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The internationalization of an application is the process of making the application usable by people in countries other than one's own.
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:
On Windows NT/2000/XP and Unix/X11 with Xft (client side font support) the following languages are also supported:
Many of these writing systems exhibit special features:
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:
The following sections give some information on the status of the internationalization (i18n) support in Qt.
See also the Qt Linguist manual.
Writing multi-platform international software with Qt is a gentle, incremental process. Your software can become internationalized in the following stages:
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.
Wherever your program uses "quoted text" for text that will be presented to the user, ensure that it is processed by the QApplication::translate() function. Essentially all that is necessary to achieve this is to use QObject::tr(). For example, assuming the LoginWidget is a subclass of QWidget:
LoginWidget::LoginWidget()
{
QLabel *label = new QLabel( tr("Password:"), this );
...
}
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:
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 );
}
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 lupdate utility described below. The macros expand to just the text (without the context).
Example of QT_TR_NOOP():
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] );
}
Example of QT_TRANSLATE_NOOP():
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] );
}
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().
Accelerator values such as Ctrl+Q or Alt+F need to be translated too. If you hardcode CTRL+Key_Q for "Quit" in your application, translators won't be able to override it. The correct idiom is
QPopupMenu *file = new QPopupMenu( this );
file->insertItem( tr("&Quit"), this, SLOT(quit()),
QKeySequence(tr("Ctrl+Q", "File|Quit")) );
The QString::arg() functions offer a simple means for substituting arguments:
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) );
}
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:
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() );
produces the correct output in English and Norwegian:
5 of 10 files copied. Copying: somefile.txt Kopierer nu somefile.txt. Av totalt 10 filer er 5 kopiert.
Once you are using tr() throughout an application, you can start producing translations of the user-visible text in your program.
Qt Linguist's manual provides further information about Qt's translation tools, Qt Linguist, lupdate and lrelease.
Translation of a Qt application is a three-step process:
Typically, you will repeat these steps for every release of your application. The lupdate utility does its best to reuse the translations from previous releases.
Before you run lupdate, you should prepare a project file. Here's an example project file (.pro file):
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
When you run lupdate or 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 qmake, you usually don't need an extra project file for lupdate; your qmake project file will work fine once you add the TRANSLATIONS entry.
In your application, you must QTranslator::load() the translation files appropriate for the user's language, and install them using QApplication::installTranslator().
If you have been using the old Qt tools (findtr, msg2qm and mergetr), you can use qm2ts to convert your old .qm files.
linguist, lupdate and lrelease are installed in the bin subdirectory of the base directory Qt is installed into. Click Help|Manual in 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 .qm files, any system that writes .qm files is sufficient. You could make an application that adds translations to a QTranslator with QTranslator::insert() and then writes a .qm file with QTranslator::save(). This way the translations can come from any source you choose.
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 $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:
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();
}
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:
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
For converting Unicode to local 8-bit encodings, a shortcut is available: the local8Bit() method of QString returns such 8-bit data. Another useful shortcut is the utf8() 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:
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.
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 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 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.
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.
void Clock::setTime(const QTime& t)
{
if ( tr("AMPM") == "AMPM" ) {
// 12-hour clock
} else {
// 24-hour clock
}
}
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.
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 languageChange() in each widget that needs to be notified. In this slot, you should update the text displayed by widgets using the QObject::tr(){tr()} function in the usual way; for example:
void MyWidget::languageChange()
{
titleLabel->setText(tr("Document Title"));
...
okPushButton->setText(tr("&OK"));
}
The default event handler for QWidget subclasses responds to the LanguageChange event, and will call this slot when necessary; other application components can also connect signals to this slot to force widgets to update themselves.
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.
Many Unix distributions contain only partial support for some locales. For example, if you have a /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 /usr/share/locale/ja_JP.EUC directory needs to be complete. For best results, use complete locales from your system vendor.
These classes are relevant to internationalizing Qt applications.
| QBig5Codec | Conversion to and from the Big5 encoding |
| QEucJpCodec | Conversion to and from EUC-JP character sets |
| QEucKrCodec | Conversion to and from EUC-KR character sets |
| QGb18030Codec | Conversion to and from the Chinese GB18030/GBK/GB2312 encoding |
| QGb2312Codec | Conversion to and from the Chinese GB2312 encoding |
| QGbkCodec | Conversion to and from the Chinese GBK encoding |
| QHebrewCodec | Conversion to and from visually ordered Hebrew |
| QJisCodec | Conversion to and from JIS character sets |
| QSjisCodec | Conversion to and from Shift-JIS |
| QTextCodec | Conversion between text encodings |
| QTextDecoder | State-based decoder |
| QTextEncoder | State-based encoder |
| QTranslator | Internationalization support for text output |
| QTranslatorMessage | Translator message and its properties |
| QTsciiCodec | Conversion to and from the Tamil TSCII encoding |
| Copyright © 2007 Trolltech | Trademarks | Qt 3.3.8
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