Porting to Qt 4

This document describes the process of porting applications from Qt 3 to Qt 4. If you haven't yet made the decision about porting, or are unsure about whether it is worth it, take a look at the key features offered by Qt 4. See also Moving from Qt 3 to Qt 4 for tips on how to write Qt 3 code that is easy to port to Qt 4.

Other porting guides:

• Moving from Qt 3 to Qt 4 — covers some high level topics relevant to developers porting from Qt 3 to Qt 4.
• Porting to Qt 4 - Drag and Drop — covers differences in the way drag and drop is handled between Qt 3 and Qt 4.
• Porting UI Files to Qt 4 — describes the new format used to describe forms created with Qt Designer.
• Porting to Graphics View — provides a class-by-class overview of the differences between Qt 3's canvas API and Qt 4's Graphics View framework.
• qt3to4 - The Qt 3 to 4 Porting Tool — provides an overview of a tool aimed at helping developers start the process of porting an application to Qt 4.

The Qt 4 series is not binary compatible with the 3 series. This means programs compiled for Qt 3 must be recompiled to work with Qt 4. Qt 4 is also not completely source compatible with 3, however nearly all points of incompatibility cause compiler errors or run-time messages (rather than mysterious results). Qt 4 includes many additional features and discards obsolete functionality. Porting from Qt 3 to Qt 4 requires some effort, but once completed the considerable additional power and flexibility of Qt 4 is available for use in your applications.

To port code from Qt 3 to Qt 4:

1. Briefly read the porting notes below to get an idea of what to expect.
2. Be sure that your code compiles and runs well on all your target platforms with Qt 3.
3. Add the line QT += qt3support to your .pro file if you use qmake; otherwise, edit your makefile or project file to link against the Qt3Support library and add -DQT3_SUPPORT to your compiler flags. (You might also need to specify other libraries. See What's New in Qt 4 for details.)
4. Run the qt3to4 porting tool. The tool will go through your source code and adapt it to Qt 4.
5. Follow the instructions in the Porting UI Files to Qt 4 page to port Qt Designer files.
6. Recompile with Qt 4. For each error, search below for related identifiers (e.g., function names, class names). This document mentions all relevant identifiers to help you get the information you need at the cost of being a little verbose.

The qt3to4 porting tool replaces occurrences of Qt 3 classes that don't exist anymore in Qt 4 with the corresponding Qt 3 support class; for example, QListBox is turned into Q3ListBox.

At some point, you might want to stop linking against the Qt 3 support library (Qt3Support) and take advantage of Qt 4's new features. The instructions below explain how to do that for each compatibility class.

In addition to the Qt3Support classes (such as Q3Action, Q3ListBox, and Q3ValueList), Qt 4 provides compatibility functions when it's possible for an old API to cohabit with the new one. For example, QString provides a QString::simplifyWhiteSpace() compatibility function that's implemented inline and that simply calls QString::simplified(). The compatibility functions are not documented here; instead, they are documented for each class.

If you have the line QT += qt3support in your .pro file, qmake will automatically define the QT3_SUPPORT symbol, turning on compatibility function support. You can also define the symbol manually (e.g., if you don't want to link against the Qt3Support library), or you can define QT3_SUPPORT_WARNINGS instead, telling the compiler to emit a warning when a compatibility function is called. (This works only with GCC 3.2+ and MSVC 7.)

If you get stuck, ask on the qt-interest mailing list. If you are a licensed customer, you can also contact Qt's technical support team.

Table of contents:

Casting and Object Types

In Qt 3, it was possible to use the qt_cast() function to determine whether instances of QObject subclasses could be safely cast to derived types of those subclasses. For example, if a QFrame instance is passed to a function whose signature specifies a QWidget pointer as its argument, qt_cast() could be used to obtain a QFrame pointer so that the instance's functions can be accessed.

In Qt 4, much of this functionality is provided by the qobject_cast() function, and additional functions also provide similar functionality for certain non-QObject types:

Type Names

The table below lists the classes that have been renamed in Qt 4. If you compile your applications with QT3_SUPPORT defined, the old names will be available.

Whenever you see an occurrence of the name on the left, you can safely replace it with the Qt 4 equivalent in your program. The qt3to4 tool performs the conversion automatically.

The table below lists the enums and typedefs that have been renamed in Qt 4. If you compile your applications with QT3_SUPPORT defined, the old names will be available.

Whenever you see an occurrence of the name on the left, you can safely replace it with the Qt 4 equivalent in your program. The qt3to4 tool performs the conversion automatically.

Enum Values

The table below lists the enum values that have been renamed in Qt 4. If you compile your applications with QT3_SUPPORT defined, the old names will be available.

Whenever you see an occurrence of the name on the left, you can safely replace it with the Qt 4 equivalent in your program. The qt3to4 tool performs the conversion automatically.

In addition, the following window flags have been either replaced with widget attributes or have been deprecated:

In Qt 4.1, the widget flags used to determine window modality were replaced by a single enum that can be used to specify the modal behavior of top-level widgets:

Properties

Some properties have been renamed in Qt 4, to make Qt's API more consistent and more intuitive. For example, QWidget's caption property has been renamed windowTitle to make it clear that it refers to the title shown in the window's title bar.

In addition, the property system has been extended to allow properties to be redefined in subclasses with the Q_PROPERTY() macro, removing the need for a Q_OVERRIDE() macro.

The table below lists the Qt properties that have been renamed in Qt 4. Occurrences of these in Qt Designer UI files are automatically converted to the new name by uic.

A handful of properties in Qt 3 are no longer properties in Qt 4, but the access functions still exist as part of the Qt 4 API. These are not used by Qt Designer; the only case where you need to worry about them is in highly dynamic applications that use Qt's meta-object system to access properties. Here's the list of these properties with the read and write functions that you can use instead:

Some properties have been removed from Qt 4, but the associated access functions are provided if QT3_SUPPORT is defined to help porting to Qt 4. When converting Qt 3 UI files to Qt 4, uic generates calls to the Qt 3 compatibility functions. Note that this only applies to the properties of the Qt3Support library, i.e. QT3_SUPPORT properties of the other libraries must be ported manually when converting Qt 3 UI files to Qt 4.

The table below lists these properties with the read and write functions that you can use instead. The documentation for the individual functions explains how to replace them with non-compatibility Qt 4 functions.

The following Qt 3 properties and their access functions are no longer available in Qt 4. In most cases, Qt 4 provides similar functionality.

Explicit Sharing

Qt 4 is the first version of Qt that contains no explicitly shared classes. All classes that were explicitly shared in Qt 3 are implicitly shared in Qt 4:

• QImage
• QBitArray
• QByteArray
• Q3PointArray

This means that if you took a copy of an instance of the class (using operator=() or the class's copy constructor), any modification to the copy would affect the original and vice versa. Needless to say, this behavior is rarely desirable.

Fortunately, nearly all Qt 3 applications don't rely on explicit sharing. When porting, you typically only need to remove calls to detach() and/or copy(), which aren't necessary anymore.

If you deliberately rely on explicit sharing in your application, you can use pointers or references to achieve the same result in Qt 4.

For example, if you have code like

 void asciify(QByteArray array)
 {
     for (int i = 0; i < (int)array.size(); ++i) {
         if ((uchar)array[i] >= 128)
             array[i] = '?';
     }
 }

you can rewrite it as

 void asciify(QByteArray &array)
 {
     for (int i = 0; i < array.size(); ++i) {
         if ((uchar)array[i] >= 128)
             array[i] = '?';
     }
 }

(Notice the & in the parameter declaration.)

Painting and Redrawing Widgets

When implementing custom widgets in Qt 3, it was possible to use QPainter to draw on a widget outside paint events. This made it possible to integrate Qt applications with third party libraries and tools that impose their own rendering models. For example, a widget might be repainted in a slot using data obtained from an external source.

In Qt 4, it is only possible to paint on a widget from within its paintEvent() handler function. This restriction simplifies Qt's interaction with native window systems, improves the performance of applications by reducing the number of redraw operations, and also enables features to be implemented to improve the appearance of widgets, such as a backing store.

Generally, we recommend redesigning applications to perform all painting operations in paintEvent() functions, deferring actual painting until the next time this function is called. Applications can post paint events to trigger repaints, and it may be possible to examine your widget's internal state to determine which part of the widget needs to be repainted.

If asynchronous repaints are used extensively by your application, and it is not practical to redesign the rendering model to perform all painting operations from within a widget's paintEvent() function, it may be necessary to consider using an intermediate painting step. In this approach, one or more images can be updated asynchronously and painted on the widget in the paint event. To avoid excessive buffering, it may be worthwhile disabling the backing store by setting the widget's Qt::WA_PaintOnScreen widget attribute.

On certain platforms, the Qt::WA_PaintOutsidePaintEvent widget attribute can be set to allow a widget to be painted from outside paint events.

Note: Setting widget attributes to disable key features of Qt's widget rendering model may also cause other features to be disabled.

Compatibility Signals and Slots

When QT3_SUPPORT is defined, the default connection type for signals and slots is the Qt::AutoCompatConnection type. This allows so-called compatibility signals and slots (defined in Qt 3 support mode to provide Qt 3 compatibility features) to be connected to other signals and slots.

However, if Qt is compiled with debugging output enabled, and the developer uses other connection types to connect to compatibility signals and slots (perhaps by building their application without Qt 3 support enabled), then Qt will output warnings to the console to indicate that compatibility connections are being made. This is intended to be used as an aid in the process of porting a Qt 3 application to Qt 4.

QAccel

The QAccel class has been renamed Q3Accel and moved to the Qt3Support module. In new applications, you have three options:

1. You can use QAction and set a key sequence using QAction::setShortcut().
2. You can use QShortcut, a class that provides similar functionality to Q3Accel.
3. You can use QWidget::grabShortcut() and process "shortcut" events by reimplementing QWidget::event().

The Q3Accel class also supports multiple accelerators using the same object, by calling Q3Accel::insertItem() multiple times. In Qt 4, the solution is to create multiple QShortcut objects.

QAccessibleInterface

The QAccessibleInterface class has undergone some API changes in Qt 4, to make it more consistent with the rest of the Qt API.

If you have classes that inherit QAccessibleInterface or one of its subclasses (QAccessibleObject, QAccessibleWidget, etc.), you must port them the new QAccessibleInterface API.

See Virtual Functions for a list of QAccessibleInterface virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QAccessibleTitleBar

The QAccessibleTitleBar has been renamed Q3AccessibleTitleBar and moved to the Qt3Support library.

QAction

The QAction class has been redesigned in Qt 4 to integrate better with the rest of the menu system. It unifies the old QMenuItem class and the old QAction class into one class, avoiding unnecessary data duplication and the need to learn two different APIs.

The old QAction and QActionGroup classes have been renamed Q3Action and Q3ActionGroup and moved to Qt3Support. In addition, the new QAction class has compatibility functions to ease transition to Qt 4. Note that when using Q3ToolBar and Q3PopupMenu, their actions must be Q3Actions.

See Virtual Functions for a list of QAction virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QActionGroup

The QAction class has been completely redesigned in Qt 4 to integrate better with the rest of the menu system. See the section on QAction for details.

QApplication

The QApplication class has been split into two classes: QCoreApplication and QApplication. The new QApplication class inherits QCoreApplication and adds GUI-related functionality. In practice, this has no consequences for existing Qt applications.

In addition, the following API changes were made:

1. QApplication::allWidgets() and QApplication::topLevelWidgets() used to return a pointer to a QWidgetList. Now they return a QWidgetList.

   Also, QWidgetList has changed from being a typedef for QPtrList<QWidget> to being a typedef for QList<QWidget *>. See the section on QWidgetList below for details.

