you write your own text input controls, use \l
QFontMetrics::charWidth() to determine the width of a character in a
string. In some languages (e.g. Arabic or languages from the Indian
subcontinent), the width and shape of a glyph changes depending on the
surrounding characters. Writing input controls usually requires a
certain knowledge of the scripts it is going to be used in. Usually
the easiest way is to subclass QLineEdit or QTextEdit.

\endlist

The following sections give some information on the status
of the internationalization (i18n) support in Qt.

See also the \link linguist-manual.book Qt Linguist\endlink manual.

\section1 Step by Step

Writing multi-platform international software with Qt is a gentle,
incremental process. Your software can become internationalized in
the following stages:

\section2 Use QString for all User-visible Text

Since QString uses the Unicode encoding internally, every
language in the world can be processed transparently using
familiar text processing operations. Also, since all Qt
functions that present text to the user take a QString as a
parameter, there is no char* to QString conversion overhead.

Strings that are in "programmer space" (such as QObject names
and file format texts) need not use QString; the traditional
char* or the QCString class will suffice.

You're unlikely to notice that you are using Unicode;
QString, and QChar are just like easier versions of the crude
const char* and char from traditional C.

\section2 Use tr() for all Literal Text

Wherever your program uses \c{"quoted text"} for text that will
be presented to the user, ensure that it is processed by the \l
QApplication::translate() function. Essentially all that is necessary
to achieve this is to use \l QObject::tr(). For example, assuming the
\c LoginWidget is a subclass of QWidget:

\code
    LoginWidget::LoginWidget()
    {
        QLabel *label = new QLabel( tr("Password:"), this );
        ...
    }
\endcode

This accounts for 99% of the user-visible strings you're likely to
write.

If the quoted text is not in a member function of a
QObject subclass, use either the tr() function of an
appropriate class, or the QApplication::translate() function
directly:

\code
    void some_global_function( LoginWidget *logwid )
    {
        QLabel *label = new QLabel(
                LoginWidget::tr("Password:"), logwid );
    }

    void same_global_function( LoginWidget *logwid )
    {
        QLabel *label = new QLabel(
                qApp->translate("LoginWidget", "Password:"),
                logwid );
    }
\endcode

If you need to have translatable text completely
outside a function, there are two macros to help: QT_TR_NOOP()
and QT_TRANSLATE_NOOP(). They merely mark the text for
extraction by the \e lupdate utility described below.
The macros expand to just the text (without the context).

Example of QT_TR_NOOP():
\code
    QString FriendlyConversation::greeting( int greet_type )
    {
        static const char* greeting_strings[] = {
            QT_TR_NOOP( "Hello" ),
            QT_TR_NOOP( "Goodbye" )
        };
        return tr( greeting_strings[greet_type] );
    }
\endcode

Example of QT_TRANSLATE_NOOP():
\code
    static const char* greeting_strings[] = {
        QT_TRANSLATE_NOOP( "FriendlyConversation", "Hello" ),
        QT_TRANSLATE_NOOP( "FriendlyConversation", "Goodbye" )
    };

    QString FriendlyConversation::greeting( int greet_type )
    {
        return tr( greeting_strings[greet_type] );
    }

    QString global_greeting( int greet_type )
    {
        return qApp->translate( "FriendlyConversation",
                                greeting_strings[greet_type] );
    }
\endcode

If you disable the const char* to QString automatic conversion
by compiling your software with the macro QT_NO_CAST_ASCII
defined, you'll be very likely to catch any strings you are
missing. See QString::fromLatin1() for more information.
Disabling the conversion can make programming a bit cumbersome.

If your source language uses characters outside Latin-1, you
might find QObject::trUtf8() more convenient than
QObject::tr(), as tr() depends on the
QApplication::defaultCodec(), which makes it more fragile than
QObject::trUtf8().

\section2 Use QKeySequence() for Accelerator Values

Accelerator values such as Ctrl+Q or Alt+F need to be
translated too. If you hardcode \c CTRL+Key_Q for "Quit" in
your application, translators won't be able to override
it. The correct idiom is

\code
    QPopupMenu *file = new QPopupMenu( this );
    file->insertItem( tr("&Quit"), this, SLOT(quit()),
                      QKeySequence(tr("Ctrl+Q", "File|Quit")) );
\endcode

\section2 Use QString::arg() for Dynamic Text

The QString::arg() functions offer a simple means for substituting
arguments:
\code
    void FileCopier::showProgress( int done, int total,
                                   const QString& current_file )
    {
        label.setText( tr("%1 of %2 files copied.\nCopying: %3")
                        .arg(done)
                        .arg(total)
                        .arg(current_file) );
    }
\endcode

In some languages the order of arguments may need to change, and this
can easily be achieved by changing the order of the % arguments. For
example:
\code
    QString s1 = "%1 of %2 files copied. Copying: %3";
    QString s2 = "Kopierer nu %3. Av totalt %2 filer er %1 kopiert.";

    qDebug( s1.arg(5).arg(10).arg("somefile.txt").ascii() );
    qDebug( s2.arg(5).arg(10).arg("somefile.txt").ascii() );
\endcode

produces the correct output in English and Norwegian:
\code
5 of 10 files copied. Copying: somefile.txt
Kopierer nu somefile.txt. Av totalt 10 filer er 5 kopiert.
\endcode

\section2 Produce Translations

Once you are using tr() throughout an application, you can start
producing translations of the user-visible text in your program.

