Qt (software)

For the company formerly known as Qt Software, see Qt Company.

Qt

GUI designing in Qt Creator using the embedded Qt Designer
Original author(s)	Haavard Nord and Eirik Chambe-Eng
Developer(s)	Trolltech (1991–2008) Nokia (2008–2011) Digia (2012–2014) Qt Company (2014-present) Qt Project (2011–present)
Initial release	20 May 1995 (1995-05-20)
Stable release	5.6 (March 16, 2016 (2016-03-16)) [±]
Preview release	5.7 Alpha (March 11, 2016 (2016-03-11)) [±]
Development status	Active
Written in	C++
Operating system	Linux (Embedded, Wayland), Unix-like (X11)), OS X, Windows, Windows Phone …
Platform	Cross-platform
Type	Application framework
License	Qt Commercial License GPL-3.0 LGPL-3.0 LGPL-2.1
Website	qt.io

Qt (/ˈkjuːt/ "cute", or unofficially as Q-T cue-tee) is a cross-platform application framework that is widely used for developing application software that can be run on various software and hardware platforms with little or no change in the underlying codebase, while still being a native application with the capabilities and speed thereof. Qt is currently being developed both by the Qt Company, a subsidiary of Digia, and the Qt Project under open-source governance, involving individual developers and firms working to advance Qt. Digia owns the Qt trademark and copyright. Qt is available with both commercial and open source GPL v3, LGPL v3 and LGPL v2 licenses.

Purposes and abilities

Qt is used mainly for developing application software with graphical user interfaces (GUIs); however, programs without a GUI can be developed, such as command-line tools and consoles for servers. An example of a non-GUI program using Qt is the Cutelyst web framework. GUI programs created with Qt can have a native-looking interface, in which cases Qt is classified as a widget toolkit.

Qt uses standard C++ with extensions including signals and slots that simplifies handling of events, and this helps in development of both GUI and server applications which receive their own set of event information and should process them accordingly. Qt supports many compilers, including the GCC C++ compiler and the Visual Studio suite. Qt also provides Qt Quick, that includes a declarative scripting language called QML that allows using JavaScript to provide the logic. With Qt Quick, rapid application development for mobile devices became possible, although logic can be written with native code as well to achieve the best possible performance. Qt can 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, JSON parsing, thread management and network support.

Software architecture

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. 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 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.

Supported platforms

Main article: List of platforms supported by Qt

Qt works on many different platforms; the following are officially supported:

Platform	New features
Android	Qt for Android, formerly known as Necessitas.
Embedded Linux	Qt for embedded platforms: personal digital assistant, smartphone, etc.
Integrity	Qt for Integrity
iOS	Qt for iOS platforms (iPhone, iPad)
OS X	Qt for Apple OS X; supports applications on Cocoa
QNX	Qt for QNX
VxWorks	Qt for VxWorks.
Wayland	Qt for Wayland. Qt applications can switch between graphical backends like X and Wayland at load time with the -platform command line option. This allows a seamless transition of Qt applications from X11 to Wayland.
Windows	Qt for Microsoft Windows XP, Vista, 7, 8
Windows CE	Qt for Windows CE 6 and Windows Embedded Compact 7.
Windows RT	Support for WinRT-based Windows 8 apps and Windows Phone 8 With 5.4 minimum supported version: Windows Phone 8.1
X11	Qt for X Window System (Linux, FreeBSD, HP-UX, Solaris, AIX, etc.)

After Nokia opened the Qt source code to the community on Gitorious various ports appeared. There are also some ports of Qt that may be available, but are not supported anymore. These platforms are listed in List of platforms supported by Qt.

Editions

There are four editions of Qt available, Community, Indie Mobile, Professional and Enterprise. The Community version is under the open source licenses, while the Indie Mobile, Professional and Enterprise versions, which contain additional functionality and libraries, e.g. Charts and Data Visualization, Enterprise Controls, Virtual Keyboard etc. are commercially sold by The Qt Company.

Qt is available under the following copyright licenses: Qt Commercial License, GNU GPL-3.0, GNU LGPL-3.0 and GNU LGPL-2.1 (with Qt special exception)

Releases

Main article: List of Qt releases

Qt 5

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. Qt 5 brings significant improvements to the speed and ease of developing user interfaces.

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.

Version	Release date	New features
5.0	19 December 2012	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.
5.2	12 December 2013	First release with official support of Android and iOS.
5.3	20 May 2014	Focus on stability and usability
5.4	10 December 2014	Full WinRT and Windows Phone support Introduction of Qt WebEngine based on Chromium internal components that will eventually replace QtWebKit in future versions. Dynamic GL switching between graphic backends on Windows (ANGLE or OpenGL) Native "look and feel" for Qt Quick Controls backends on Android platforms Introduction of Qt WebChannels providing a QObject bridge over WebSockets. Initially only integrated with Qt WebKit, but with Qt WebEngine integration under way for 5.5
5.5	1 July 2015	Features available in Qt 5.5 according to official Qt.io website: New modules: Qt 3D, Qt Canvas 3D, Qt Location Deprecated modules: Qt WebKit, Qt Declarative (Qt Quick 1), Qt Script Bluetooth Low Energy API final release On Windows, there will be no more OpenGL-only or ANGLE-only builds and Qt will manage this dynamically GStreamer 1.0 support New video filtering framework Camera and QML MediaPlayer improvement on iOS Qt NFC for Linux SSL/TLS improvements for Qt Network
5.6	Beta release plan: 14 December 2015 Final release plan: 9 February 2016	Notable improvements: Deprecated Modules: Qt Script, Qt Enginio Removed Modules: Qt WebKit, Qt Declarative (Qt Quick 1) Qt Core: Several performance improvements, including reduction of memory usage in dynamic properties, and performance optimisations in QString Qt Network: Support HTTP redirection Qt Gui: Improved cross-platform OpenGL ES 3.0 and 3.1 Qt Multimedia: Adding playlist QML type Qt WebEngine: Based on Chromium 45, with support for pepper plugins including Flash, API for custom URL schemes, intercepting and blocking network requests and also tracking or blocking cookies Qt QML: Reduced memory consumption Embedded platforms: support for Intel Atom-based NUCs

