Qt (toolkit)

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Template:Infobox Software In computer programming, Qt is a cross-platform graphical widget toolkit for the development of GUI programs. Qt is most notably used in the K Desktop Environment. It is produced by the Norwegian company Trolltech, formerly Quasar Technologies. Trolltech insiders pronounce Qt as "cute".

Qt uses an extended version of the [[C++]] programming language, but bindings exist for Python, Ruby, C, Perl and Pascal. It runs on all major platforms, and has extensive internationalization support. Non-GUI features include SQL database access, XML parsing, thread management, and a unified cross-platform API for file handling.

1 Varieties
2 Current
3 History
4 Design
- 4.1 Complete abstraction of the GUI
- 4.2 Meta Object Compiler
5 See also
6 External links

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Varieties

Qt is released by Trolltech on the following platforms:

Qt/X11 — Qt for X Window System
Qt/Mac — Qt for Apple Mac OS X
Qt/Windows — Qt for Microsoft Windows
Qt/Embedded — Qt for embedded platforms (PDA, Smartphone, ...)

There are four editions of Qt available on each of these platforms, namely:

Qt Console — edition for non-GUI development.
Qt Desktop Light — entry level GUI edition, stripped of network and database support.
Qt Desktop — complete edition.
Qt Open Source Edition — complete edition, for open-source development.

The first three editions are proprietary, and released under a commercial license; while the Open Source edition is available under the GPL license and additionally under the Q Public License (QPL) for the Qt/X11 version.

In case of the X11 platform, the QPL allows the final application to be licensed under various open source licenses, such as the LGPL or the Artistic license. For the Windows and Mac OS X platforms, the GPL is the only Open Source license available so the applications developed with it must be GPL as well.

All editions support a wide range of compilers, including the GCC C++ compiler, and in the case of the commercial Qt/Windows, the Visual Studio suite [1].

There is a project to port the Open source Qt/X11 version of Qt to Windows, along with KDE. The project was started for Qt3 to provide a version of Qt on Windows suitable for free software programs :

Qt/Windows Free Edition — a free version of Qt released by the KDE on Cygwin project. This version is based entirely on the Qt/X11 source code and licensed under GPL.

The release of Qt4 under the GPL for the Unix, MacOS and Windows platforms makes the project less relevant.

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Current

Trolltech released Qt 4 on June 28, 2005 and introduced five new technologies in the framework:

Tulip A set of template container classes.
Interview A model/view architecture for item views.
Arthur A 2D painting framework.
Scribe A Unicode text renderer with a public API for performing low-level text layout.
MainWindow A modern action-based main window, toolbar, menu, and docking architecture.

Qt 4 is dual-licensed under GPL and proprietary licenses on all supported platforms including Windows (while Qt/Windows 3.3 is only released under a proprietary license).

Qt 4.1, released on December 19, 2005, introduced integrated SVG Tiny support, a PDF backend to Qt's printing system, and a few other features.

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History

Haavard Nord and Eirik Chambe-Eng (the original developers of Qt and the CEO and President of Trolltech respectively) began development of "Qt" in 1991, three years before the company was incorporated as Quasar Technologies, then changed the name to Troll Tech, and then to Trolltech.

The toolkit was called Qt because the letter Q looked beautiful in Haavard's Emacs font, and T was inspired by Xt, the X toolkit.

Controversy erupted around 1998 when it became clear that KDE was going to become one of the leading desktop environments for GNU/Linux. As KDE was based on Qt, many people in the open source and free software movements were worried that an essential piece of one of their major operating systems would be under commercial control.

This gave rise to two efforts: the Harmony toolkit which sought to duplicate the Qt Toolkit under a free software license and the GNOME desktop that was meant to supplant KDE entirely. The GNOME Desktop uses the [[GTK+]] toolkit which was written for the GIMP, and mainly uses the C programming language.

Until version 1.45, source code for Qt was released under the FreeQt license — which was viewed as not compliant to the open source principle by the Open Source Initiative and Free Software Foundation because while the source was available it did not allow the redistribution of modified versions. With the release of version 2.0 of the toolkit, the license was changed to the Q Public License (QPL), a free software license but one regarded by the Free Software Foundation as incompatible with the GPL. Compromises were sought between KDE and Trolltech wherein Qt would not be able to fall under a more restrictive license than the QPL, even if Trolltech was bought out or went bankrupt. This led to the creation of the KDE Free Qt foundation, which guarantees that Qt would fall under a BSD license should no open source version of Qt be released during 12 months.

The first versions of Qt had only two flavours: Qt/X11 for Unix and Qt/Windows for the Windows platform. The Windows platform was only available under the commercial license. In the end of 2001, Trolltech released Qt 3.0 which added support for the Mac OS X platform. The Mac OS X support was available only in the commercial license, until June 2003, where Trolltech released the version 3.2 with Mac OS X support available under the GPL license. Then came June 2005, with the addition of the Windows platform to the Open Source edition. Qt4 hence support the same set of platforms in the commercial edition as in the Open Source editions.

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Design

The innovation of Qt when it was first released relied on a few key concepts.

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Complete abstraction of the GUI

Qt uses its own paint engine and controls. It emulates the look of the different platforms it runs on. This made the porting work easier because very few classes in Qt depended really on the target platform. The drawback is that Qt had to emulate precisely the look of the different platforms. This drawback however no longer applies because the latest versions of Qt use the native styles API of the different platforms to draw the Qt controls.

Other portable graphical toolkits have made a different design decision, such as wxWidgets and the Java based SWT [2] which use the toolkit of the target platform for their implementation.

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Meta Object Compiler

Known as the moc, this is a tool that one must run on the sources of a Qt program prior to compiling it. The tool will generate "Meta Information" about the classes used in the program. This meta information is used by Qt to provide programming features not available in C++: Introspection, signal/slot system.

The use of an additional tool has been criticised by part of the C++ community, stating that Qt programming is making a "moc"ery of C++. In particular, the choice of an implementation based on macros has been criticized for its absence of type safety and pollution of the namespace. This is viewed by Trolltech as a necessary trade-off to provide introspection and dynamically generated slots or signals. Further, when Qt 1.x was released, consistency between compiler template implementations could not be relied upon.

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