\link linguist-manual.book Qt Linguist\endlink's manual provides
further information about Qt's translation tools, \e{Qt Linguist}, \e
lupdate and \e lrelease.

Translation of a Qt application is a three-step process:

\list 1

\i Run \e lupdate to extract translatable text from the C++ source
code of the Qt application, resulting in a message file for
translators (a \c .ts file). The utility recognizes the tr() construct
and the QT_*_NOOP macros described above and produces \c .ts files
(usually one per language).

\i Provide translations for the source texts in the \c .ts file, using
\e{Qt Linguist}. Since \c .ts files are in XML format, you can also
edit them by hand.

\i Run \e lrelease to obtain a light-weight message file (a \c .qm
file) from the \c .ts file, suitable only for end use. Think of the \c
.ts files as "source files", and \c .qm files as "object files". The
translator edits the \c .ts files, but the users of your application
only need the \c .qm files. Both kinds of files are platform and
locale independent.

\endlist

Typically, you will repeat these steps for every release of your
application. The \e lupdate utility does its best to reuse the
translations from previous releases.

Before you run \e lupdate, you should prepare a project file. Here's
an example project file (\c .pro file):

\code
    HEADERS         = funnydialog.h \
                      wackywidget.h
    SOURCES         = funnydialog.cpp \
                      main.cpp \
                      wackywidget.cpp
    FORMS           = fancybox.ui
    TRANSLATIONS    = superapp_dk.ts \
                      superapp_fi.ts \
                      superapp_no.ts \
                      superapp_se.ts
\endcode

When you run \e lupdate or \e lrelease, you must give the name of the
project file as a command-line argument.

In this example, four exotic languages are supported: Danish, Finnish,
Norwegian and Swedish. If you use \link qmake-manual.book
qmake\endlink, you usually don't need an extra project
file for \e lupdate; your \c qmake project file will work fine once
you add the \c TRANSLATIONS entry.

In your application, you must \l QTranslator::load() the translation
files appropriate for the user's language, and install them using \l
QApplication::installTranslator().

If you have been using the old Qt tools (\c findtr, \c msg2qm and \c
mergetr), you can use \e qm2ts to convert your old \c .qm files.

\e linguist, \e lupdate and \e lrelease are installed in the \c bin
subdirectory of the base directory Qt is installed into. Click Help|Manual
in \e{Qt Linguist} to access the user's manual; it contains a tutorial
to get you started.

While these utilities offer a convenient way to create \c .qm files,
any system that writes \c .qm files is sufficient. You could make an
application that adds translations to a QTranslator with
QTranslator::insert() and then writes a \c .qm file with
QTranslator::save(). This way the translations can come from any
source you choose.

\target qt-itself
Qt itself contains over 400 strings that will also need to be
translated into the languages that you are targeting. You will find
translation files for French and German in \c $QTDIR/translations as
well as a template for translating to other languages. (This directory
also contains some additional unsupported translations which may be
useful.)

Typically, your application's main() function will look like this:
\code
    int main( int argc, char **argv )
    {
        QApplication app( argc, argv );

        // translation file for Qt
        QTranslator qt( 0 );
        qt.load( QString( "qt_" ) + QTextCodec::locale(), "." );
        app.installTranslator( &qt );

        // translation file for application strings
        QTranslator myapp( 0 );
        myapp.load( QString( "myapp_" ) + QTextCodec::locale(), "." );
        app.installTranslator( &myapp );

        ...

        return app.exec();
    }
\endcode

\section2 Support for Encodings

The QTextCodec class and the facilities in QTextStream make it easy to
support many input and output encodings for your users' data. When an
application starts, the locale of the machine will determine the 8-bit
encoding used when dealing with 8-bit data: such as for font
selection, text display, 8-bit text I/O and character input.

The application may occasionally require encodings other than the
default local 8-bit encoding. For example, an application in a
Cyrillic KOI8-R locale (the de-facto standard locale in Russia) might
need to output Cyrillic in the ISO 8859-5 encoding. Code for this
would be:

\code
    QString string = ...; // some Unicode text

    QTextCodec* codec = QTextCodec::codecForName( "ISO 8859-5" );
    QCString encoded_string = codec->fromUnicode( string );

    ...; // use encoded_string in 8-bit operations
\endcode

For converting Unicode to local 8-bit encodings, a shortcut is
available: the \link QString::local8Bit() local8Bit\endlink() method
of QString returns such 8-bit data. Another useful shortcut is the
\link QString::utf8() utf8\endlink() method, which returns text in the
8-bit UTF-8 encoding: this perfectly preserves Unicode information
while looking like plain US-ASCII if the text is wholly US-ASCII.

For converting the other way, there are the QString::fromUtf8() and
QString::fromLocal8Bit() convenience functions, or the general code,
demonstrated by this conversion from ISO 8859-5 Cyrillic to Unicode
conversion:

\code
    QCString encoded_string = ...; // Some ISO 8859-5 encoded text.

    QTextCodec* codec = QTextCodec::codecForName("ISO 8859-5");
    QString string = codec->toUnicode(encoded_string);

    ...; // Use string in all of Qt's QString operations.
\endcode

Ideally Unicode I/O should be used as this maximizes the portability
of documents between users around the world, but in reality it is
useful to support all the appropriate encodings that your users will
need to process existing documents. In general, Unicode (UTF-16 or
UTF-8) is best for information transferred between arbitrary people,
while within a language or national group, a local standard is often
more appropriate. The most important encoding to support is the one
returned by QTextCodec::codecForLocale(), as this is the one the user
is most likely to need for communicating with other people and
applications (this is the codec used by local8Bit()).

Qt supports most of the more frequently used encodings natively. For a
complete list of supported encodings see the \l QTextCodec
documentation.

In some cases and for less frequently used encodings it may be
necessary to write your own QTextCodec subclass. Depending on the
urgency, it may be useful to contact Trolltech technical support or
ask on the \c qt-interest mailing list to see if someone else is
already working on supporting the encoding. A useful interim measure
can be to use the QTextCodec::loadCharmapFile() function to build a
data-driven codec, although this approach has a memory and speed
penalty, especially with dynamically loaded libraries. For details of
writing your own QTextCodec, see the main QTextCodec class
documentation.

\keyword localization

\section2 Localize

Localization is the process of adapting to local conventions, for
example presenting dates and times using the locally preferred
formats. Such localizations can be accomplished using appropriate tr()
strings.

\code
    void Clock::setTime(const QTime& t)
    {
        if ( tr("AMPM") == "AMPM" ) {
            // 12-hour clock
        } else {
            // 24-hour clock
        }
    }
\endcode

In the example, for the US we would leave the translation of "AMPM" as
it is and thereby use the 12-hour clock branch; but in Europe we would
translate it as something else (anything else, e.g. "EU") and this
will make the code use the 24-hour clock branch.

Localizing images is not recommended. Choose clear icons that are
appropriate for all localities, rather than relying on local puns or
stretched metaphors.

\section1 Dynamic Translation

Some applications, such as Qt Linguist, must be able to support changes
to the user's language settings while they are still running. To make
widgets aware of changes to the system language, implement a public
slot called \c languageChange() in each widget that needs to be notified.
In this slot, you should update the text displayed by widgets using the
\l{QObject::tr()}{tr()} function in the usual way; for example:

\code
void MyWidget::languageChange()
{
    titleLabel->setText(tr("Document Title"));
    ...
    okPushButton->setText(tr("&OK"));
}
\endcode

The default event handler for QWidget subclasses responds to the
\link QEvent::Type LanguageChange\endlink event, and will call this slot
when necessary; other application components can also connect signals
to this slot to force widgets to update themselves.

\section1 System Support

Some of the operating systems and windowing systems that Qt runs on
only have limited support for Unicode. The level of support available
in the underlying system has some influence on the support that Qt can
provide on those platforms, although in general Qt applications need
not be too concerned with platform-specific limitations.

\section2 Unix/X11

\list
\i Locale-oriented fonts and input methods. Qt hides these and
    provides Unicode input and output.
\i Filesystem conventions such as
    \link http://www.ietf.org/rfc/rfc2279.txt UTF-8 \endlink
    are under development
    in some Unix variants. All Qt file functions allow Unicode,
    but convert filenames to the local 8-bit encoding, as
    this is the Unix convention
    (see QFile::setEncodingFunction()
        to explore alternative encodings).
\i File I/O defaults to the local 8-bit encoding,
    with Unicode options in QTextStream.
\endlist

\section2 Windows

\list
\i Qt provides full Unicode support, including input methods, fonts,
    clipboard, drag-and-drop and file names.
\i File I/O defaults to Latin-1, with Unicode options in QTextStream.
    Note that some Windows programs do not understand big-endian
    Unicode text files even though that is the order prescribed by
    the Unicode Standard in the absence of higher-level protocols.
\i Unlike programs written with MFC or plain winlib, Qt programs
    are portable between Windows 95/98 and Windows NT.
    \e {You do not need different binaries to support Unicode.}
\endlist

\section1 Note about Locales on X11

Many Unix distributions contain only partial support for some locales.
For example, if you have a \c /usr/share/locale/ja_JP.EUC directory,
this does not necessarily mean you can display Japanese text; you also
need JIS encoded fonts (or Unicode fonts), and the \c
/usr/share/locale/ja_JP.EUC directory needs to be complete. For best
results, use complete locales from your system vendor.

\section1 Relevant Qt Classes

These classes are relevant to internationalizing Qt applications.
*/