Software modules

Starting with Qt 4.0 the framework was split into individual modules. With Qt 5.0 the architecture was modularized even further. Qt is now split into essential and add-on modules.

Qt essentials

Module	Description
Qt Core	The only required Qt module, containing classes used by other modules, including the meta-object system, concurrency and threading, containers, event system, plugins and I/O facilities.
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 Qt 5.
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 intended mainly for desktop applications.
Qt Quick Layouts	Layouts 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

Module	Description
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 NFC	Classes accessing NFC hardware. Only officially supported on BlackBerry hardware so far (or N9 in the MeeGo port).
Qt OpenGL	Legacy module containing the OpenGL classes from Qt 4. In Qt 5 the similar functionality in Qt GUI is recommended.
Qt Positioning	Classes for accessing GPS and other location services. Split off from the Qt 4 Mobile module of Qt Location. Supported on Android, BlackBerry, iOS and Linux (using GeoClue).
Qt Script	Legacy module for scripting Qt application using ECMAScript/JavaScript. In Qt 5, using similar classes in Qt QML is recommended.
Qt Sensors	Classes for accessing various mobile hardware sensors. Used to be part of Qt Mobile in Qt 4. Supported on Android, BlackBerry, iOS, WinRT, Mer and Linux.
Qt Serial Port	Classes for access to hardware and virtual serial ports. Supported on Windows, Linux and Mac OS X.
Qt WebChannel	Provides access to Qt objects to HTML/Js over WebSockets.
Qt WebEngine	A new set of Qt Widget and QML webview APIs based on Chromium.
Qt WebSockets	Provides a WebSocket implementation.
Qt XML	Legacy module containing classes for SAX and DOM style XML APIs. Replaced with QXmlStreamReader and QXmlStreamWriter classes in Qt Core.
Qt XML Patterns	Support for XPath, XQuery, XSLT and XML Schema validation.

Tools

Qt comes with its own set of tools to ease cross-platform development, which can otherwise be cumbersome due to different set of development tools. Qt Creator is a cross-platform IDE for C++ and QML. Qt Designer's GUI layout/design functionality is integrated into the IDE, although Qt Designer can still be started as a standalone tool.

In addition to Qt Creator, Qt provides qmake, a cross-platform build script generation tool that automates the generation of Makefiles for development projects across different platforms. Without such a tool, one would have to write different Makefiles for each platform, so it is useful for abstracting away the differences of various platforms.

There are other tools available in Qt, including the Qt Designer interface builder and the Qt Assistant help browser (which are both embedded in Qt Creator), the Qt Linguist translation tool, uic (user interface compiler), and moc (Meta-Object Compiler).

Programming language bindings

Main articles: List of language bindings for Qt 4 and List of language bindings for Qt 5

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

Uses

Organizations using Qt

Because of simplicity, robustness, native performance, cross-platform compatibility and both commercial and open source licenses, many organizations in many parts of the world use Qt. These include but are not limited to European Space Agency, DreamWorks, Lucasfilm, Panasonic, Philips, Samsung, Siemens, Volvo, Walt Disney Animation Studios, Blizzard Entertainment, Electronic Arts, AMD.

Software using Qt

Main category: Software that uses Qt

Example applications using Qt are Autodesk Maya, Mathematica, Google Earth, the RStudio IDE for the R programming language, Spotify for Linux, the VirtualBox OS virtualization software package, the VLC media player, and AMD's Radeon Software Crimson Edition driver tool application.

The KDE desktop environment for UNIX-like operating systems uses Qt as its widget toolkit.

History

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.

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.

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.

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. 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.

In February 2011, Nokia announced its decision to drop Symbian technologies and base their future smartphones on the Windows Phone 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.

In March 2011, Nokia sold the commercial licensing part of Qt to Digia creating Qt Commercial. In August 2012, Digia announced that it will acquire Qt from Nokia The Qt team started working in Digia in September 2012. Qt team at Digia, released Qt 5.0 within a month, and released newer versions every 6 months with new features and additional supported platforms.

Licensing

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.

On 20 May 1995 Troll Tech publicly release Qt 0.90 for X11/Linux with the source code under the FreeQt license. This license 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. Trolltech used this license until version 1.45. 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, 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. 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. 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 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, which should make Qt even more attractive for non-GPL open source projects and for closed applications.

In March 2011, Nokia sold the commercial licensing part of Qt to Digia creating Qt Commercial. In September 2014, Digia transferred the Qt business and copyrights to their wholly owned subsidiary, the Qt Company.

See also

• List of widget toolkits
• Android software development
• iOS SDK

External links

• Official website
• Official blog
• Official wiki