Qt (software)

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For the company also known as Qt Software, see Qt Development Frameworks.
Qt Designer 4 4 3.png
The Qt designer used for GUI designing
Original author(s) Haavard Nord and Eirik Chambe-Eng[1]
Initial release 20 May 1995; 19 years ago (1995-05-20)[1]
Stable release 5.3.2 (September 16, 2014; 0 days ago (2014-09-16)) [±][2]
Preview release 5.4 alpha (September 8, 2014; 8 days ago (2014-09-08)) [±][3]
Development status Active
Written in C++
Operating system Linux (Embedded, Wayland, X11), OS X, Windows,
Platform Cross-platform
Type Application framework
License LGPL-2.1 (Qt open-source version)[4]
Qt Commercial License (Qt Commercial version)[5]
Website qt-project.org

Qt (/ˈkjuːt/ "cute", or unofficially as Q-T cue-tee[6][7]) is a cross-platform application framework that is widely used for developing application software with a graphical user interface (GUI) (in which cases Qt is classified as a widget toolkit), and also used for developing non-GUI programs such as command-line tools and consoles for servers.

Qt uses standard C++ but makes extensive use of a special code generator (called the Meta Object Compiler, or moc) together with several macros to enrich the language. Qt can also be used in several other programming languages via language bindings. It runs on the major desktop platforms and some of the mobile platforms. It has extensive internationalization support. Non-GUI features include SQL database access, XML parsing, thread management, network support, and a unified cross-platform application programming interface (API) for file handling.

Qt is available under a commercial license, GPL v3 and LGPL v2. All editions support many compilers, including the GCC C++ compiler and the Visual Studio suite.

Qt is developed by Digia, who owns the Qt trademark, and the Qt Project under open-source governance, involving individual developers and firms working to advance Qt. Before the launch of the Qt Project, it was produced by Nokia's Qt Development Frameworks division, which came into existence after Nokia's acquisition of the Norwegian company Trolltech, the original producer of Qt.[8] In February 2011, Nokia announced its decision to drop Symbian technologies and base their future smartphones on Microsoft platform instead. One month later, Nokia announced the sale of Qt's commercial licensing and professional services to Digia, with the immediate goal of taking Qt support to Android, iOS and Windows 8 platforms, and to continue focusing on desktop and embedded development, although Nokia was to remain the main development force behind the framework at that time.[9]


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.[1]

The toolkit was called Qt because the letter Q looked appealing in Haavard's Emacs typeface, and "t" was inspired by Xt, the X toolkit.[1]

The first two versions of Qt had only two flavors: Qt/X11 for Unix and Qt/Windows for Windows. The Windows platform was only available under a proprietary license, which meant free/open source applications written in Qt for X11 could not be ported to Windows without purchasing the proprietary edition.

At the end of 2001, Trolltech released Qt 3.0, which added support for Mac OS X. The Mac OS X support was available only in the proprietary license until June 2003, when Trolltech released Qt 3.2 with Mac OS X support available under the GPL.

In June 2005, Trolltech released Qt 4.0.[10]

Nokia acquired Trolltech ASA on 17 June 2008 and changed the name first to Qt Software, then to Qt Development Frameworks. Since then it focused on Qt development to turn it into the main development platform for its devices, including a port to the Symbian S60 platform. Version 1.0 of the Nokia Qt SDK was released on 23 June 2010.[11] The source code was made available over Gitorious, a community oriented git source code repository, to gather an even broader community that is not only using Qt but also helping to improve it.


At all times, Qt was available under a commercial license that allows developing proprietary applications with no restrictions on licensing. In addition, Qt has been gradually made available under several increasingly free licenses.

Until version 1.45, source code for Qt was released under the FreeQt license. This 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.

Controversy erupted around 1998 when it became clear that the K Desktop Environment (now known as the KDE Software Compilation) was going to become one of the leading desktop environments for Linux. As it 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.

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/open source version of Qt be released during 12 months.

In 2000, Qt/X11 2.2 was released under the GPL v2,[12] ending all controversy regarding GPL compatibility.

In 2002, members of the KDE on Cygwin project began porting the GPL licensed Qt/X11 code base to Windows.[13] This was in response to Trolltech's refusal to license Qt/Windows under the GPL on the grounds that Windows was not a free/open source software platform.[14][15] 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. The GPL v3 with special exception[16] was later added as an added licensing option. The GPL exception allows the final application to be licensed under various GPL-incompatible free software/open source licenses such as the Mozilla Public License 1.1.

On 14 January 2009, Qt version 4.5 added another option, the LGPL,[17] which should make Qt even more attractive for non-GPL open source projects and for closed applications.[18]

In March 2011, Nokia sold the commercial licensing part of Qt to Digia creating Qt Commercial.


Qt works on the following platforms:

External ports[edit]

Since Nokia opened the Qt source code to the community on Gitorious various ports have been appearing. Here are some of them:

Deprecated ports[edit]


There are two editions of Qt available, the Qt Enterprise Edition (additional functionality and libraries included, e.g. Charts and Data Visualization[52]), a commercial edition sold by Digia, and the Open Source edition.

Qt is available under the following copyright licenses:[53] GNU LGPL[4] (2.1 version with Qt special exception), GNU GPL[54] 3.0 version, and the Commercial Developer License[5]


Main article: List of Qt releases

Qt 4[edit]

Trolltech released Qt 4.0 on 28 June 2005 and introduced five new technologies in the framework:

  • Tulip A set of template container classes.
  • Interview A model–view–controller 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 5[edit]

Qt 5 was officially released on 19 December 2012. This new version marked a major change in the platform, with hardware-accelerated graphics, QML and JavaScript playing a major role. The traditional C++-only QWidgets continued to be supported, but did not benefit from the performance improvements available through the new architecture.[55] Qt5 brings significant improvements to the speed and ease of developing user interfaces.[56]

Framework development of Qt 5 moved to open governance, taking place at qt-project.org. It is now possible for developers outside Digia to submit patches and have them reviewed.[57]

Version Release date New features
5.0 19 December 2012[58] Major overhaul of the Qt 4.x series.
Complete Wayland support, including the client-side decorations.
5.1 3 July 2013 New modules and experimental Android and iOS support as technology preview.[59]
5.2 12 December 2013 First release with official support of Android and iOS.
5.3 20 May 2014 http://blog.qt.digia.com/blog/2014/05/20/qt-5-3-released/
5.4 Planned for 18 Nov 2014 [60]
  • Full WinRT support
  • Introduction of Qt WebEngine based on Chromium that will eventually replace Qt WebKit in future versions
  • Dynamic switching between graphic backends on Windows (ANGLE or OpenGL)
  • Android Style for Qt Quick Controls backends[61]

Current version (5.2)

Qt 5.2 provides several improvements, including a new Scene graph renderer that has much better performance for drawing vector objects by using an OpenGL backend and minimizing GPU overdraws. Benchmarks of development versions shows significant improvements in speed and a visible decrease in CPU usage, because of the better usage of GPU rendering. The vision is to have game-like performance for the drawing canvas and QML renderer.[62] Additionally, Qt 5.2 introduces new functionalities for KDE Frameworks 5 and many other improvements.[63]

Software architecture[edit]

Example of Qt usage in Linux-based systems

Qt, when it was first released, relied on a few key concepts:

  • Complete abstraction of the GUI – When first released, Qt used its own paint engine and controls, emulating the look of the different platforms it runs on when it drew its widgets. This made the porting work easier because very few classes in Qt depended really on the target platform; however, this occasionally led to slight discrepancies where that emulation was imperfect.
Recent versions of Qt use the native style APIs of the different platforms, on platforms that have a native widget set, to query metrics and draw most controls, and do not suffer from such issues as much.[64]
On some platforms (such as MeeGo and KDE) Qt is the native API.
Some other portable graphical toolkits have made different design decisions; for example, wxWidgets uses the toolkits of the target platform for its implementations.
  • Signals and slots - a language construct introduced in Qt for communication between objects[65] which makes it easy to implement the Observer pattern while avoiding boilerplate code. The concept is that GUI widgets can send signals containing event information which can be received by other controls using special functions known as slots.
  • Metaobject compiler - The metaobject compiler, termed moc, is a tool that is run on the sources of a Qt program. It interprets certain macros from the C++ code as annotations, and uses them to generate added C++ code with Meta Information about the classes used in the program. This meta information is used by Qt to provide programming features not available natively in C++: signals and slots, introspection and asynchronous function calls.


