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Porting to Qt 3.x

You're probably looking at this page because you want to port your application from Qt 2.x to Qt 3.x, but to be sure, let's review the good reasons to do this:

The Qt 3.x series is not binary compatible with the 2.x series. This means programs compiled for Qt 2.x must be recompiled to work with Qt 3.x. Qt 3.x is also not completely source compatible with 2.x, however all points of incompatibility cause compiler errors (rather than mysterious results), or produce run-time messages. The result is that Qt 3.x includes many additional features, discards obsolete functionality that is easily converted to use the new features, and that porting an application from Qt 2.x to Qt 3.x is a simple task well worth the amount of effort required.

To port code using Qt 2.x to use Qt 3.x:

  1. Briefly read the porting notes below to get an idea of what to expect.
  2. Be sure your code compiles and runs well on all your target platforms with Qt 2.x.
  3. Recompile with Qt 3.x. For each error, search below for related identifiers (eg. function names, class names) - this documented is structured to mention all relevant identifiers to facilitate such searching, even if that makes it a little verbose.
  4. If you get stuck, ask on the qt-interest mailing list, or Trolltech Technical Support if you're a Professional Edition licensee.

Many very major projects, such as ### have been ported, so there is plenty of expertise in the collective conscious that is the Qt Developer Community!


The Porting Notes


Header file inclusion changes

Qt 3.x remove some unnecessary nested #include directives from header files. This speeds up compilation when you don't need those nested header files. But in some cases you will find you need to add an extra #include to your files.

For example, if you get a message about QStringList or its functions not being defined, then add #include <qstringlist.h> at the top of the file giving the error.

Header files that you might need to add #include directives for include:

Namespace

Qt 3.x is namespace-clean. A few global identifiers that had been left in Qt 2.x have been discarded.

Enumeration QCursorShape and it values are now part of the special Qt class defined in qnamespace.h. If you get compilation errors about these being missing (unlikely, since most of your code will be in classes that inherit from the Qt namespace class), then change:

The name of some debugging macro variables has been changed. We have tried not to break source compatibility as much as possible. If you observe error messages on the UNIX console or the Windows debugging stream that were previously disabled, please check these macro variables:

The name of some debugging macro functions has been changed as well but source compatibility should not be affected if the macro variable QT_CLEAN_NAMESPACE is not defined:

For the record, undocumented macro variables that are not part of the API have been changed:

Removed functions

All these functions have been removed in Qt 3.x:

Additionally, the following preprocessor directives have been removed:

See the changes-3.0 document for motivation why this had to be done. You might have been relying on the non-portable and unpredictable behavior resulting from these directives. We strongly recommend to either make use of the safe qstr* variants directly or ensure that no 0 pointer is passed to the standard C functions in your current and future code base.

Collection class renaming

The classes QArray, QCollection, QList, QListIterator, QQueue, QStack and QVector have been renamed QMemArray, QPtrCollection, QPtrList, QPtrListIterator, QPtrQueue and QPtrStack, and they are found in the header files <qmemarray.h>, <qptrcollection.h>, <qptrlist.h>, <qptrqueue.h>, <qptrstack.h> and <qptrvector.h>. To ease porting, the old names and the old header-file names are still supported.

QDate

Two member functions of QDate were virtual in Qt 2.0 and are not anymore in Qt 3.0. The information here is relevent only if you subclassed QDate and reimplemented these functions:

QString QDate::monthName( int month ) const
To override the default English short month names ("Jan", "Feb", ..., "Dec"), you can now translate the English strings (in the "QDate::shortMonth" context, see Internationalization with Qt for details) or even call QDate::setShortMonthNames().

QString QDate::dayName( int weekday ) const
To override the default English short day names ("Mon", "Tue", ..., "Sun"), you can now translate the English strings (in the "QDate::shortDay" context) or even call QDate::setShortDayNames().

Apart from losing their virtualness, QDate::monthName() and QDate::dayName() have been renamed QDate::shortMonthName() and QDate::shortDayName() and have been made static (as they always should had been). The old names are still provided for source compatibility.

QFont

The internals of QFont have changed significantly between Qt 2.2 and Qt 3.0, to give better Unicode support and make developing internationalized applications easier. The external behaviour has however not changed a lot. The main change is that the CharSet enum in QFont and functions related to it have disappeared. This comes about because Qt does now handle all charset related issues internally, and takes this burden away from the developer.

If you used the CharSet enum anywhere or any functions as QFont::charSet(), QFont::setCharSet() just remove them from your code to get things working again. There are a few other functions that took a QFont::CharSet as parameter. Just remove the charset from the parameterlist.

QMultiLineEdit

The QMultiLineEdit was a simple editor widget in former Qt versions. Since Qt 3.0, which comes with a new richtext engine, which also supports editing, QMultiLineEdit is obsolete. It is still included for compatibility reasons. It is now a subclass of QTextEdit which wrappes the old QMultiLineEdit so that it is mostly source compatible to keep old applications working.

