Qt (pronounced "cute" by its creators) is a cross-platform application development framework, widely used for the development of GUI programs (in which case it is known as a widget toolkit), and also used for developing non-GUI programs such as console tools and servers. Qt is most notably used in KDE, Opera, Google Earth, Skype, Qtopia, Photoshop Elements, VirtualBox and OPIE. It is produced by the Norwegian company Trolltech. Nokia acquired Trolltech on January 28, 2008.
Qt uses C++ with several non-standard extensions implemented by an additional pre-processor that generates standard C++ code before compilation. Qt can also be used in several other programming languages; bindings exist for Ada (QtAda), C# (Qyoto/Kimono), Java (Qt Jambi), Pascal, Perl, PHP (PHP-Qt), Ruby (RubyQt), and Python (PyQt). It runs on all major platforms, and has extensive internationalization support. Non-GUI features include SQL database access, XML parsing, thread management, network support and a unified cross-platform API for file handling.
There are four editions of Qt available on each of these platforms, namely:
Qt is available under a dual license, the GPL v2 or v3 with special exception and a proprietary commercial license on all supported platforms. The commercial license allows the final application to be licensed under various free software/open source licenses such as the LGPL or the Artistic License, or a proprietary software license.
Qt 4.2, released on October 4, 2006, introduced Windows Vista support, introduced native CSS support for widget styling, as well as the QGraphicsView framework for efficient rendering of thousands of 2D objects onscreen, to replace Qt 3.x's QCanvas class.
Qt 4.4, released on May 6, 2008. Features included are improved multimedia support using Phonon, enhanced XML support, a concurrency framework to ease the development of multi-threaded applications, an IPC framework with a focus on shared memory, and WebKit integration.
Haavard Nord and Eirik Chambe-Eng (the original developers of Qt and the CEO and President, respectively, of Trolltech) 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.
Controversy erupted around 1998 when it became clear that KDE was going to become one of the leading desktop environments for Linux. As KDE was based on Qt, many people in the free software movement worried that an essential piece of one of their major operating systems would be proprietary.
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, which intended to supplant KDE entirely. The GNOME Desktop uses the GTK+ toolkit, which was originally written for the GIMP, and primarily 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 with the open source principle by the Open Source Initiative and the free software definition by 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 whereby 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-style license should no free software/open source version of Qt be released during 12 months.
The first two 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 proprietary license which meant free/open source applications written in Qt for X11 could not be ported to Windows without purchasing the QPL edition. 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 proprietary license, until June 2003, where Trolltech released Qt 3.2 with Mac OS X support available under the GPL.
In 2002 members of the KDE on Cygwin project began porting the GPL licensed Qt/X11 code base to Windows. This was in response to Trolltech's refusal to license Qt/Windows under the GPL on the grounds that Windows was not a free software/open source platform. The project achieved reasonable success although it never reached production quality.
This was resolved when Trolltech released Qt/Windows 4 under the GPL in June 2005. Qt 4 now supports the same set of platforms in the free software/open source editions as in the proprietary edition, so it is now possible to create GPL-licensed free/open source applications using Qt on all supported platforms.
Qt used to emulate the native look of its intended platforms, which occasionally led to slight discrepancies where that emulation wasn't perfect. Recent versions of Qt use the native APIs of the different platforms to draw the Qt controls, and so do not suffer from such issues. (See also wxWidgets, which relies on native APIs for most widget functionality.)
Known as the moc, this is a tool that is 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++: The signal/slot system, introspection and asynchronous function calls.
The use of an additional tool has been criticized for making Qt programming different from pure C++ programming. 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 the dynamically generated slot and signal mechanism.
From Qt 4.3.0 onward, the scripting API , which is based on QSA is integrated as a core part of Qt and is no longer a separate library.
Popular examples of applications which use Qt include: