GUI designing in Qt Creator using embedded Qt Designer
|Original author(s)||Haavard Nord and Eirik Chambe-Eng|
|Initial release||20 May 1995|
|Stable release||5.3.2 (September 16, 2014) |
|Preview release||5.4 RC (November 27, 2014) |
|Operating system||Linux (Embedded, Wayland, X11), OS X, Windows, Windows Phone …|
Qt Commercial License 
Qt ( "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 codebase, while having the power and speed of native applications. 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 1
- Software architecture 2
- Supported platforms 3
- Editions 4
- Qt 5 5.1
Software modules 6
- Qt Essentials 6.1
- Qt add-ons 6.2
- Tools 7
- Programming language bindings 8
- Organizations using Qt 9.1
- Software using Qt 9.2
- Licensing 10.1
- Future directions 11
- See also 12
- Bibliography 13.1
- External links 14
Purposes and abilities
Qt is used for developing application software. Programs created with Qt can have a native-looking graphical user interface (GUI), in which cases Qt is classified as a widget toolkit, but they are not limited to GUI. Qt is also used for developing non-GUI programs such as command-line tools and consoles for servers.
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 several different platforms, which makes it attractive for those who want a single code base to work virtually everywhere in Write once, compile anywhere fashion. qt.io website, the new home for Qt developers, introduces Qt as: "1 framework, 15 platforms, 95% satisfaction, 800,000+ users". The following platforms are officially supported by Digia:
|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 / BlackBerry 10||Qt for QNX and the QNX-based BlackBerry 10 platform.|
|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 (GNU/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.
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.
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||http://blog.qt.digia.com/blog/2014/05/20/qt-5-3-released/|
|5.4||Planned for 2 December 2014||
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 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 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.|
|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 Qt4. In Qt5 the similar functionality in Qt GUI is recommended.|
|Qt Positioning||Classes for accessing GPS and other location services. Split off from the Qt4 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 Qt4. 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 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 scheme validation.|
Qt comes with its own set of tools to ease cross-platform development, that is otherwise 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 this relatively new IDE, although Qt Designer can still be called as a standalone tool.
In addition to Qt Creator, Qt provides a handy makefile generation tool,qmake, a tool that automates the generation of Makefiles for development project across different platforms. Without qmake, one should write different makefiles for each platform, so it is a useful tool for transparent handeling of differences in various platforms.
There are other tools available in Qt, including Qt designer, Qt assistant (both are embedded in Qt creator now), Qt assistant (for translating GUI), uic (user interface compiler), moc (Meta-object System compiler for handling Signals and slots). Various other converters, compiling and linking also released with Qt.
Programming language bindings
Qt has a range of bindings for various languages, which implement some or all of its feature set.
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
Software using Qt
Example applications using Qt are Altera Quartus, a design and simulation tool for programmable logic; Autodesk Maya; Bitcoin Core; Cameleon (programming language); Mathematica; Google Earth; KDE, a desktop environment for UNIX-like operating systems; Skype; Spotify for Linux; Ubuntu; VirtualBox, an OS virtualization software package; Musescore music score writing software; and the VLC media player.
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 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 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.
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.
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, 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.
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