If you implement something new with QMultiLineEdit, you should use QTextEdit instead.

Although most of the old QMultiLineEdit API is still available, there is one important difference. Because of a design flaw of the old QMultiLineEdit it operated on lines and not on paragraphs. As lines do change all the time during wordwrap, the new richtext engine only knows paragraphs as elements in the data structure. So all functions (like numLines(), textLine()), which worked on lines, now work on paragraphs.

Also the function getString() has been removed as this function published the internal data structure.

So, applications which made normal usage of the QMultiLineEdit, should keep working without problems. Programs which did some special stuff with it might require some porting.

The source code for the old 2.x version of QMultiLineEdit can be found in $QTDIR/src/attic/qtmultilineedit.h/cpp. Note that the class has been renamed to QtMultiLineEdit to avoid name clashes. If you really need to keep compatibility with the old QMultiLineEdit, simply include this class in your project and rename all references from QMultiLineEdit to QtMultiLineEdit.

QPrinter

QPrinter has undergone some changes, to make it more flexible and force the same runtime behaviour on Unix and on Windows. In 2.x, QPrinter behaved differently on Windows and Unix, when using view transformations on the QPainter. This has changed now, and QPrinter behaves consistent across all platforms. A compatibilty mode has been added that behaves exactly as in Qt-2.x, to ease porting from Qt-2.x to Qt-3. This compatibilty mode can be enabled by passing a QPrinter::Compatible flag to the QPrinter constructor.

On X11, QPrinter used to generate encapsulated postscript when fullPage() was TRUE and only one page was printed. This does not happen by default anymore, giving a more consistent printing output.

QRegExp

The QRegExp class has been rewritten to support most of the features of Perl regular expressions. Both the regular expression syntax and the QRegExp interface were modified.

Be also aware that <qregexp.h> is no longer included automatically when you include <qstringlist.h>. See above for details.

New special characters
There are five new special characters: (, ), {, | and } (parentheses, braces and pipe). When porting old regular expressions, you have to add \ (backslash) in front of any of these (actually, \\ in C++ strings), unless it is already there.

Example: Old code like

    QRegExp rx( "([0-9|]*\\)" );        // works in Qt 2.x

should be converted into

    QRegExp rx( "\\([0-9]\\|]*\\)" );   // works in Qt 2.x and 3.x

However, wildcard patterns need no conversion. Here are two examples:

    QRegExp wild( "(*.*)" );
    wild.setWildcard( TRUE );

    // TRUE as third argument means wildcard
    QRegExp wild( "(*.*)", FALSE, TRUE );

Usually, QRegExp is easy to spot in programs. However, in one unfortunate case, it is subtle: as first argument to QString::replace(). Code like

    QString text = fetch_it_from_somewhere();
    text.replace( QString("([^)]*)"), "" );

involves an implicit QRegExp (namely, "([^)]*)") that should be changed.

QRegExp& QRegExp::operator= ( const QString & pattern )
This function has been replaced by QRegExp::setPattern() in Qt 2.2. Old code such as

    QRegExp rx( "alpha" );
    rx.setCaseSensitive( FALSE );
    rx.setWildcard( TRUE );
    rx = "beta";

still compiles with Qt 3, but produces a different result (the case sensitivity and wildcard options are forgotten). This way,

    rx = "beta";

is the same as

    rx = QRegExp( "beta" );

which is what one expects.

bool QRegExp::match( const QString & str, int index = 0, int * len = 0, bool indexIsStart = TRUE ) const
This function is now obsolete, as it has an unwieldy parameter list and a bad name. It will be removed in a future version of Qt. Its documentation explains how to accomplish the same.

int QRegExp::find( const QString & str, int index )
This function was removed, after a brief appearance in Qt 2.2. Its name is confusing. Use QRegExp::search() or QString::find() instead.

QTableView

The QTableView class has been obsoleted and is no longer a part of the Qt API. You should use the QTable class in any new code you create. If you really need this class for compatibility issues you can find it in $QTDIR/src/attic/qttableview.{cpp,h}. Note that the class has been renamed from QTableView to QtTableView to avoid name clashes. To use it, simply include it in your project and rename all references in your code from QTableView to QtTableView.

QToolButton

The QToolButton class used to distinguish between "on" and "off" icons. In 3.0, this mechanism was moved into the QIconSet class (see QIconSet::State).

The old QToolButton::onIconSet and QToolButton::offIconSet properties are still provided for old source to compile, but their semantics have changed: They are now synonyms for QToolButton::iconSet. If you used that distinction in Qt 2, you will need to adjust your code to use the QIconSet On/Off mechanism.

Likewise, the on parameter of these two functions is now ignored: