GUI designing in Qt Creator using the embedded Qt Designer
|Original author(s)||Haavard Nord and Eirik Chambe-Eng|
|Initial release||20 May 1995|
|Stable release||5.7.0 (June 16, 2016[±])|
|Written in||C++, Objective-C, Objective-C++, Java|
|Operating system||Android, iOS, Linux (Embedded, Wayland, X11), OS X, Windows, Windows Phone, …|
|License||Qt Commercial License
GPL 2.0, 3.0
Qt (// "cute",) 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 2.0, GPL 3.0, and LGPL 3.0.  
- 1 Purposes and abilities
- 2 History
- 3 Licensing
- 4 Software architecture
- 5 Supported platforms
- 6 Use Cases
- 7 Editions
- 8 Software modules
- 9 Tools
- 10 Programming language bindings
- 11 See also
- 12 References
- 13 External links
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.
In the summer of 1990, Haavard Nord and Eirik Chambe-Eng (the original developers of Qt and the CEO and President, respectively, of Trolltech) were working together on a database application for ultrasound images written in C++ and running on Mac OS, Unix, and Windows. They 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 first two versions of Qt had only two flavors: Qt/X11 for Unix and Qt/Windows for Windows.
On 20 May 1995 Troll Tech publicly released 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 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.
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.
Becoming Free Software-friendly
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.
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 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 4.0 also for Windows under the GPL in June 2005. Qt 4 supported the same set of platforms in the free software/open source editions as in the proprietary edition, so it is possible, with Qt 4.0 and later releases, 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.
Acquisition by Nokia
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.
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.
In September 2014, Digia transferred the Qt business and copyrights to their wholly owned subsidiary, The Qt Company.
Qt has always been available under a commercial license that allows developing proprietary applications with no restrictions on licensing.
In addition, Qt is also licensed under a combination of GPL 2.0, GPL 3.0, LGPL 2.1 and LGPL 3.0, depending on the Qt module and version. Starting from Qt 5.7 the LGPL 2.1 license was dropped from most Qt modules, therefore prohibiting to distribute closed embedded devices except by acquiring a commercial license (see also Tivoization).
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.
Qt works on many different platforms; the following are officially supported:
|X11||Qt for X Window System (Linux, FreeBSD, HP-UX, Solaris, AIX, etc.)|
|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.|
|Embedded Linux||Qt for embedded platforms: personal digital assistant, smartphone, etc. Exists as multiple platforms depending on display technology. DirectFB, LinuxFB and EGLFS (EGL Full Screen).|
|Android||Qt for Android, formerly known as Necessitas.|
|OS X||Qt for Apple OS X; supports applications on Cocoa|
|iOS||Qt for iOS platforms (iPhone, iPad)|
|Windows||Qt for Microsoft Windows XP, Vista, 7, 8 and 10|
|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|
|Other Embedded Platforms|
|Integrity||Qt for Integrity|
|QNX||Qt for QNX|
|VxWorks||Qt for VxWorks.|
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.
Organizations using Qt
Qt is used by many organizations, including but not limited to European Space Agency, DreamWorks, Lucasfilm, Panasonic, Philips, Samsung, Siemens, Volvo, Walt Disney Animation Studios, Blizzard Entertainment, Electronic Arts, AMD.
GUI and Desktop environments
Several GUIs and desktop environments utilize Qt as widget toolkit.
- KDE Plasma, a desktop environment for various form factors like computers, tablets and smartphones
- Unity8, the upcoming desktop environment of Ubuntu
- Ubuntu Touch, the phone UI from Canonical
- LXQt (Lightweight X11 Desktop Environment), the successor of LXDE based on Qt
- Lumina, a desktop environment designed for PC-BSD
- Hawaii, a Wayland and Qt Quick-based desktop environment
- Sailfish OS, a mobile operating system developed by Jolla
- webOS, a multitask operating system for smart devices like TVs and smartwatches
- Sky Q, the home entertainment system of Sky plc
- Avionics, Panasonic's Inflight Entertainment System
- Tesla Model S in-car UI
- Papyros, a modern GUI based on Qt5 and Material Design principles
- Deepin Desktop Environment, a GUI for the Deepin distribution
- OPIE, a GUI for former Sharp Zaurus
Applications using Qt
Example applications using Qt are
- AMD's Radeon Software Crimson Edition driver tool application.
- Autodesk Maya
- Dragonframe stop motion animation software
- Google Earth
- Scribus desktop publishing software 
- Spotify for Linux
- Stellarium, a planetarium program
- Subsurface, a software for logging and planning scuba dives initially designed and developed by Linus Torvalds
- Telegram desktop messaging client
- VirtualBox OS virtualization software package
- VLC media player
- XnView MP
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 General Public License 3.0, GNU Lesser General Public License 3.0 and GNU Lesser General Public License 2.1 (with Qt special exception)
|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 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 WebEngine||A new set of Qt Widget and QML webview APIs based on Chromium.|
|Qt Test||Classes for unit testing Qt applications and libraries.|
|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 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 WebKit||Qt's WebKit implementation and API.|
|Qt WebKit Widgets||The widget API for Qt WebKit|
|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.|
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
Qt has a range of bindings for various languages, which implement some or all of its feature set.
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Qt Wiki provides a comprehensive list of English books about Qt.  This is a list of notable books:
- Bocklage-Ryannel, Juergen; Thelin, Johan (12 May 2015). "Qt 5 Cadaques" (1st ed.).
- Blanchette, Jasmin; Summerfield, Mark (14 February 2008). "C++ GUI Programming with Qt 4" (2nd ed.). Prentice Hall. ISBN 978-0-13-235416-5.
- Summerfield, Mark (23 August 2010). "Advanced Qt Programming: Creating Great Software with C++ and Qt 4" (1st ed.). Addison-Wesley. ISBN 978-0-321-63590-7.
- Fitzek, Frank H. P.; Mikkonen, Tommi; Torp, Tony (17 May 2010). "Qt for Symbian" (1st ed.). Wiley. ISBN 0-470-75010-3.
- Summerfield, Mark (28 October 2007). "Rapid GUI Programming with Python and Qt" (1st ed.). Prentice Hall. ISBN 978-0-13-235418-9.
- Molkentin, Daniel (19 July 2007). "The Book of Qt 4: The Art of Building Qt Applications" (1st ed.). No Starch Press. ISBN 978-1-59327-147-3.
- Thelin, Johan (3 August 2007). "Foundations of Qt Development" (1st ed.). Apress. ISBN 978-1-59059-831-3.
- Dalheimer, Matthias (January 2002). "Programming with Qt" (2nd ed.). O'Reilly Media. ISBN 978-0-596-00064-6.
- Ezust, Alan; Ezust, Paul (10 September 2006). "An Introduction to Design Patterns in C++ with Qt 4" (2nd ed.). Prentice Hall. ISBN 978-0-13-187905-8.
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