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Key Features in Qt 3.0

Qt 3.0 adds a lot of new features and improvements over the Qt 2.x series. Some internals have undergone major redesign and new classes and methods have been added.

We have tried to keep the API of Qt 3.0 as compatible as possible with the Qt 2.x series. For most applications only minor changes will be needed to compile and run them successfully using Qt 3.0.

One of the major new features that has been added in the 3.0 release is a module allowing you to easily work with databases. The API is platform independent and database neutral. This module is seamlessly integrated into Qt Designer, greatly simplifying the process of building database applications and using data aware widgets.

Other major new features include a plugin architecture. You can use your own and third party plugins your own applications. The Unicode support of Qt 2.x has been greatly enhanced, it now includes full support for scripts written from right to left (e.g. Arabic and Hebrew) and also provides improved support for Asian languages.

Many new classes have been added to the Qt Library. Amongst them are classes that provide a docking architecture (QDockArea/QDockWindow), a powerful rich text editor (QTextEdit), a class to store and access application settings (QSettings) and a class to create and communicate with processes (QProcess).

Apart from the changes in the library itself a lot has been done to make the development of Qt applications with Qt 3.0 even easier than before. Two new applications have been added: Qt Linguist is a tool to help you translate your application into different languages; Qt Assistant is an easy to use help browser for the Qt documentation that supports bookmarks and can search by keyword.

Another change concerns the Qt build system, which has been reworked to make it a lot easier to port Qt to new platforms. You can use this platform independent build system for your own applications.

The Qt Library

A large number of new features has been added to Qt 3.0. The following list gives an overview of the most important new and changed aspects of the Qt library. A full list of every new method follows the overview.

Database support

One of the major new features in Qt 3.0 is the SQL module that provides multiplatform access to SQL databases, making database application programming with Qt seamless and portable. The API, built with standard SQL, is database-neutral and software development is independent of the underlying database.

A collection of tightly focused C++ classes are provided to give the programmer direct access to SQL databases. Developers can send raw SQL to the database server or have the Qt SQL classes generate SQL queries automatically. Drivers for Oracle, PostgreSQL, MySQL and ODBC are available and writing new drivers is straightforward.

Tying the results of SQL queries to GUI components is fully supported by Qt's SQL widgets. These classes include a tabular data widget (for spreadsheet-like data presentation with in-place editing), a form-based data browser (which provides data navigation and edit functions) and a form-based data viewer (which provides read-only forms). This framework can be extended by using custom field editors, allowing for example, a data table to use custom widgets for in-place editing. The SQL module fully supports Qt's signal/slots mechanism, making it easy for developers to include their own data validation and auditing code.

Qt Designer fully supports Qt's SQL module. All SQL widgets can be laid out within Qt Designer, and relationships can be established between controls visually. Many interactions can be defined purely in terms of Qt's signals/slots mechanism directly in Qt Designer.


The QLibrary class provides a platform independent wrapper for runtime loading of shared libraries. QPluginManager makes it trivial to implement plugin support in applications. The Qt library is able to load additional styles, database drivers and text codecs from plugins.

Qt Designer supports custom widgets in plugins, and will use the widgets both when designing and previewing forms.

See the plugins documentation.

Rich text engine and editor

The rich text engine originally introduced in Qt 2.0 has been further optimized and extended to support editing. It allows editing formatted text with different fonts, colors, paragraph styles, tables and images. The editor supports different word wrap modes, command-based undo/redo, multiple selections, drag and drop, and many other features. The new QTextEdit engine is highly optimized for proccesing and displaying large documents quickly and efficiently.


Apart from the rich text engine, another new feature of Qt 3.0 that relates to text handling is the greatly improved Unicode support. Qt 3.0 includes an implementation of the bidirectional algorithm (BiDi) as defined in the Unicode standard and a shaping engine for Arabic, which gives full native language support to Arabic and Hebrew speaking people. At the same time the support for Asian languages has been greatly enhanced.

The support is almost transparent for the developer using Qt to develop their applications. This means that developers who developed applications using Qt 2.x will automatically gain the full support for these languages when switching to Qt 3.0. Developers can rely on their application to work for people using writing systems different from Latin1, without having to worry about the complexities involved with these scripts, as Qt takes care of this automatically.

Docked and Floating Windows

Qt 3.0 introduces the concept of Dock Windows and Dock Areas. Dock windows are widgets, that can be attached to, and detached from, dock areas. The commonest kind of dock window is a tool bar. Any number of dock windows may be placed in a dock area. A main window can have dock areas, for example, QMainWindow provides four dock areas (top, left, bottom, right) by default. The user can freely move dock windows and place them at a convenient place in a dock area, or drag them out of the application and have them float freely as top level windows in their own right. Dock windows can also be minimized or hidden.

For developers, dock windows behave just like ordinary widgets. QToolbar for example is now a specialized subclass of a dock window. The API of QMainWindow and QToolBar is source compatible with Qt 2.x, so existing code which uses these classes will continue to work.

Regular Expressions

Qt has always provided regular expression support, but that support was pretty much limited to what was required in common GUI control elements such as file dialogs. Qt 3.0 introduces a new regular expression engine, QRegExp, that supports most of Perl's regex features and is Unicode based. The most useful additions are support for parentheses (capturing and non-capturing) and backreferences.

Storing application settings

Most programs will need to store some settings between runs, for example, user selected fonts, colors and other preferences, or a list of recently used files. The new QSettings class provides a platform independent way to achieve this goal. The API makes it easy to store and retrieve most of the basic data types used in Qt (such as basic C++ types, strings, lists, colors, etc). The class uses the registry on the Windows platform and traditional resource files on Unix.

Creating and controlling other processes

QProcess is a class that allows you to start other programs from within a Qt application in a platform independent manner. It gives you full control over the started program, for example you can redirect the input and output of console applications.


Accessibility means making software usable and accessible to a wide range of users, including those with disabilities. In Qt 3.0, most widgets provide accessibility information for assistive tools that can be used by a wide range of disabled users. Qt standard widgets like buttons or range controls are fully supported. Support for complex widgets, like e.g. QListView, is in development. Existing applications that make use of standard widgets will become accessible just by using Qt 3.0.

Qt uses the Active Accessibility infrastructure on Windows, and needs the MSAA SDK, which is part of most platform SDKs. With improving standardization of accessibility on other platforms, Qt will support assistive technologies on other systems, too.

XML Improvements

The XML framework introduced in Qt 2.2 has been vastly improved. Qt 2.2 already supported level 1 of the Document Object Model (DOM), a W3C standard for accessing and modifying XML documents. Qt 3.0 has added support for DOM Level 2 and XML namespaces.

The XML parser has been extended to allow incremental parsing of XML documents. This allows you to start parsing the document directly after the first parts of the data have arrived, and to continue whenever new data is available. This is especially useful if the XML document is read from a slow source, e.g. over the network, as it allows the application to start working on the data at a very early stage.

SVG support

SVG is a W3C standard for "Scalable Vector Graphics". Qt 3.0's XML support means that QPicture can optionally generate and import static SVG documents. All the SVG features that have an equivalent in QPainter are supported.

Multihead support

Many professional applications, such as DTP and CAD software, are able to display data on two or more monitors. In Qt 3.0 the QDesktopWidget class provides the application with runtime information about the number and geometry of the desktops on the different monitors and such allows applications to efficiently use a multi-monitor setup.

The virtual desktop of Mac OS X, Windows 98, and 2000 is supported, as well as the traditional multi-screen and the newer Xinerama multihead setups on X11.

X11 specific enhancements

Qt 3.0 now complies with the NET WM Specification, recently adopted by KDE 2.0. This allows easy integration and proper execution with desktop environments that support the NET WM specification.

The font handling on X11 has undergone major changes. QFont no longer has a one-to-one relation with window system fonts. QFont is now a logical font that can load multiple window system fonts to simplify Unicode text display. This completely removes the burden of changing/setting fonts for a specific locale/language from the programmer. For end-users, any font can be used in any locale. For example, a user in Norway will be able to see Korean text without having to set their locale to Korean.

Qt 3.0 also supports the new render extension recently added to XFree86. This adds support for anti aliased text and pixmaps with alpha channel (semi transparency) on the systems that support the rendering extension (at the moment XFree 4.0.3 and later).


Printing support has been enhanced on all platforms. The QPrinter class now supports setting a virtual resolution for the painting process. This makes WYSIWYG printing trivial, and also allows you to take full advantage of the high resolution of a printer when painting on it.

The postscript driver built into Qt and used on Unix has been greatly enhanced. It supports the embedding of true/open type and type1 fonts into the document, and can correctly handle and display Unicode. Support for fonts built into the printer has been enhanced and Qt now knows about the most common printer fonts used for Asian languages.


This class provides a simple interface for HTTP downloads and uploads.

Compatibility with the Standard Template Library (STL)

Support for the C++ Standard Template Library has been added to the Qt Template Library (QTL). The QTL classes now contain appropriate copy constructors and typedefs so that they can be freely mixed with other STL containers and algorithms. In addition, new member functions have been added to QTL template classes which correspond to STL-style naming conventions (e.g., push_back()).

Qt Designer

Qt Designer was a pure dialog editor in Qt 2.2 but has now been extended to provide the full functionality of a GUI design tool.

This includes the ability to lay out main windows with menus and toolbars. Actions can be edited within Qt Designer and then plugged into toolbars and menu bars via drag and drop. Splitters can now be used in a way similar to layouts to group widgets horizontally or vertically.

In Qt 2.2, many of the dialogs created by Qt Designer had to be subclassed to implement functionality beyond the predefined signal and slot connections. Whilst the subclassing approach is still fully supported, Qt Designer now offers an alternative: a plugin for editing slots. The editor offers features such as syntax highlighting, completion, parentheses matching and incremental search.

The functionality of Qt Designer can now be extended via plugins. Using Qt Designer's interface or by implementing one of the provided interfaces in a plugin, a two way communication between plugin and Qt Designer can be established. This functionality is used to implement plugins for custom widgets, so that they can be used as real widgets inside the designer.

Basic support for project management has been added. This allows you to read and edit *.pro files, add and remove files to/from the project and do some global operations on the project. You can now open the project file and have one-click access to all the *.ui forms in the project.

In addition to generating code via uic, Qt Designer now supports the dynamic creation of widgets directly from XML user interface description files (*.ui files) at runtime. This eliminates the need of recompiling your application when the GUI changes, and could be used to enable your customers to do their own customizations. Technically, the feature is provided by a new class, QWidgetFactory in the QResource library.

Qt Linguist

Qt Linguist is a GUI utility to support translating the user-visible text in applications written with Qt. It comes with two command-line tools: lupdate and lrelease.

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

Qt Linguist is a tool suitable for use by translators. Each user-visible (source) text is characterized by the text itself, a context (usually the name of the C++ class containing the text), and an optional comment to help the translator. The C++ class name will usually be the name of the relevant dialog, and the comment will often contain instructions that describe how to navigate to the relevant dialog.

You can create phrase books for Qt Linguist to provide common translations to help ensure consistency and to speed up the translation process. Whenever a translator navigates to a new text to translate, Qt Linguist uses an intelligent algorithm to provide a list of possible translations: the list is composed of relevant text from any open phrase books and also from identical or similar text that has already been translated.

Once a translation is complete it can be marked as "done"; such translations are included in the *.qm file. Text that has not been "done" is included in the *.qm file in its original form. Although Qt Linguist is a GUI application with dock windows and mouse control, toolbars, etc., it has a full set of keyboard shortcuts to make translation as fast and efficient as possible.

When the Qt application that you're developing evolves (e.g. from version 1.0 to version 1.1), the utility lupdate merges the source texts from the new version with the previous translation source file, reusing existing translations. In some typical cases, lupdate may suggest translations. These translations are marked as unfinished, so you can easily find and check them.

Qt Assistant

Thanks to the positive feedback we received about the help system built into Qt Designer, we decided to offer this part as a separate application called Qt Assistant. Qt Assistant can be used to browse the Qt class documentation as well as the manuals for Qt Designer and Qt Linguist. It offers index searching, a contents overview, bookmarks history and incremental search. Qt Assistant is used by both Qt Designer and Qt Linguist for browsing their help documentation.


To ease portability we now provide the qmake utility to replace tmake. QMake is a C++ version of tmake which offers additional functionallity that is difficult to reproduce in tmake. Trolltech uses qmake in its build system for Qt and related products and we have released it as free software.

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Qt version 3.1.1