Starting with Qt 4.0 the framework was split into individual modules.[66][67] With Qt 5.0 the architecture was modularized even further.[68][69] Qt is now split into essential and add-on modules.[70]

Qt Essentials[edit]

  • Qt Core The only required Qt module, containing non-graphical classes used by other modules
  • Qt GUI The central gui module. In Qt 5 this module now depends on OpenGL, but no longer contains any widget classes.
  • Qt Widgets Contains classes for classic widget based GUI applications and the QSceneGraph classes. Was split off from QtGui in Qt5.
  • Qt QML Module for QML and JavaScript languages.
  • Qt Quick The module for GUI application written using QML2.
  • Qt Quick Controls Widget like controls for Qt Quick
  • Qt Quick Layouts Layout for arranging items in Qt Quick
  • Qt Network Network abstraction layer. Complete with TCP, UDP, HTTP, SSL and since Qt 5.3 SPDY support.
  • Qt Multimedia Classes for audio, video, radio and camera functionality.
  • Qt Multimedia Widgets The widgets from Qt Multimedia.
  • Qt SQL Contains classes for database integration using SQL.
  • Qt WebKit Qt's WebKit implementation and API.
  • Qt WebKit Widgets The widget API for Qt WebKit
  • Qt Test Classes for unit testing Qt applications and libraries.

Qt Add-Ons[edit]

  • Active Qt Classes for applications which use ActiveX.
  • Qt Bluetooth Classes accessing Bluetooth hardware.
  • Qt D-Bus Classes for IPC using the D-Bus protocol.
  • Qt OpenGL Legacy module containing the OpenGL classes from Qt4. In Qt5 the similar functionality in Qt GUI is recommended.
  • Qt Script Legacy module for scripting Qt application using ECMAScript/JavaScript. In Qt5, using similar classes in Qt QML is recommended.


CodeEditor, a Qt-based source code editor/IDE. The screenshot depicts "bracket matching" and syntax highlighting.
  • Qt Creator, a cross-platform IDE for C++ and QML. Qt Designer's GUI layout/design functionality is integrated into this relatively new IDE, although Qt Designer can still be called as a standalone tool.
  • qmake, a tool that automates the generation of Makefiles for development project across different platforms

Programming language bindings[edit]

Qt has a range of bindings for various languages,[71] which implement some or all of its feature set.


Organizations using Qt[edit]

Software using Qt[edit]

Main category: Software that uses Qt

Example applications using Qt are Altera Quartus, a design and simulation tool for programmable logic; Autodesk Maya;[84][85] Bitcoin Core; Cameleon (programming language); Mathematica;[86] Google Earth;[87] KDE,[88] a desktop environment for UNIX-like operating systems; Skype;[89] Spotify for Linux;[90] Ubuntu;[91] VirtualBox, an OS virtualization software package; Musescore music score writing software; and the VLC media player.[92]

See also[edit]


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  2. ^ Tuukka Turunen (2014-09-16). "Qt 5.3.2 Released with Qt Creator 3.2.1". Retrieved 2014-09-16. 
  3. ^ Tuukka Turunen (2014-09-08). "Qt 5.4 Alpha Released". Retrieved 2014-09-08. 
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  12. ^ "Trolltech offers a choice in licensing with the addition of GPL licensing for the upcoming release of Qt". Qt.nokia.com. Retrieved 13 November 2011. 
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  14. ^ E-mail to the kde-cygwin mailing list by Chris January, 4 February 2003
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External links[edit]