   For example, if you have code like

        QWidgetList *list = QApplication::topLevelWidgets();
        QWidgetListIt it(*list);
        QWidget *widget;
        while ((widget = it.current())) {
            if (widget->inherits("MainWindow"))
                ((MainWindow *)widget)->updateRecentFileItems();
            ++it;
        }
        delete list;

   you can rewrite it as

        QWidgetList list = QApplication::topLevelWidgets();
        for (int i = 0; i < list.size(); ++i) {
            if (MainWindow *mainWin = qobject_cast<MainWindow *>(list.at(i)))
                mainWin->updateRecentFileItems();
        }

2. QApplication::setMainWidget() is no longer used. When all an application's windows are closed, the application will exit normally.

QAquaStyle

The QAquaStyle class first appeared in Qt 3.0, when the Qt for Mac OS X port was first released. It emulated Apple's "Aqua" theme. In Qt 3.1, QAquaStyle was obsoleted by QMacStyle, which uses Appearance Manager to perform its drawing.

The QAquaStyle class is no longer provided in Qt 4. Use QMacStyle instead.

QAsciiCache<T>

QAsciiCache<T> has been renamed Q3AsciiCache<T> and moved to the Qt3Support library. It has been replaced by QCache<QByteArray, T>.

For details, read the section on QCache<T>, mentally substituting QByteArray for QString.

QAsciiDict<T>

QAsciiDict<T> and QAsciiDictIterator<T> have been renamed Q3AsciiDict<T> and Q3AsciiDictIterator<T> and moved to the Qt3Support library. They have been replaced by the more modern QHash<Key, T> and QMultiHash<Key, T> classes and their associated iterator classes.

When porting old code that uses Q3AsciiDict<T> to Qt 4, there are four classes that you can use:

• QMultiHash<QByteArray, T *>
• QMultiHash<QByteArray, T>
• QHash<QByteArray, T *>
• QHash<QByteArray, T>

For details, read the section on QDict<T>, mentally substituting QByteArray for QString.

QAsyncIO

The QAsyncIO class was used internally in Qt 2.x in conjunction with QImageConsumer. It was obsoleted in Qt 3.0.

If you use this mechanism in your application, please submit a report to the Task Tracker on the Qt website and we will try to find a satisfactory substitute.

QBackInsertIterator

The undocumented QBackInsertIterator class has been removed from the Qt library. If you need it in your application, feel free to copy the source code from the Qt 3 <qtl.h> header file.

QBitArray

In Qt 3, QBitArray inherited from QByteArray. In Qt 4, QBitArray is a totally independent class. This makes very little difference to the user, except that the new QBitArray doesn't provide any of QByteArray's byte-based API anymore. These calls will result in a compile-time error, except calls to QBitArray::truncate(), whose parameter was a number of bytes in Qt 3 and a number of bits in Qt 4.

QBitArray was an explicitly shared class in Qt 3. See Explicit Sharing for more information.

The QBitVal class has been renamed QBitRef.

QButton

The QButton class has been replaced by QAbstractButton in Qt 4. Classes like QPushButton and QRadioButton inherit from QAbstractButton. As a help when porting older Qt applications, the Qt3Support library contains a Q3Button class implemented in terms of the new QAbstractButton.

If you used the QButton class as a base class for your own button type and want to port your code to the newer QAbstractButton, you need to be aware that QAbstractButton has no equivalent for the Q3Button::drawButton(QPainter *) virtual function. The solution is to reimplement QWidget::paintEvent() in your QAbstractButton subclass as follows:

 void MyButton::paintEvent(QPaintEvent *)
 {
     QPainter painter(this);
     drawButton(&painter);
 }

Remarks:

1. In Qt 3, QButton had a "toggle type", which could be QButton::SingleShot, QButton::Toggle, or QButton::Tristate. The new QAbstractButton class doesn't support "tristate" directly; this feature is implemented in QCheckBox instead. The two other "toggle types" (QButton::SingleShot and QButton::Toggle) are replaced by a QAbstractButton::checkable property.
2. In Qt 3, QButton had a "toggle state", which could be QButton::Off, QButton::NoChange, or QButton::On. In Qt 4, this mechanism has been moved to QCheckBox.

See Virtual Functions for a list of QButton virtual member functions in Qt 3 that aren't virtual in Qt 4.

See Properties for a list of QButton properties in Qt 3 that have changed in Qt 4.

QButtonGroup

The QButtonGroup class has been completely redesigned in Qt 4. For compatibility, the old QButtonGroup class has been renamed Q3ButtonGroup and has been moved to Qt3Support. Likewise, the QHButtonGroup and QVButtonGroup convenience subclasses have been renamed Q3HButtonGroup and Q3VButtonGroup and moved to the Qt3Support library.

The old QButtonGroup, as well as Q3ButtonGroup, can be used in two ways:

1. The button group is the parent widget of a number of buttons, i.e. the button group is the parent argument in the button constructor. The buttons are assigned identifiers 0, 1, 2, etc., in the order they are created. A Q3ButtonGroup can display a frame and a title because it inherits Q3GroupBox.
2. The button group is an invisible widget and the contained buttons have some other parent widget. In this usage, each button must be manually inserted, using Q3ButtonGroup::insert(), into the button group and given an ID number.

Unlike Q3ButtonGroup, the new QButtonGroup doesn't inherit QWidget. It is very similar to a "hidden Q3ButtonGroup".

If you use a Q3ButtonGroup, Q3HButtonGroup, or Q3VButtonGroup as a widget and want to port to Qt 4, you can replace it with QGroupBox. In Qt 4, radio buttons with the same parent are automatically part of an exclusive group, so you normally don't need to do anything else. See also the section on QGroupBox below.

See Virtual Functions for a list of QButtonGroup virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QByteArray

In Qt 3, QByteArray was simply a typedef for QMemArray<char>. In Qt 4, QByteArray is a class in its own right, with a higher-level API in the style of QString.

Here are the main issues to be aware of when porting to Qt 4:

1. The QMemArray(int size) constructor has been replaced with QByteArray(int size, char ch). The second argument specifies which character should be used for initializing the array; pass '\0' if you have no specific needs.

   For example, if you have code like

    QByteArray ba(64);

   you can rewrite it as

    QByteArray ba(64, '\0');

2. QMemArray::at() returned a non-const reference, whereas the new QByteArray::at() returns a const value. Code like

    ba.at(0) = 'X';

   will no longer compile. Instead, use QByteArray::operator[]:

    ba[0] = 'X';

3. The QMemArray::contains(char) function has been renamed QByteArray::count(char). In addition, there now exists a QByteArray::contains(char) function that returns a boolean value. Replace old calls to contains() with either count() or contains(), depending on whether you care about the specific number of occurrences of a character in the byte array or only care about whether the array contains that character or not.
4. The new QByteArray has no assign() function. Calls to QMemArray::assign(const QMemArray &) can be replaced by calls to QByteArray::operator=(). Calls to QMemArray::assign(const T *, uint) have no equivalent in Qt 4; if you use it, the solution is either to use QByteArray::fromRawData() and to call free() yourself to avoid a memory leak, or to use the QByteArray(const char *, int) constructor, which will take a deep copy of the data.
5. QMemArray::bsearch() and QMemArray::sort() have no equivalent in the new QByteArray class. Use qBinaryFind() and qSort() if you need that functionality.

QByteArray was an explicitly shared class in Qt 3. See Explicit Sharing for more information.

QCache<T>

QCache<T> has been renamed Q3Cache<T> and moved to Qt3Support. The new QCache class has a different API, and takes different template parameters: QCache<Key, T>.

When porting to Qt 4, QCache<QString, T> is the obvious substitute for Q3Cache<T>. The following table summarizes the API differences.

Remarks:

1. Q3Cache requires the user to allocate a specific number of buckets by passing a prime number (17 by default) to the constructor. In contrast, the new QCache's hash table automatically grows and shrinks as needed, and the constructor doesn't take a prime number.
2. Q3Cache supportes case-insensitive lookups by passing false as second argument to the constructor. This feature has no equivalent in QMultiHash. Instead, call QString::toLower() before you insert or lookup a key in the hash.
3. The Q3Cache::insert() function returns a bool value that indicates whether or not the item actually was inserted in the cache. If the item wasn't inserted, it was the caller's responsibility to delete the item. The new QCache::insert() function returns void and either adds it to the cache or deletes it right away. Old code like

    if (!cache.insert(key, object))
        delete object;

   becomes

    cache.insert(key, object);

4. The new QCache class always takes ownership of the items it stores (i.e. auto-delete is always on). If you use Q3Cache with auto-delete turned off (the rarely useful default), you cannot use QCache as a direct substitute. One unelegant trick that works well in practice is to use QCache<QString, T *> instead of QCache<QString, T>. In that case, QCache owns the pointers, not the objects that the pointers refer to. For example,

    Q3Cache<QWidget> cache;
    cache.insert(widget->name(), widget);
    ...
    QWidget *foo = cache.take("foo");
    if (foo)
        foo->show();

   becomes

    typedef QWidget *QWidgetPtr;
    QCache<QString, QWidgetPtr> cache;
    cache.insert(widget->name(), new QWidgetPtr(widget));
    ...
    QWidgetPtr *ptr = cache.take("foo");
    if (ptr) {
        QWidget *foo = *ptr;
        delete ptr;
        foo->show();
    }

   An alternative is to stick to using Q3Cache.

QCacheIterator<T> has been renamed Q3CacheIterator<T> and moved to the Qt3Support library. The new QCache class doesn't offer any iterator types.

QCanvas

The canvas module classes have been renamed and moved to the Qt3Support library.

The Graphics View Framework replaces QCanvas. For more on porting to Graphics View, see Porting to Graphics View.

QColor

In Qt 4, QColor is a value type like QPoint or QRect. Graphics system-specific code has been implemented in QColormap.

The QColor::maxColors() function has been replaced by QColormap::size().

The QColor::numBitPlanes() function has been replaced by QColormap::depth().

The QColor::setNamedColor() function no longer supports the named color in the same way as Qt 3. Qt 4's setNamedColor() uses the new W3C convention as stated here.

The predefined colors listed in the table above were static QColor objects in Qt 3. In Qt 4, they are enum values of type Qt::GlobalColor. Thanks to the implicit QColor(Qt::GlobalColor) constructor, the enum values are automatically converted to QColors in most contexts. Occasionally, you might need a cast.

For example, if you have code like

 QColor lightCyan = Qt::cyan.light(180);

you can rewrite it as

 QColor lightCyan = QColor(Qt::cyan).light(180);

QColorGroup

In Qt 3, a QPalette consisted of three QColorGroup objects. In Qt 4, the (rarely used) QColorGroup abstraction has been eliminated. For source compatibility, a QColorGroup class is available when QT3_SUPPORT is defined.

The new QPalette still works in terms of color groups, specified through enum values (QPalette::Active, QPalette::Disabled, and QPalette::Inactive). It also has the concept of a current color group, which you can set using QPalette::setCurrentColorGroup().

The QPalette object returned by QWidget::palette() returns a QPalette initialized with the correct current color group for the widget. This means that if you had code like

 painter.setBrush(colorGroup().brush(QColorGroup::Text));

you can simply replace colorGroup() with palette():

 painter.setBrush(palette().brush(QPalette::Text));

QColorDrag

The QColorDrag class has been renamed Q3ColorDrag and moved to the Qt3Support library. In Qt 4, use QMimeData instead and call QMimeData::setColor() to set the color.

QComboBox

In Qt 3, the list box used to display the contents of a QComboBox widget could be accessed by using the listBox() function. In Qt 4, the standard list box is provided by a QListView widget, and can be accessed with the view() function.

See Virtual Functions for a list of QComboBox virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QCString

In Qt 3, QCString inherited from QByteArray. The main drawback of this approach is that the user had the responsibility of ensuring that the string is '\0'-terminated. Another important issue was that conversions between QCString and QByteArray often gave confusing results. (See the Achtung! Binary and Character Data article in Qt Quarterly for an overview of the pitfalls.)

Qt 4 solves that problem by merging the QByteArray and QCString classes into one class called QByteArray. Most functions that were in QCString previously have been moved to QByteArray. The '\0' issue is handled by having QByteArray allocate one extra byte that it always sets to '\0'. For example:

 QByteArray ba("Hello");
 ba.size();             // returns 5 (the '\0' is not counted)
 ba.length();           // returns 5
 ba.data()[5];          // returns '\0'

The Qt3Support library contains a class called Q3CString that inherits from the new QByteArray class and that extends it to provide an API that is as close to the old QCString class as possible. Note that the following functions aren't provided by Q3CString:

• QCString::find(const QRegExp &, int)
• QCString::findRev(const QRegExp &, int)
• QCString::contains(const QRegExp &)
• QCString::replace(const QRegExp &, const char *)

The following functions have lost their last parameter, which specified whether the search was case sensitive or not:

• QByteArray::find(char, int)
• QByteArray::find(const char *, int)
• QByteArray::findRev(char, int)
• QByteArray::findRev(const char *, int)
• QByteArray::contains(char)
• QByteArray::contains(const char *)

In both cases, the solution is to convert the QCString to a QString and use the corresponding QString functions instead.

Also be aware that QCString::size() (inherited from QByteArray) used to return the size of the character data including the '\0'-terminator, whereas the new QByteArray::size() is just a synonym for QByteArray::length(). This brings QByteArray in line with QString.

When porting to Qt 4, occurrences of QCString should be replaced with QByteArray or QString. The following table summarizes the API differences between the Q3CString class and the Qt 4 QByteArray and QString classes:

Remarks:

1. Q3CString(const char *str, uint max) constructs a string of length strlen(str) or max - 1, whichever is shorter. QByteArray(const char *data, int size) constructs a byte array containing exactly size bytes.

   For example, if you have code like

        QCString str1("Hello", 4);           // "Hel"
        QCString str2("Hello world!", n);

   you can rewrite it as

        QByteArray str1("Hello", 3);
        QByteArray str2("Hello world!");
        str2.truncate(n - 1);

2. Q3CString::setExpand(uint index, char ch) has no equivalent in Qt 4.

   For example, if you have code like

        QCString str("Hello world");
        str.setExpand(16, '\n');            // "Hello world     \n"

   you can rewrite it as

        QByteArray str("Hello world");
        while (str.size() < 16)
            str += ' ';
        str += '\n';

Since the old QCString class inherited from QByteArray, everything that is said in the QByteArray section applies for QCString as well.

QCustomEvent

In Qt 3, developers could create a custom event by constructing a new QCustomEvent, and send relevant data to other components in the application by passing a void pointer, either on construction or using the setData() function. Objects could receive custom events by reimplementing the customEvent() function, and access the stored data using the event's data() function.

In Qt 4, custom events are created by subclassing QEvent. Event-specific data can be stored in a way that is appropriate for your application. Custom events are still delivered to each object's customEvent() handler function, but as QEvent objects rather than as deprecated QCustomEvent objects.

QDataBrowser

The QDataBrowser class has been renamed Q3DataBrowser and moved to the Qt3Support library. In Qt 4.2, you should use the QDataWidgetMapper class to create data-aware forms.

See QtSql Module for an overview of the new SQL classes.

QDataPump

The QDataPump class was used internally in Qt 2.x in conjunction with QImageConsumer. It was obsoleted in Qt 3.0.

If you use this mechanism in your application, please submit a report to the Task Tracker on the Qt website and we will try to find a satisfactory substitute.

QDataSink

The QDataSink class was used internally in Qt 2.x in conjunction with QImageConsumer. It was obsoleted in Qt 3.0.

If you use this mechanism in your application, please submit a report to the Task Tracker on the Qt website and we will try to find a satisfactory substitute.

QDataSource

The QDataSource class was used internally in Qt 2.x in conjunction with QImageConsumer. It was obsoleted in Qt 3.0. If you use this mechanism in your application, please submit a report to the Task Tracker on the Qt website and we will try to find a satisfactory substitute.

QDataTable

The QDataTable class has been renamed Q3DataTable and moved to the Qt3Support library. In Qt 4.2, you should use the QDataWidgetMapper class to create data-aware forms.

See QtSql Module for an overview of the new SQL classes.

QDataView

The QDataView class has been renamed Q3DataView and moved to the Qt3Support library. In Qt 4.2, you should use the QDataWidgetMapper class to create data-aware forms.

See QtSql Module for an overview of the new SQL classes.

QDateEdit

The QDateEdit class in Qt 4 is a convenience class based on QDateTimeEdit. The old class has been renamed Q3DateEdit and moved to the Qt3Support library.

See Virtual Functions for a list of QDateEdit virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QDateTimeEditBase

The QDateTimeEditBase class has been renamed Q3DateTimeEditBase and moved to Qt3Support. Use QDateTimeEdit or QAbstractSpinBox instead.

QDateTimeEdit

The old QDateTimeEdit class has been renamed Q3DateTimeEditBase and moved to Qt3Support. The new QDateTimeEdit in Qt 4 has been rewritten from scratch to provide a more flexible and powerful API.

See Virtual Functions for a list of QDateTimeEdit virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QDeepCopy<T>

The QDeepCopy<T> class in Qt 3 provided a means of ensuring that implicitly shared and explicitly shared classes referenced unique data. This was necessary because the reference counting in Qt's container classes was done in a thread-unsafe manner.

With Qt 4, QDeepCopy<T> has been renamed Q3DeepCopy<T> and moved to the Qt3Support library. Removing it from existing code is straightforward.

For example, if you have code like

 QString str1 = "I am a string";
 QDeepCopy<QString> str2 = str1;
 QString str3 = QDeepCopy<QString>(str2);

you can rewrite it as

 QString str1 = "I am a string";
 QString str2 = str1;
 QString str3 = str2;

QDial

See Virtual Functions for a list of QDial virtual member functions in Qt 3 that are no longer virtual in Qt 4.

See Properties for a list of QDial properties in Qt 3 that have changed in Qt 4.

QDict<T>

QDict<T> has been renamed Q3Dict<T> and moved to Qt3Support. It has been replaced by the more modern QHash<Key, T> and QMultiHash<Key, T> classes.

When porting old code that uses QDict<T> to Qt 4, there are four classes that you can use:

The APIs of Q3Dict<T> and QMultiHash<QString, T *> are quite similar. The main issue is that Q3Dict supports auto-delete whereas QMultiHash doesn't.

The following table summarizes the API differences between the two classes:

Remarks:

1. Q3Dict requires the user to allocate a specific number of buckets by passing a prime number (17 by default) to the constructor and/or calling Q3Dict::resize() later on. In contrast, QMultiHash's hash table automatically grows and shrinks as needed, and the constructor doesn't take a prime number.
2. Q3Dict supportes case-insensitive lookups by passing false as second argument to the constructor. This feature has no equivalent in QMultiHash. Instead, call QString::toLower() before you insert or lookup a key in the hash.
3. Q3Dict::size() and QMultiHash::size() have different semantics. The former returns the number of buckets in the container, whereas the latter returns the number of items in the container.
4. If there are multiple items with the same key, Q3Dict::remove() removes only the most recently inserted item, whereas QMultiHash::remove() removes all items that share a particular key. To remove only the most recently inserted item, call QMultiHash::take().
5. Q3Dict has only one [] operator (Q3Dict::operator[]()), providing const access to an item's value. QMultiHash also has a non-const overload that can be used on the left side of the assignment operator. If you use the [] operator on a non-const QHash with an unexisting item, QHash will created an element and initialize it to be a null pointer. For that reason, Q3Dict::operator[] should be converted to QMultiHash::value(), not QMultiHash::operator[].

If you use Q3Dict's auto-delete feature (by calling Q3Dict::setAutoDelete(true)), you need to do some more work. You have two options: Either you call delete yourself whenever you remove an item from the container, or you use QMultiHash<QString, T> instead of QMultiHash<QString, T *> (i.e. store values directly instead of pointers to values). Here, we'll see when to call delete.

The following table summarizes the idioms that you need to watch out for if you want to call delete yourself.

 dict.replace(key, value);

 delete hash.take(key);
 hash.insert(key, value);

 dict.remove(key, value);

 delete hash.take(key);

 dict.clear();

 while (!hash.isEmpty()) {
     T *value = *hash.begin();
     hash.erase(hash.begin());
     delete value;
 }

 qDeleteAll(hash);
 hash.clear();

Be aware that Q3Dict's destructor automatically calls clear(). If you have a Q3Dict data member in a custom class and use the auto-delete feature, you will need to call delete on all the items in the container from your class destructor to avoid a memory leak.

Finally, QDictIterator<T> (renamed Q3DictIterator<T>) must also be ported. There are no fewer than four iterator classes that can be used as a replacement: QHash::const_iterator, QHash::iterator, QHashIterator, and QMutableHashIterator. The most straightforward class to use when porting is QHashIterator<QString, T *>. The following table summarizes the API differences:

Be aware that QHashIterator has a different way of iterating than Q3DictIterator. A typical loop with Q3DictIterator looks like this:

 Q3DictIterator<QWidget> i(dict);
 while (i.current() != 0) {
     do_something(i.currentKey(), i.current());
     ++i;
 }

Here's the equivalent QHashIterator loop:

 QHashIterator<QString, QWidget *> i(hash);
 while (i.hasNext()) {
     i.next();                   // must come first
     do_something(i.key(), i.value());
 }

See Java-style iterators for details.

QDir

The following functions used to have a boolean acceptAbsPath parameter that defaulted to true:

• QDir::filePath()
• QDir::absFilePath()
• QDir::cd()
• QDir::mkdir()
• QDir::rmdir()
• QDir::remove()
• QDir::rename()
• QDir::exists()

In Qt 3, if acceptAbsPath is true, a file name starting with '/' is be returned without change; if acceptAbsPath is false, an absolute path is prepended to the file name. For example:

In Qt 4, this parameter is no longer available. If you use it in your code, you can check that QDir::isRelativePath() returns false instead.

For example, if you have code like

 QDir dir("/home/tsmith");
 QString path = dir.filePath(fileName, false);

you can rewrite it as

 QDir dir("/home/tsmith");
 QString path;
 if (dir.isRelativePath(fileName))
     path = dir.filePath(fileName);
 else
     path = fileName;

QDir::encodedEntryList() has been removed.

fileInfoList(), entryInfoList(), and drives() now return a QList<QFileInfo> and not a QPtrList<QFileInfo> *. Code using these methods will not work with the Qt3Support library and must be adapted instead.

See Virtual Functions for a list of QDir virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QDir::match() now always matches case insensitively.

QDir::homeDirPath() has been removed. Use QDir::home() instead, and extract the path separately.

QDns

Qt 3 used its own implementation of the DNS protocol and provided a low-level QDns class. Qt 4's QHostInfo class uses the system's gethostbyname() function from a thread instead.

The old QDns class has been renamed Q3Dns and moved to the Qt3Support library. The new QHostInfo class has a radically different API: It consists mainly of two static functions, one of which is blocking (QHostInfo::fromName()), the other non-blocking (QHostInfo::lookupHost()). See the QHostInfo class documentation for details.

QDockArea

The QDockArea class has been renamed Q3DockArea and moved to the Qt3Support library. In Qt 4, QMainWindow handles the dock and toolbar areas itself. See the QMainWindow documentation for details.

QDockWindow

The old QDockWindow class has been renamed Q3DockWindow and moved to the Qt3Support library. In Qt 4, there is a new QDockWidget class with a different API. See the class documentation for details.

See Virtual Functions for a list of QDockWidget virtual member functions in Qt 3 that are no longer virtual in Qt 4.

Note: Q3DockWindow's horizontallyStretchable property can be achieved in QDockWidget with size policies.

QDragObject

The QDragObject class has been renamed Q3DragObject and moved to the Qt3Support library. In Qt 4, it has been replaced by the QMimeData class. See the class documentation for details.

Note that the Q3DragObject::DragCopyOrMove drag and drop mode is interpreted differently to Qt 3's QDragObject::DragCopyOrMove mode. In Qt 3, a move operation was performed by default, and the user had to hold down the Ctrl key to perform a copy operation. In Qt 4, a copy operation is performed by default; the user has to hold down the Shift key to perform a move operation.

See Porting to Qt 4 - Drag and Drop for a comparison between the drag and drop APIs in Qt 3 and Qt 4.

QDropSite

The QDropSite class has been renamed Q3DropSite and moved to the Qt3Support library.

The QDropSite class has been obsolete ever since Qt 2.0. The only thing it does is call QWidget::setAcceptDrops(true).

For example, if you have code like

 class MyWidget : public QWidget, public QDropSite
 {
 public:
     MyWidget(const QWidget *parent)
         : QWidget(parent), QDropSite(this)
     {
     }
     ...
 }

you can rewrite it as

 class MyWidget : public QWidget
 {
 public:
     MyWidget(const QWidget *parent)
         : QWidget(parent)
     {
         setAcceptDrops(true);
     }
     ...
 }

See Porting to Qt 4 - Drag and Drop for a comparison between the drag and drop APIs in Qt 3 and Qt 4.

QEditorFactory

The QEditorFactory class has been renamed Q3EditorFactory and moved to the Qt3Support library.

See QtSql Module for an overview of the new SQL classes.

QEventLoop

In Qt 3, QEventLoop combined the Qt event loop and the event dispatching. In Qt 4, these tasks are now assigned to two distinct classes: QEventLoop and QAbstractEventDispatcher.

If you subclassed QEventLoop to integrate with another library's event loop, you must subclass QAbstractEventDispatcher instead. See the class documentation for details.

Developers using QEventLoop::loopLevel() in Qt 3 should use QCoreApplication::loopLevel() instead. Note that this function is marked as obsolete, but it is expected to be available for the lifetime of Qt 4.

QFileDialog

The QFileDialog class in Qt 4 has been totally rewritten. It provides most of the functionality of the old QFileDialog class, but with a different API. Some functionality, such as the ability to preview files, is expected to be added in a later Qt 4 release.

The old QFileDialog, QFileIconProvider, and QFilePreview classes has been renamed Q3FileDialog, Q3FileIconProvider, and Q3FilePreview and have been moved to Qt3Support. You can use them if you need some functionality not provided yet by the new QFileDialog class.

The following table lists which functions have been renamed or removed in Qt 4.

Remarks:

1. The Q3FileDialog::addFilter(const QString &) function has no direct equivalent in the new QFileDialog. Use QFileDialog::setFilters() instead.

   For example, if you have code like

        fileDialog->addFilter(tr("JPEG files (*.jpg *.jpeg)"));

   you can rewrite it as

        QStringList filters = fileDialog->filters();
        filters << tr("JPEG files (*.jpg *.jpeg)");
        fileDialog->setFilters(filters);

2. The Q3FileDialog::setSelectedFilter(int) overload has no direct equivalent in the new QFileDialog. Use QFileDialog::selectFilter(const QString &) instead.

   For example, if you have code like

        fileDialog->setSelectedFilter(3);

   you can rewrite it as

        fileDialog->selectFilter(fileDialog->filters().at(3));

There are no equivalent virtual functions to the two Q3FileDialog::setSelectedFilter() virtual functions in the QFileDialog API. In addition, these functions have been renamed or removed, as described above.

QFocusData

The QFocusData class is not available in Qt 4. Some of its functionality is available via the QWidget::nextInFocusChain() and QWidget::focusNextPrevChild() functions.

QFocusEvent

The setReason() function is no longer present in Qt 4. It is necessary to define the reason when constructing a focus event.

QFont

QFont::Script has been moved to QFontDatabase::WritingSystem.

QFrame

The QFrame class has been made more lightweight in Qt 4, by reducing the number of properties and virtual functions. The reduction in the number of virtual functions is significant because QFrame is the base class of many Qt classes.

Here's an overview of the changes:

• QFrame no longer has a margin property (which wasn't honored by Qt's layout managers anyway).
• QFrame no longer has a frameChanged() function, reimplement QFrame::resizeEvent() instead.
• QFrame used to have drawFrame(QPainter *) and drawContents(QPainter *) virtual functions. These are now gone. In Qt 4, the frame is drawn by the QFrame::paintEvent() function. If you want to change the way QFrame paints itself, reimplement this function. To draw the contents of the frame, reimplement QFrame:paintEvent() and call the base class implementation of the function before you use the contentsRect() function inherited from QWidget, to retrieve the rectangle to paint on.

To help with porting, the Qt3Support library contains a Q3Frame class that inherits QFrame and provides a similar API to the old QFrame class. If you derived from QFrame in your application, you might want to use Q3Frame as a base class as a first step in the porting process, and later move on to the new QFrame class.

See Virtual Functions for a list of QFrame virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QFtp

QFtp no longer inherits from QNetworkProtocol. See the section on QNetworkProtocol for details.

The old QFtp class has been renamed Q3Ftp and moved to the Qt3Support library.

QGLayoutIterator

The QGLayoutIterator class no longer exists in Qt 4. This makes only a difference if you implemented custom layout managers (i.e., QLayout subclasses).

The new approach is much simpler: It consists in reimplementing QLayout::itemAt() and QLayout::takeAt(). These functions operate on indexes, eliminating the need for a layout iterator class.

QGrid

The QGrid class is now only available as Q3Grid in Qt 4. You can achieve the same result as QGrid by creating a QWidget with a grid layout:

For example, if you have code like

 QGrid *grid = new QGrid(2, Qt::Horizontal);
 QPushButton *child1 = new QPushButton(grid);
 QPushButton *child2 = new QPushButton(grid);
 QPushButton *child3 = new QPushButton(grid);
 QPushButton *child4 = new QPushButton(grid);

you can rewrite it as

 QWidget *grid = new QWidget;
 QPushButton *child1 = new QPushButton(grid);
 QPushButton *child2 = new QPushButton(grid);
 QPushButton *child3 = new QPushButton(grid);
 QPushButton *child4 = new QPushButton(grid);

 QVBoxLayout *layout = new QVBoxLayout;
 layout->addWidget(child1, 0, 0);
 layout->addWidget(child2, 0, 1);
 layout->addWidget(child3, 1, 0);
 layout->addWidget(child4, 1, 1);
 grid->setLayout(layout);

QGridLayout

See Virtual Functions for a list of QGridLayout virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QGridView

The QGridView class has been renamed Q3GridView and moved to the Qt3Support library. In Qt 4, we recommend that you use QTableView or QAbstractItemView for presenting tabular data.

See Model/View Programming for an overview of the new item view classes.

QGroupBox

The QGroupBox class has been redesigned in Qt 4. Many of the features of the old QGroupBox class can be obtained by using the Q3GroupBox class from the Qt3Support library.

The new QGroupBox is more lightweight. It doesn't attempt to duplicate functionality already provided by QGridLayout, and it does not inherit from QFrame. As a result, the following members have been removed:

• Q3GroupBox::setColumns(), Q3GroupBox::columns()
• Q3GroupBox::setOrientation(), Q3GroupBox::orientation()
• Q3GroupBox::setInsideMargin(), Q3GroupBox::insideMargin()
• Q3GroupBox::addSpace()

Naturally, the columns and orientation properties have also been removed.

If you rely on some of the missing functionality in your application, you can use Q3GroupBox instead of QGroupBox as a help to porting.

See Virtual Functions for a list of QGroupBox virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QHBox

The QHBox class is now only available as Q3HBox in Qt 4. You can achieve the same result as QHBox by creating a QWidget with an horizontal layout:

For example, if you have code like

 QHBox *hbox = new QHBox;
 QPushButton *child1 = new QPushButton(hbox);
 QPushButton *child2 = new QPushButton(hbox);

you can rewrite it as

 QWidget *hbox = new QWidget;
 QPushButton *child1 = new QPushButton;
 QPushButton *child2 = new QPushButton;

 QHBoxLayout *layout = new QHBoxLayout;
 layout->addWidget(child1);
 layout->addWidget(child2);
 hbox->setLayout(layout);

Note that child widgets are not automatically placed into the widget's layout; you will need to manually add each widget to the QHBoxLayout.

QHeader

The QHeader class has been renamed Q3Header and moved to the Qt3Support library. In Qt 4, it is replaced by the QHeaderView class.

See Model/View Programming for an overview of the new item view classes.

QHGroupBox

The QHGroupBox class has been renamed Q3HGroupBox and moved to the Qt3Support library. Qt 4 does not provide a specific replacement class for QHGroupBox since QGroupBox is designed to be a generic container widget. As a result, you need to supply your own layout for any child widgets.

See #QGroupBox for more information about porting code that uses group boxes.

QHttp

QHttp no longer inherits from QNetworkProtocol. See the See the section on QNetworkProtocol for details.

The old QHttp, QHttpHeader, QHttpRequestHeader, and QHttpResponseHeader classes have been renamed Q3Http, Q3HttpHeader, Q3HttpRequestHeader, and Q3HttpResponseHeader and have been moved to the Qt3Support library.

QIconFactory

The QIconFactory class is no longer part of Qt. It has been replaced by the QIconEngine class.

QIconSet

The QIconSet class is no longer part of Qt. It has been replaced by the QIcon class.

QIconView

The QIconView, QIconViewItem, QIconDrag, and QIconDragItem classes has been renamed Q3IconView, Q3IconViewItem, Q3IconDrag, and Q3IconDragItem and moved to the Qt3Support library. New Qt applications should use QListWidget or its base class QListView instead, and call QListView::setViewMode(QListView::IconMode) to obtain an "icon view" look.

See Model/View Programming for an overview of the new item view classes.

QImageDrag

The QImageDrag class has been renamed Q3ImageDrag and moved to the Qt3Support library. In Qt 4, use QMimeData instead and call QMimeData::setImage() to set the image.

See Porting to Qt 4 - Drag and Drop for a comparison between the drag and drop APIs in Qt 3 and Qt 4.

QImageIO

The QImageIO class has been split into two classes: QImageReader and QImageWriter. The table below shows the correspondance between the two APIs:

QIntCache<T>

QIntCache<T> has been moved to Qt3Support. It has been replaced by QCache<int, T>.

For details, read the section on QCache<T>, mentally substituting int for QString.

QIntDict<T>

QIntDict<T> and QIntDictIterator<T> have been moved to Qt3Support. They have been replaced by the more modern QHash<Key, T> and QMultiHash<Key, T> classes and their associated iterator classes.

When porting old code that uses QIntDict<T> to Qt 4, there are four classes that you can use:

• QMultiHash<int, T *>
• QMultiHash<int, T>
• QHash<int, T *>
• QHash<int, T>

For details, read the section on QDict<T>, mentally substituting int for QString.

QIODevice

The QIODevice class's API has been simplified to make it easier to subclass and to make it work more smoothly with asynchronous devices such as QTcpSocket and QProcess.

The following virtual functions have changed name or signature:

Note: QIODevice::open(QIODevice::OpenMode) is no longer pure virtual.

The following functions are no longer virtual or don't exist anymore:

The IO_xxx flags have been revised, and the protected setFlags() function removed. Most of the flags have been eliminated because errors are best handled by implementing certain functions in QIODevice subclasses rather than through the base classes. The file access flags, such as IO_ReadOnly and IO_WriteOnly, have been moved to the QIODevice class to avoid polluting the global namespace. The table below shows the correspondence between the Qt 3 IO_xxx flags and the Qt 4 API:

QIODeviceSource

The QIODeviceSource class was used internally in Qt 2.x in conjunction with QImageConsumer. It was obsoleted in Qt 3.0. If you use this mechanism in your application, please submit a report to the Task Tracker on the Qt website and we will try to find a satisfactory substitute.

QLabel

QLabel doesn't enable word-wrap automatically anymore when rich text is used. You can enable it by calling QLabel::setWordWrap() or by setting the wordWrap property. The reason for this change is that the old behavior was confusing to many users.

Also, QLabel no longer offers an autoResize property. Instead, you can call QWidget::setFixedSize() on the label, with QLabel::sizeHint() as the argument, whenever you change the contents of the QLabel.

See also Virtual Functions for a list of QLabel virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QLayout

In Qt 4, margins are always handled by layouts; there is no QLayout::setSupportsMargin() function anymore.

The deleteAllItems() function is now only available if QT3_SUPPORT is defined. If you maintain a QList of layout items, you can use qDeleteAll() to remove all the items in one go.

In Qt 3, it was possible to change the resizing behavior for layouts in top-level widgets by adjusting the layout's resizeMode property. In Qt 4, this property has been replaced by the QLayout::sizeConstraint property which provides more control over how the layout behaves when resized.

See also the section on QLayoutIterator and the section on QGLayoutIterator.

QLayoutIterator

The QLayoutIterator class is obsoleted in Qt 4. It is available only if QT3_SUPPORT is defined. It can be replaced by the QLayout::itemAt() and QLayout::takeAt() functions, which operate on indexes.

For example, if you have code like

     QLayoutIterator it = layout()->iterator();
     QLayoutItem *child;
     while ((child = it.current()) != 0) {
         if (child->widget() == myWidget) {
             it.takeCurrent();
             return;
         ++it;
     }

you can rewrite it as

     int i = 0;
     QLayoutItem *child;
     while ((child = layout()->itemAt(i)) != 0) {
         if (child->widget() == myWidget) {
             layout()->takeAt(i);
             return;
         }
         ++i;
     }

QLineEdit

See Properties for a list of QLineEdit properties in Qt 3 that have changed in Qt 4.

The default value of QLineEdit's dragEnabled property was true in Qt 3. In Qt 4, the default value is false.

Note that QLineEdit in Qt 4 is no longer a subclass of QFrame. If you need to visually style a line edit with a frame, we recommend either using a QFrame as a container for a QLineEdit or customizing the line edit with a style sheet.

QListBox

The QListBox, QListBoxItem, QListBoxText, and QListBoxPixmap classes have been renamed Q3ListBox, Q3ListBoxItem, Q3ListBoxText, and Q3ListBoxPixmap and have been moved to the Qt3Support library. New Qt applications should use QListWidget or its base class QListView instead.

See Model/View Programming for an overview of the new item view classes.

QListView

The QListView, QListViewItem, QCheckListItem, and QListViewItemIterator classes have been renamed Q3ListView, Q3ListViewItem, Q3CheckListItem, and Q3ListViewItemIterator, and have been moved to the Qt3Support library. New Qt applications should use one of the following four classes instead: QTreeView or QTreeWidget for tree-like structures; QListWidget or the new QListView class for one-dimensional lists.

See Model/View Programming for an overview of the new item view classes.

QLocalFs

The QLocalFs class is no longer part of the public Qt API. It has been renamed Q3LocalFs and moved to Qt3Support. Use QDir, QFileInfo, or QFile instead.

QMainWindow

The QMainWindow class has been redesigned in Qt 4 to provide a more modern look and feel and more flexibility. The API has changed to reflect that. The old QMainWindow class has been renamed Q3MainWindow and moved to Qt3Support. See the QMainWindow class documentation for details.

QMemArray<T>

QMemArray<T> has been moved to Qt3Support. It has been replaced by the QVector<T> class.

The following table summarizes the API differences between the two classes.

Remarks:

1. QMemArray::assign(const T *, uint) and QMemArray::duplicate(const T *, uint) can be replaced by QVector::resize() and qCopy().

   For example, if you have code like

        QMemArray<QSize> array;
        ...
        array.assign(data, size);

   you can rewrite it as

        QVector<QSize> vector;
        ...
        vector.resize(size);
        qCopy(data, data + size, vector.begin());

2. QMemArray is an explicitly shared class, whereas QVector is implicitly shared. See Explicit Sharing for more information.

QMenuBar

In Qt 3, QMenuBar inherited from QFrame and QMenuData; in Qt 4, it is a direct subclass of QWidget. Applications that provided customized menu bars will need to take advantage of the styling features described in the Qt Style Sheets document.

It is not possible to add widgets to menu bars in Qt 4.

QMenuData

In Qt 4, the QMenu class provides a menu widget that can be used in all the places where menus are used in an application. Unlike QMenuData, QMenu is designed around the concept of actions, provided by the QAction class, instead of the identifiers used in Qt 3.

In Qt 3, it was possible to insert widgets directly into menus by using a specific QMenuData::insertItem() overload. In Qt 4.2 and later, the QWidgetAction class can be used to wrap widgets for use in Qt 4's action-based APIs.

QMessageBox

The QMessageBox::iconPixmap() function used to return a "const QPixmap *". In Qt 4, it returns a QPixmap.

QMimeSourceFactory

The QMimeSourceFactory has been renamed Q3MimeSourceFactory and moved to the Qt3Support library. New Qt applications should use Qt 4's Resource System instead.

QMovie

The QMovie API has been revised in Qt 4 to make it more consistent with the other Qt classes (notably QImageReader). The table below summarizes the changes.

QMultiLineEdit

The QMultiLineEdit class in Qt 3 was a convenience QTextEdit subclass that provided an interface compatible with Qt 2's QMultiLineEdit class. In Qt 4, it is called Q3MultiLineEdit, it inherits Q3TextEdit, and it is part of Qt3Support. Use QTextEdit in new code.

QNetworkProtocol

The QNetworkProtocol, QNetworkProtocolFactoryBase, QNetworkProtocolFactory<T>, and QNetworkOperation classes are no longer part of the public Qt API. They have been renamed Q3NetworkProtocol, Q3NetworkProtocolFactoryBase, Q3NetworkProtocolFactory<T>, and Q3NetworkOperation and have been moved to the Qt3Support library.

In Qt 4 applications, you can use classes like QFtp and QNetworkAccessManager directly to perform file-related actions on a remote host.

QObject

QObject::children() now returns a QObjectList instead of a pointer to a QObjectList. See also the comments on QObjectList below.

Use QObject::findChildren() (or qFindChildren() if you need MSVC 6 compatibility) instead of QObject::queryList(). For example:

 QList<QWidget *> myWidgets = qFindChildren<QWidget *>(myParent);

QObject::killTimers() has been removed because it was unsafe to use in subclass. (A subclass normally doesn't know whether the base class uses timers or not.)

The QObject::name property has been renamed QObject::objectName.

QObject::objectTrees() has been removed. If you are primarly interested in widgets, use QApplication::allWidgets() or QApplication::topLevelWidgets().

QObjectDictionary

The QObjectDictionary class is a synonym for QAsciiDict<QMetaObject>. See the section on QAsciiDict<T>.

QObjectList

In Qt 3, the QObjectList class was a typedef for QPtrList<QObject>. In Qt 4, it is a typedef for QList<QObject *>. See the section on QPtrList<T>.

QPaintDevice

To reimplement painter backends one previously needed to reimplement the virtual function QPaintDevice::cmd(). This function is taken out and should is replaced with the function QPaintDevice::paintEngine() and the abstract class QPaintEngine. QPaintEngine provides virtual functions for all drawing operations that can be performed on a painter backend.

bitBlt() and copyBlt() are now only compatibility functions. Use QPainter::drawPixmap() instead.

QPaintDeviceMetrics

All functions that used to be provided by the QPaintDeviceMetrics class have now been moved to QPaintDevice.

For example, if you have code like

     QPaintDeviceMetrics metrics(widget);
     int deviceDepth = metrics.depth();

you can rewrite it as

     int deviceDepth = widget->depth();

For compatibility, the old QPaintDeviceMetrics class has been renamed Q3PaintDeviceMetrics and moved to Qt3Support.

QPainter

The QPainter class has undergone some changes in Qt 4 because of the way rectangles are drawn. In Qt 4, the result of drawing a QRect with a pen width of 1 pixel is 1 pixel wider and 1 pixel taller than in Qt 3.

For compatibility, we provide a Q3Painter class in Qt3Support that provides the old semantics. See the Q3Painter documentation for details and for the reasons why we had to make this change.

The QPainter::CoordinateMode enum has been removed in Qt 4. All clipping operations are now defined using logical coordinates and are subject to transformation operations.

The QPainter::RasterOP enum has been replaced with QPainter::CompositionMode.

QPicture

In Qt 3, a QPicture could be saved in the SVG file format. In Qt 4, the SVG support is provided by the QtSvg module, which includes classes for displaying the contents of SVG files.

If you would like to generate SVG files, you can use the Q3Picture compatibility class or the QSvgGenerator class introduced in Qt 4.3.

QPixmap

The mask() function has been changed to return a reference to a QBitmap rather than a pointer. As a result, it is no longer possible simply to test for a null pointer when determining whether a pixmap has a mask. Instead, you need to explicitly test whether the mask bitmap is null or not.

For example, if you have code like

     if (pixmap.mask())
         widget->setMask(*pixmap.mask());

you can rewrite it as

     if (!pixmap.mask().isNull())
         widget->setMask(pixmap.mask());

The QPixmap::setOptimization() and QPixmap::setDefaultOptimization() mechanism is no longer available in Qt 4.

QPointArray

The QPointArray class has been renamed QPolygon in Qt 4 and has undergone significant changes. In Qt 3, QPointArray inherited from QMemArray<QPoint>. In Qt 4, QPolygon inherits from QVector<QPoint>. Everything mentioned in the section on QMemArray<T> apply for QPointArray as well.

The Qt3Support library contains a Q3PointArray class that inherits from QPolygon and provides a few functions that existed in QPointArray but no longer exist in QPolygon. These functions include Q3PointArray::makeArc(), Q3PointArray::makeEllipse(), and Q3PointArray::cubicBezier(). In Qt 4, we recommend that you use QPainterPath for representing arcs, ellipses, and Bezier curves, rather than QPolygon.

The QPolygon::setPoints() and QPolygon::putPoints() functions return void in Qt 4. The corresponding Qt 3 functions returned a bool indicating whether the array was successfully resized or not. This can now be checked by checking QPolygon::size() after the call.

QPopupMenu

For most purposes, QPopupMenu has been replaced by QMenu in Qt 4. For compatibility with older applications, Q3PopupMenu provides the old API and features that are specific to pop-up menus. Note that, when using Q3PopupMenu, the menu's actions must be Q3Actions.

In Qt 3, it was common practice to add entries to pop-up menus using the insertItem() function, maintaining identifiers for future use; for example, to dynamically change menu items. In Qt 4, menu entries are completely represented by actions for consistency with other user interface components, such as toolbar buttons. Create new menus with the QMenu class, and use the overloaded QMenu::addAction() functions to insert new entries. If you need to manage a set of actions created for a particular menu, we suggest that you construct a QActionGroup and add them to that.

The Main Window Examples provided show how to use Qt's action system to construct menus, toolbars, and other common user interface elements.

QPrinter

The QPrinter class now expects printing to be set up from a QPrintDialog.

QProcess

The QProcess class has undergone major improvements in Qt 4. It now inherits QIODevice, which makes it possible to combine QProcess with a QTextStream or a QDataStream.

The old QProcess class has been renamed Q3Process and moved to the Qt3Support library.

QProgressBar

The QProgressBar API has been significantly improved in Qt 4. The old QProgressBar API is available as Q3ProgressBar in the Qt3Support library.

QProgressDialog

The QProgressDialog API has been significantly improved in Qt 4. The old QProgressDialog API is available as Q3ProgressDialog in the Qt3Support library.

See Properties for a list of QProgressDialog properties in Qt 3 that have changed in Qt 4.

QPtrCollection<T>

The QPtrCollection<T> abstract base class has been renamed Q3PtrCollection<T> moved to the Qt3Support library. There is no direct equivalent in Qt 4.

See Container Classes for a list of Qt 4 containers.

QPtrDict<T>

QPtrDict<T> and QPtrDictIterator<T> have been renamed Q3PtrDict<T> and Q3PtrDictIterator<T> and have been moved to the Qt3Support library. They have been replaced by the more modern QHash<Key, T> and QMultiHash<Key, T> classes and their associated iterator classes.

When porting old code that uses Q3PtrDict<T> to Qt 4, there are four classes that you can use:

• QMultiHash<void *, T *>
• QMultiHash<void *, T>
• QHash<void *, T *>
• QHash<void *, T>

(You can naturally use other types than void * for the key type, e.g. QWidget *.)

To port Q3PtrDict<T> to Qt 4, read the section on QDict<T>, mentally substituting void * for QString.

QPtrList<T>

QPtrList<T>, QPtrListIterator<T>, and QPtrListStdIterator<T> have been moved to the Qt3Support library. They have been replaced by the more modern QList and QLinkedList classes and their associated iterator classes.

When porting to Qt 4, you have the choice of using QList<T> or QLinkedList<T> as alternatives to QValueList<T>. QList<T> has an index-based API and provides very fast random access (QList::operator[]), whereas QLinkedList<T> has an iterator-based API.

The following table summarizes the API differences between QPtrList<T> and QList<T *>:

Remarks:

1. QPtrList::toVector(QGVector *) can be replaced by QVector::resize() and qCopy().

   For example, if you have code like

        QPtrList<QWidget> list;
        ...
        QPtrVector<QWidget> vector;
        list.toVector(&vector);

   you can rewrite it as

        QList<QWidget *> list;
        ...
        QVector<QWidget *> vector;
        vector.resize(list.size());
        qCopy(list.begin(), list.end(), vector.begin());

2. QPtrList::sort() relied on the virtual compareItems() to sort items. In Qt 4, you can use qSort() instead and pass your "compare item" function as an argument.
3. QPtrList::find(const T *) returns an iterator, whereas QList::indexOf(T *) returns an index. To convert an index into an iterator, add the index to QList::begin().
4. QPtrList::removeFirst() and QPtrList::removeLast() return a bool that indicates whether the element was removed or not. The corresponding QList functions return void. You can achieve the same result by calling QList::isEmpty() before attempting to remove an item.

If you use QPtrList's auto-delete feature (by calling QPtrList::setAutoDelete(true)), you need to do some more work. You have two options: Either you call delete yourself whenever you remove an item from the container, or you can use QList<T> instead of QList<T *> (i.e. store values directly instead of pointers to values). Here, we'll see when to call delete.

The following table summarizes the idioms that you need to watch out for if you want to call delete yourself.

 list.replace(index, value);

 delete list[index];
 list[index] = value;

 list.removeFirst();

 delete list.takeFirst();

 list.removeLast();

 delete list.takeLast();

 list.remove(index);

 delete list.takeAt(index);

 list.remove(value);

 int i = list.indexOf(value);
 if (i != -1)
     delete list.takeAt(i);

 list.remove();

 QMutableListIterator<T *> i;
 ...
 delete i.value();
 i.remove();

 list.clear();

 while (!list.isEmpty())
     delete list.takeFirst();

 qDeleteAll(list);
 list.clear();

Be aware that QPtrList's destructor automatically calls clear(). If you have a QPtrList data member in a custom class and use the auto-delete feature, you will need to call delete on all the items in the container from your class destructor to avoid a memory leak.

QPtrList had the concept of a "current item", which could be used for traversing the list without using an iterator. When porting to Qt 4, you can use the Java-style QListIterator<T *> (or QMutableListIterator<T *>) class instead. The following table summarizes the API differences:

Be aware that QListIterator has a different way of iterating than QPtrList. A typical loop with QPtrList looks like this:

 QPtrList<QWidget> list;
 ...
 while (list.current() != 0) {
     do_something(list.current());
     list.next();
 }

Here's the equivalent QListIterator loop:

 QList<QWidget *> list;
 ...
 QListIterator<QWidget *> i(list);
 while (i.hasNext())
     do_something(i.next());

Finally, QPtrListIterator<T> must also be ported. There are no fewer than four iterator classes that can be used as a replacement: QList::const_iterator, QList::iterator, QListIterator, and QMutableListIterator. The most straightforward class to use when porting is QMutableListIterator<T *> (if you modify the list through the iterator) or QListIterator<T *> (if you don't). The following table summarizes the API differences:

Again, be aware that QListIterator has a different way of iterating than QPtrList. A typical loop with QPtrList looks like this:

 QPtrList<QWidget> list;
 ...
 QPtrListIterator<QWidget> i;
 while (i.current() != 0) {
     do_something(i.current());
     i.next();
 }

Here's the equivalent QListIterator loop:

 QList<QWidget *> list;
 ...
 QListIterator<QWidget *> i(list);
 while (i.hasNext())
     do_something(i.next());

Finally, QPtrListStdIterator<T> must also be ported. This is easy, because QList also provides STL-style iterators (QList::iterator and QList::const_iterator).

QPtrQueue<T>

QPtrQueue has been moved to the Qt3Support library. It has been replaced by the more modern QQueue class.

The following table summarizes the differences between QPtrQueue<T> and QQueue<T *>:

If you use QPtrQueue's auto-delete feature (by calling QPtrQueue::setAutoDelete(true)), you need to do some more work. You have two options: Either you call delete yourself whenever you remove an item from the container, or you can use QQueue<T> instead of QQueue<T *> (i.e. store values directly instead of pointers to values). Here, we will show when to call delete.

 queue.dequeue();

 delete queue.dequeue();

 queue.remove();

 delete queue.dequeue();

 queue.clear();

 while (!queue.isEmpty())
     delete queue.dequeue();

 qDeleteAll(queue);
 queue.clear();

QPtrStack<T>

QPtrStack has been moved to the Qt3Support library. It has been replaced by the more modern QStack class.

The following table summarizes the differences between QPtrStack<T> and QStack<T *>:

If you use QPtrStack's auto-delete feature (by calling QPtrStack::setAutoDelete(true)), you need to do some more work. You have two options: Either you call delete yourself whenever you remove an item from the container, or you can use QStack<T> instead of QStack<T *> (i.e. store values directly instead of pointers to values). Here, we will show when to call delete.

 stack.pop();

 delete stack.pop();

 stack.remove();

 delete stack.pop();

 stack.clear();

 while (!stack.isEmpty())
     delete stack.pop();

 qDeleteAll(stack);
 stack.clear();

QPtrVector<T>

QPtrVector<T> has been moved to Qt3Support. It has been replaced by the more modern QVector class.

When porting to Qt 4, you can use QVector<T *> as an alternative to QPtrVector<T>. The APIs of QPtrVector<T> and QVector<T *> are somewhat similar. The main issue is that QPtrVector supports auto-delete whereas QVector doesn't.

The following table summarizes the API differences between the two classes:

Remarks:

1. QPtrVector::insert(uint, T *) sets an item to store a certain pointer value. This is not the same as QVector::insert(int, T *), which creates space for the item by moving following items by one position. Use vect[i] = ptr to set a QVector item to a particular value.
2. QPtrVector::remove(uint) sets an item to be 0. This is not the same as QVector::removeAt(int), which entirely erases the item, reducing the size of the vector. Use vect[i] = 0 to set a QVector item to 0.
3. Likewise, QPtrVector::take(uint) sets an item to be 0 and returns the previous value of the item. Again, this is easy to achieve using QVector::operator[]().
4. QPtrVector::count() returns the number of non-null items in the vector, whereas QVector::count() (like QVector::size()) returns the number of items (null or non-null) in the vector. Fortunately, it's not too hard to simulate QPtrVector::count().

   For example, if you have code like

        int numValidItems = vect.count();

   you can rewrite it as

        int numValidItems = vect.size() - vect.count(0);

If you use QVector's auto-delete feature (by calling QVector::setAutoDelete(true)), you need to do some more work. You have two options: Either you call delete yourself whenever you remove an item from the container, or you use QVector<T> instead of QVector<T *> (i.e. store values directly instead of pointers to values). Here, we'll see when to call delete.

The following table summarizes the idioms that you need to watch out for if you want to call delete yourself.

 vect.insert(i, ptr);

 delete vect[i];
 vect[i] = ptr;

 vect.remove(i);

 delete vect[i];
 vect[i] = 0;

 T *ptr = vect.take(i);

 T *ptr = vect[i];
 vect[i] = 0;

 vect.resize(n)

 while (n > vect.size())
     vect.append(0);
 while (n < vect.size() {
     T *ptr = vect.last();
     vect.remove(vect.size() - 1);
     delete ptr;
 }

 vect.clear();

 for (int i = 0; i < vect.size(); ++i)
     T *ptr = vect[i];
     vect[i] = 0;
     delete ptr;
 }

 qDeleteAll(vect);
 vect.clear();

Be aware that QPtrVector's destructor automatically calls clear(). If you have a QPtrVector data member in a custom class and use the auto-delete feature, you will need to call delete on all the items in the container from your class destructor to avoid a memory leak.

QPushButton

See Properties for a list of QPushButton properties in Qt 3 that have changed in Qt 4.

QRangeControl

In Qt 3, various "range control" widgets (QDial, QScrollBar, QSlider, and QSpin) inherited from both QWidget and QRangeControl.

In Qt 4, QRangeControl has been replaced with the new QAbstractSlider and QAbstractSpinBox classes, which inherit from QWidget and provides similar functionality. Apart from eliminating unnecessary multiple inheritance, the new design allows QAbstractSlider to provide signals, slots, and properties.

The old QRangeControl class has been renamed Q3RangeControl and moved to the Qt3Support library, together with the (undocumented) QSpinWidget class.

If you use QRangeControl as a base class in your application, you can switch to use QAbstractSlider or QAbstractSpinBox instead.

For example, if you have code like

     class VolumeControl : public QWidget, public QRangeControl
     {
         ...
     protected:
         void valueChange() {
             update();
             emit valueChanged(value());
         }
         void rangeChange() {
             update();
         }
         void stepChange() {
             update();
         }
     };

you can rewrite it as

     class VolumeControl : public QAbstractSlider
     {
         ...
     protected:
         void sliderChange(SliderChange change) {
             update();
             if (change == SliderValueChange)
                 emit valueChanged(value());
         }
     };

QRegExp

The search() and searchRev() functions have been renamed to indexIn() and lastIndexIn() respectively.

QRegion

The following changes have been made to QRegion in Qt 4:

• There is no longer any difference between a null region and an empty region. Use isEmpty() in most places where you would have used a null QRegion.
• QRegion::rects() used to return a QMemArray<QRect>. It now returns a QVector<QRect>.

QScrollBar

See Properties for a list of QScrollBar properties in Qt 3 that have changed in Qt 4.

QScrollView

The QScrollView class has been renamed Q3ScrollView and moved to the Qt3Support library. It has been replaced by the QAbstractScrollArea and QScrollArea classes.

Note that Qt 4 in general uses the QScrollArea::widget() function where Qt 3 used QScrollView::viewport(). The rationale for this is that it is no longer possible to draw directly on a scroll area. The QScrollArea::widget() function returns the widget set on the scroll area.

QScrollView was designed to work around the 16-bit limitation on widget coordinates found on most window systems. In Qt 4, this is done transparently for all widgets, so there is no longer a need for such functionality in QScrollView. For that reason, the new QAbstractScrollArea and QScrollArea classes are much more lightweight, and concentrate on handling scroll bars.

QServerSocket

The QServerSocket class has been renamed Q3ServerSocket and moved to the Qt3Support library. In Qt 4, it has been replaced by QTcpServer.

With Q3ServerSocket, connections are accepted by reimplementing a virtual function (Q3ServerSocket::newConnection()). With QTcpServer, on the other hand, you don't need to subclass. Instead, simply connect to the QTcpServer::newConnection() signal.

QSettings

The QSettings class has been rewritten to be more robust and to respect existing standards (e.g., the INI file format). The API has also been extensively revised. The old API is still provided when Qt 3 support is enabled.

Since the format and location of settings have changed between Qt 3 and Qt 4, the Qt 4 version of your application won't recognize settings written using Qt 3.

QShared

The QShared class has been obsoleted by the more powerful QSharedData and QSharedDataPointer as a means of creating custom implicitly shared classes. It has been renamed Q3Shared and moved to the Qt3Support library.

An easy way of porting to Qt 4 is to include this class into your project and to use it instead of QShared:

 struct Shared
 {
     Shared() : count(1) {}
     void ref() { ++count; }
     bool deref() { return !--count; }
     uint count;
 };

If possible, we recommend that you use QSharedData and QSharedDataPointer instead. They provide thread-safe reference counting and handle all the reference counting behind the scenes, eliminating the risks of forgetting to increment or decrement the reference count.

QSignal

The QSignal class has been renamed to Q3Signal and moved to the Qt3Support library. The preferred approach is to create your own QObject subclass with a signal that has the desired signature. Alternatively, you can call QMetaObject::invokeMethod() if you want to invoke a slot.

QSimpleRichText

QSimpleRichText has been obsoleted by QTextDocument. It has been renamed Q3SimpleRichText and moved to the Qt3Support library.

Previously, you would do the following with Q3SimpleRichText:

 // Declare the object
 QSimpleRichText richText(text, font);

 // Set the width of the paragraph to w
 richText.setWidth(w);

 // Or set a reasonable default size
 richText.adjustSize();

 // Query for its used size
 int width = richText.widthUsed();
 int height = richText.height();

 // Draw
 richText.draw(painter, x, y, clipRect, colorGroup);

However, with QTextDocument, you use the following code instead:

 // Declare the object
 QTextDocument doc;

 // If text is rich text, use setHtml()
 doc.setHtml(text);

 // Otherwise, use setPlainText()
 doc.setPlainText(text);

 // Set the width of the paragraph of text to w
 doc.setTextWidth(w);

 // Query for the used size
 int width = doc.idealWidth();
 int height = doc.size().height();

 // Draw
 painter.translate(x, y);
 doc.drawContents(painter, clipRect);

 // If you have a palette/colorgroup you can draw using lower-level functions:
 QAbstractTextDocumentLayout::PaintContext context;
 context.palette = myPalette;
 doc.documentLayout()->draw(painter, context);

See Rich Text Processing for an overview of the Qt 4 rich text classes.

QSlider

The QSlider::sliderStart() and QSlider::sliderRect() functions have been removed.

The slider's rect can now be retrieved using the code snippet below:

 QSlider *slider;
 slider->style()->subControlRect(CC_Slider, sliderOption, SC_SliderHandle, slider);

In addition, the direction of a vertical QSlider has changed, i.e. the bottom is now the minimum, and the top the maximum. You can use the QAbstractSlider::invertedAppearance property to control this behavior.

See Properties for a list of QSlider properties in Qt 3 that have changed in Qt 4.

QSocket

The QSocket class has been renamed Q3Socket and moved to the Qt3Support library. In Qt 4, it has been replaced by the QTcpSocket class, which inherits most of its functionality from QAbstractSocket.

QSocketDevice

The QSocketDevice class has been renamed Q3SocketDevice and moved to the Qt3Support library. In Qt 4, there is no direct equivalent to Q3SocketDevice:

• If you use Q3SocketDevice in a thread to perform blocking network I/O (a technique encouraged by the Qt Quarterly article Unblocking Networking), you can now use QTcpSocket, QFtp, or QNetworkAccessManager, which can be used from non-GUI threads.
• If you use Q3SocketDevice for UDP, you can now use QUdpSocket instead.
• If you use Q3SocketDevice for other uses, Qt 4 offers no alternative right now. However, there is a QAbstractSocketEngine internal class that offers a low-level socket API similar to Q3SocketDevice. Should the need for such functionality arise in Qt 4 applications, we will consider making this class public in a future release.

QSortedList

The QSortedList<T> class has been deprecated since Qt 3.0. In Qt 4, it has been moved to the Qt3Support library.

In new code, we recommend that you use QList<T> instead and use qSort() to sort the items.

QSplitter

The function setResizeMode() has been moved into Qt3Support. Set the stretch factor in the widget's size policy to get equivalent functionality.

The obsolete function drawSplitter() has been removed. Use QStyle::drawPrimitive() to acheive similar functionality.

QSpinBox

See Properties for a list of QSpinBox properties in Qt 3 that have changed in Qt 4.

QSqlCursor

The QSqlCursor class has been renamed Q3SqlCursor and moved to the Qt3Support library. In Qt 4, you can use QSqlQuery, QSqlQueryModel, or QSqlTableModel, depending on whether you want a low-level or a high-level interface for accessing databases.

See QtSql Module for an overview of the new SQL classes.

QSqlDatabase

QSqlDatabase is now a smart pointer that is passed around by value. Simply replace all QSqlDatabase pointers by QSqlDatabase objects.

QSqlEditorFactory

The QSqlEditorFactory class has been renamed Q3SqlEditorFactory and moved to Qt3Support.

See QtSql Module for an overview of the new SQL classes.

QSqlError

The enum Type was renamed to ErrorType, The values were renamed as well:

• None - use NoError instead
• Connection - use ConnectionError instead
• Statement - use StatementError instead
• Transaction - use TransactionError instead
• Unknown - use UnknownError instead

QSqlFieldInfo

The QSqlFieldInfo class has been moved to Qt3Support. Its functionality is now provided by the QSqlField class.

See QtSql Module for an overview of the new SQL classes.

QSqlForm

The QSqlForm class has been renamed Q3SqlForm and moved to the Qt3Support library.

See QtSql Module for an overview of the new SQL classes.

QSqlPropertyMap

The QSqlPropertyMap class has been renamed Q3SqlPropertyMap moved to the Qt3Support library.

See QtSql Module for an overview of the new SQL classes.

QSqlQuery

QSqlQuery::prev() was renamed to QSqlQuery::previous(). QSqlQuery::prev() remains, but it just calls previous(). QSqlQuery no longer has any virtual methods, i.e., exec(), value(), seek(), next(), prev(), first(), last(), and the destructor are no longer virtual.

QSqlRecord

QSqlRecord behaves like a vector now, QSqlRecord::insert() will actually insert a new field instead of replacing the existing one.

QSqlRecordInfo

The QSqlRecordInfo class has been moved to Qt3Support. Its functionality is now provided by the QSqlRecord class.

See QtSql Module for an overview of the new SQL classes.

QSqlSelectCursor

The QSqlSelectCursor class has been renamed Q3SqlSelectCursor and moved to the Qt3Support library.

See QtSql Module for an overview of the new SQL classes.

QStoredDrag

The QStoredDrag class has been renamed Q3StoredDrag and moved to the Qt3Support library. In Qt 4, use QMimeData instead and call QMimeData::setData() to set the data.

See Porting to Qt 4 - Drag and Drop for a comparison between the drag and drop APIs in Qt 3 and Qt 4.

QStr(I)List

The QStrList and QStrIList convenience classes have been deprecated since Qt 2.0. In Qt 4, they have been moved to the Qt3Support library. If you used any of these, we recommend that you use QStringList or QList<QByteArray> instead.

QStr(I)Vec

The QStrVec and QStrIVec convenience classes have been deprecated since Qt 2.0. In Qt 4, they have been moved to Qt3Support. If you used any of these, we recommend that you use QStringList or QList<QByteArray> instead.

QString

Here are the main issues to be aware of when porting QString to Qt 4:

1. The QString::QString(QChar) constructor performed implicit conversion in Qt 3. Now, you will need a cast to convert a QChar to a QString.
2. The QString::QString(const QByteArray &) constructor used to stop at the first '\0' it encountered, for compatibility with Qt 1. This quirk has now been fixed; in Qt 4, the resulting QString always has the same length as the QByteArray that was passed to the constructor.
3. The QString::null static constant has been deprecated in Qt 4. For compatibility, Qt 4 provides a QString::null symbol that behaves more or less the same as the old constant. The new idiom is to write QString() instead of QString::null, or to call clear().

   For example, if you have code like

            str1 = QString::null;
            if (str2 == QString::null)
                do_something(QString::null);

   you can rewrite it as

            str1.clear();
            if (str2.isNull())
                do_something(QString());

   In new code, we recommend that you don't rely on the distinction between a null string and a (non-null) empty string. See Distinction Between Null and Empty Strings for details.

4. QString::latin1() and QString::ascii() have been replaced with QString::toLatin1() and QString::toAscii(), which return a QByteArray instead of a (non-reentrant) const char *. For consistency, QString::utf8() and QString::local8Bit(), which already returned a QByteArray (actually a QCString), have been renamed QString::toUtf8() and QString::toLocal8Bit().

   To obtain a const char * pointer to ASCII or Latin-1 data, use QString::toAscii() or QString::toLatin1() to obtain a QByteArray containing the data, then call QByteArray::constData() to access the character data directly. Note that the pointer returned by this function is only valid for the lifetime of the byte array; you should avoid taking a pointer to the data contained in temporary objects.

    QString greeting = "Hello";
    const char *badData = greeting.toAscii().constData(); // data is invalid
    QByteArray asciiData = greeting.toAscii();
    const char *goodData = asciiData.constData();

   In the above example, the goodData pointer is valid for the lifetime of the asciiData byte array. If you need to keep a copy of the data in a non-Qt data structure, use standard C memory allocation and string copying functions to do so before destroying the byte array.

5. QString::at() returned a non-const reference, whereas the new QString::at() returns a const value. Code like

    str.at(0) = 'X';

   will no longer compile. Instead, use QString::operator[]:

    str[0] = 'X';

6. The QString::contains(x) function (where x is a character or a string) has been renamed QString::count(x). In addition, there now exists a set of QString::contains() functions that returns a boolean value. Replace old calls to contains() with either count() or contains(), depending on whether you care about the specific number of occurrences of a character in the string or only care about whether the string contains that character or not.
7. Many functions in QString had a bool parameter that specified case sensitivity. In Qt 4, in the interest of code readability and maintainability, the bool parameters have been replaced by the Qt::CaseSensitivity enum, which can take the values Qt::CaseSensitive and Qt::CaseInsensitive.

   For example, if you have code like

            if (url.startsWith("http:", false))
                ...

   you can rewrite it as

            if (url.startsWith("http:", Qt::CaseInsensitive))
                ...

8. The QString::setExpand(uint, QChar) function, which already was obsolete in Qt 3, is no longer available. Use QString::operator[] instead.

   For example, if you have code like

            str.setExpand(32, '$');

   you can rewrite it as

            str[32] = '$';

9. The QT_NO_ASCII_CAST and QT_NO_CAST_ASCII macros have been renamed QT_NO_CAST_TO_ASCII and QT_NO_CAST_FROM_ASCII, respectively.
10. The QString::data() used to return the same as QString::ascii(). It now returns a pointer to the Unicode data stored in the QString object. Call QString::ascii() if you want the old behavior.
11. QString::arg() now converts two-digit place markers, allowing up to 99 place markers to be used in any given string.
12. Comparisons between QStrings and NULL in order to determine whether strings are empty are no longer allowed. Use isEmpty() instead.

QStringList

QStringList now inherits from QList<QString> and can no longer be converted to a QValueList<QString>. Since QValueList inherits QList a cast will work as expected.

This change implies some API incompatibilities for QStringList. For example, at() returns the string, not an iterator. See the section on QValueList for details.

The static QStringList::split() function for splitting strings into lists of smaller strings has been replaced by QString::split(), which returns a QStringList.

QStyle

The QStyle API has been overhauled and improved. Most of the information on why this change was done is described in the QStyle overview.

Since QStyle is mostly used internally by Qt's widgets and styles and since it is not essential to the good functioning of an application, there is no compatibility path. This means that we have changed many enums and functions and the qt3to4 porting tool will not change much in your qstyle code. To ease the pain, we list some of the major changes here.

QStyleOption has taken on a more central role and is no longer an optional argument, please see the QStyleOption documentation for more information.

The QStyle::StyleFlags have been renamed QStyle::StateFlags and are now prefixed State_ instead of Style_, in addition the Style_ButtonDefault flag has moved to QStyleOptionButton.

The QStyle::PrimitiveElement enumeration has undergone extensive change. Some of the enums were moved to QStyle::ControlElement, some were removed and all were renamed. This renaming is not done by the qt3to4 porting tool, so you must do it yourself. The table below shows how things look now.

The QStyle::drawControlMask() and QStyle::drawComplexControlMask() functions have been removed. They are replaced with a style hint.

The QStyle::drawItem() overloads that took both a pixmap and a string have been removed. Use QStyle::drawItemText() and QStyle::drawItemPixmap() directly.

The QStyle::itemRect() overload that took both a pixmap and a string is also removed, use either QStyle::itemTextRect() or QStyle::itemPixmapRect() instead.

QStyleSheet

The QStyleSheet and QStyleSheetItem classes have been renamed Q3StyleSheet and Q3StyleSheetItem, and have been moved to the Qt3Support library.

See Rich Text Processing for an overview of the Qt 4 rich text classes, and Qt Style Sheets for a description of CSS-like style sheet support in Qt 4.2 and above.

QSyntaxHighlighter

The QSyntaxHighlighter class from Qt 3 has been renamed Q3SyntaxHighlighter and moved to the Qt3Support library. Since Qt 4.1, it has been replaced by a new QSyntaxHighlighter class based on Qt 4's new rich text engine.

QTabBar

See Properties for a list of QTabBar properties in Qt 3 that have changed in Qt 4.

QTabDialog

The QTabDialog class is no longer part of the public Qt API. It has been renamed Q3TabDialog and moved to Qt3Support. In Qt 4 applications, you can easily obtain the same result by combining a QTabWidget with a QDialog and provide QPushButtons yourself.

See also the dialogs/tabdialog example, which shows how to implement tab dialogs in Qt 4.

QTabWidget

See Properties for a list of QTabWidget properties in Qt 3 that have changed in Qt 4.

QTable

The QTable, QTableItem, QComboTableItem, QCheckTableItem, and QTableSelection classes have been renamed Q3Table, Q3TableItem, Q3ComboTableItem, Q3CheckTableItem, and Q3TableSelection and moved to the Qt3Support library. New Qt applications should use the new QTableWidget or QTableView class instead.

Some of these classes behave differently with respect to the way they handle NULL pointers. For example, Q3TableItem::setPixmap() no longer accepts NULL or 0 to indicate that the item should contain a null pixmap; in this case, a null pixmap should be constructed and passed explicitly to the function.

See Model/View Programming for an overview of the new item view classes.

QTextCodec

The loadCharmap() and loadCharmapFromFile() functions are no longer available in Qt 4. You need to create your own codec if you want to create a codec based on a POSIX2 charmap definition.

QTextDrag

The QTextDrag class has been renamed Q3TextDrag and moved to the Qt3Support library. In Qt 4, use QMimeData instead and call QMimeData::setText() to set the data.

See Porting to Qt 4 - Drag and Drop for a comparison between the drag and drop APIs in Qt 3 and Qt 4.

QTextEdit

The old QTextEdit and QTextBrowser classes have been renamed Q3TextEdit and Q3TextBrowser, and have been moved to Qt3Support. The new QTextEdit and QTextBrowser have a somewhat different API.

The QTextEdit::setWrapPolicy() function has been renamed to setWordWrapMode() and the QTextEdit::setWrapColumnOrWidth() function has been renamed to setLineWrapColumnOrWidth(). The Q3TextEdit::setWrapPolicy() and Q3TextEdit::setWrapColumnOrWidth() still provide this functionality in the Q3TextEdit class.

See Rich Text Processing for an overview of the Qt 4 rich text classes.

QTextIStream

The QTextIStream convenience class is no longer provided in Qt 4. Use QTextStream directly instead.

QTextOStream

The QTextOStream convenience class is no longer provided in Qt 4. Use QTextStream directly instead.

QTextOStreamIterator

The undocumented QTextOStreamIterator class has been removed from the Qt library. If you need it in your application, feel free to copy the source code from the Qt 3 <qtl.h> header file.

QTextStream

QTextStream has undergone a number of API and implementation enhancements, and some of the changes affect QTextStream's behavior:

• QTextStream now uses buffered writing, which means that you need to call QTextStream::flush(), or use the streaming manipulators endl or flush if you need QTextStream to flush its write buffer. The stream is flushed automatically if QTextStream is deleted or when the device is closed.
• QTextStream now uses buffered reading, so if you read a line from the stream, QTextStream will read as much as it can from the device to fill up its internal read buffer. This speeds up reading significantly, but Qt 3 code that mixed QTextStream access and direct device access may need to be updated.
• While QTextStream in Qt 3 always translated end-of-line characters from Windows style ("\r\n") to Unix style ("\n") on Windows, QTextStream in Qt 4 only does this on devices opened with the QIODevice::Text mode (formerly IO_Translate).

Note that when using a QTextStream on a QFile in Qt 4, calling QIODevice::reset() on the QFile will not have the expected result because QTextStream now buffers the file. Use the QTextStream::seek() function instead.

QTextView

The QTextView class has been renamed Q3TextView and moved to the Qt3Support library.

QTimeEdit

The QTimeEdit class in Qt 4 is a convenience class based on QDateTimeEdit. The old class has been renamed Q3TimeEdit and moved to the Qt3Support library.

See Virtual Functions for a list of QTimeEdit virtual member functions in Qt 3 that are no longer virtual in Qt 4.

QTimer

Windows restricts the granularity of timers, but starting with Qt 4, we emulate a finer time resolution. On Windows XP we use the multimedia timer API, which gives us 1 millisecond resolution for QTimer.

Note that Windows 2000 has a lower timer resolution, and that code relying on underlying system timer restrictions encounters no such limitations using Qt 4 (e.g., setting an interval of 0 millisecond results in Qt occupying all of the processor time when no GUI events need processing).

QToolBar

The old QToolBar class, which worked with the old QMainWindow and QDockArea classes and inherited from QDockWindow, has been renamed Q3ToolBar and moved to Qt3Support. Note that, when using Q3ToolBar, the toolbar's actions must be Q3Actions.

Use the new QToolBar class in new applications.

Note: Q3ToolBar's horizontallyStretchable property can be achieved in QToolBar with size policies.

QToolButton

See Properties for a list of QToolButton properties in Qt 3 that have changed in Qt 4.

Note that many of the properties that could previously be set in the constructor must now be set separately.

QToolTip

The QToolTip::setGloballyEnabled() function no longer exists. Tooltips can be disabled by installing an event filter on qApp (the unique QApplication object) to block events of type QEvent::ToolTip.

QUriDrag

The QUriDrag class has been renamed Q3UriDrag and moved to the Qt3Support library. In Qt 4, use QMimeData instead and call QMimeData::setUrl() to set the URL.

See Porting to Qt 4 - Drag and Drop for a comparison between the drag and drop APIs in Qt 3 and Qt 4.

QUrl

The QUrl class has been rewritten from scratch in Qt 4 to be more standard-compliant. The old QUrl class has been renamed Q3Url and moved to the Qt3Support library.

The new QUrl class provides an extensive list of compatibility functions to ease porting from Q3Url to QUrl. A few functions require you to change your code:

• Q3Url::Q3Url(const Q3Url &, const QString &, bool) can be simulated by combining the URLs manually (using QString::operator+(), for example).
• Q3Url::setEncodedPathAndQuery(const QString &) is replaced by QUrl::setPath() and QUrl::setEncodedQuery().
• Q3Url::encodedPathAndQuery() is replaced by QUrl::path() and QUrl::encodedQuery().
• Q3Url::isLocalFile() can be simulated by checking that QUrl::protocol() is "file".
• Q3Url::toString(bool, bool) is replaced by QUrl::toString(int), where the int parameter specifies a combination of formatting options.

QUrlOperator

The QUrlOperator class is no longer part of the public Qt API. It has been renamed Q3UrlOperator and moved to Qt3Support.

From Qt 4.4, the Network Access API provides a subset of the features provided by QUrlOperator that are mostly intended for use with applications that use the HTTP and FTP protocols. See the QNetworkRequest, QNetworkReply, and QNetworkAccessManager documentation for further details.

QValueList<T>

The QValueList<T> class has been replaced by QList<T> and QLinkedList<T> in Qt 4. As a help when porting older Qt applications, the Qt3Support library contains a QValueList<T> class implemented in terms of the new QLinkedList<T>. Similarly, it contains QValueListIterator<T> and QValueListConstIterator<T> classes implemented in terms of QLinkedList<T>::iterator and QLinkedList<T>::const_iterator.

When porting to Qt 4, you have the choice of using QList<T> or QLinkedList<T> as alternatives to QValueList<T>. QList<T> has an index-based API and provides very fast random access (QList::operator[]), whereas QLinkedList<T> has an iterator-based API.

Here's a list of problem functions:

• QValueList(const std::list<T> &) doesn't exist in QList or QLinkedList. You can simulate it by calling append() in a loop.
• QValueList::insert(iterator, size_type, const T& x) doesn't exist in QList or QLinkedList. Call insert() repeatedly instead.
• QValueList::fromLast() doesn't exist in QList or QLinkedList. Use QValueList::end() instead.

  For example, if you have code like

       for (QValueList<T>::iterator i = list.fromLast(); i != list.begin(); --i)
           do_something(*i);

  you can rewrite it as

       QLinkedList<T>::iterator i = list.end();
       while (i != list.begin()) {
           --i; // decrement i before using it
           do_something(*i);
       }

• QValueList::append() and QValueList::prepend() return an iterator to the inserted item. QList's and QLinkedList's corresponding functions don't, but it's not a problem because QValueList::prepend() always returns begin() and append() always returns QValueList::end() - 1.
• QValueList::at(i) return an iterator to the item at index i. This corresponds to QList::begin() + i.
• QValueList::contains(const T &) corresponds to QList::count(const T &) and QLinkedList::count(const T &).

QValueVector<T>

The QValueVector<T> class has been replaced by QVector<T> in Qt 4. As a help when porting older Qt applications, the Qt3Support library contains a Q3ValueVector<T> class implemented in terms of the new QVector<T>.

When porting from QValueVector<T> to QVector<T>, you might run into the following incompatibilities:

• QValueVector(const std::vector<T> &) doesn't exist in QVector. You can simulate it by calling QVector::append()} in a loop.
• QValueVector::resize(int, const T &) doesn't exist in QVector. If you want the new items to be initialized with a particular value, use QVector::insert() instead.
• QValueVector::at() on a non-const vector returns a non-const reference. This corresponds to QVector::operator[]().
• Both QValueVector::at() functions have an ok parameter of type bool * that is set to true if the index is within bounds. This functionality doesn't exist in QVector; instead, check the index against QVector::size() yourself.

See Container Classes for an overview of the Qt 4 container classes.

QVariant

Some changes to the rest of the Qt library have implications on QVariant:

1. The QVariant::ColorGroup enum value is defined only if QT3_SUPPORT is defined.
2. The QVariant::IconSet enum value has been renamed QVariant::Icon.
3. The QVariant::CString enum value is now a synonym for QVariant::ByteArray.

Also, the QVariant(bool, int) constructor has been replaced by QVariant(bool). Old code like QVariant(true, 0) should be replaced with QVariant(true); otherwise, the QVariant(int, void *) overload might accidentally be triggered.

Many of QVariant's convenience functions in Qt 3, such as toColor() and toKeySequence(), have been removed to enable QVariant to be part of the QtCore module. QVariant is still able to hold values of these types.

Types which are not supported by any of the QVariant constructors can be stored as variants with the QVariant::fromValue() function. Types with no suitable convenience function for unpacking can be retrieved with the QVariant::value() function or passed directly to classes that implement the QVariant() operator.

See the QVariant::Type enum for a list of types supported by QVariant.