Qt

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Qt is a cross-platform application and widget toolkit that 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. Some of its more important features include:

  • Running on the major desktop platforms and some of the mobile platforms.
  • Extensive internationalization support.
  • A complete library that provides SQL database access, XML parsing, thread management, network support, and a unified cross-platform application programming interface (API) for file handling.

The Qt framework is the basis of the KDE software community, as well as other important open source and proprietary applications such as VLC, VirtualBox, Mathematica and many others.

Installation

Qt 6.x and 5.x are available in the official repositories. Legacy versions of Qt (4.x and 3.x) are available from the AUR. They can be installed with the following packages:

  • Qt 6.x is available in the qt6-base package, with documentation in the qt6-doc package.
  • Qt 5.x is available in the qt5-base package, with documentation in the qt5-doc package.
  • Qt 4.x is available in the qt4AUR package, with documentation on AUR in the qt4-docAUR package.
  • Qt 3.x is available from the AUR in the qt3AUR package, with documentation on AUR in the qt3-docAUR package.

Default Qt toolkit

By installing qtchooserAUR you can make the Qt binaries in /usr/bin (e.g. qmake), which point to the Qt 5 versions by default (e.g. qmake-qt5), point to older versions of those tools (e.g. qmake-qt4 or qmake-qt3) instead.

Warning: qtchooserAUR is now in conflict with qt5-base. You can install it to /usr/local if you really need it, but it is not officially supported anymore. See FS#51308.

Using environment variables

To define the default Qt toolkit, you can create QT_SELECT environment variable. For example, to use Qt 4, set QT_SELECT=4.

Using configuration files

You can set the default Qt toolkit by creating a symlink ~/.config/qtchooser/default.conf to one of .conf files in /etc/xdg/qtchooser/ directory. For example, to set Qt 4 symlink /etc/xdg/qtchooser/4.conf to ~/.config/qtchooser/default.conf: 

$ ln -s /etc/xdg/qtchooser/4.conf ~/.config/qtchooser/default.conf

Configuration

Styles in Qt 5

Qt 5 decides the style to use based on what desktop environment is used:

  • In KDE Plasma, it uses the actually selected Qt style. It can be configured using KDE System Settings (systemsettings), the settings can be found in Appearance > Application Style > Widget Style.
  • In Cinnamon, GNOME, MATE, LXDE, Xfce, it uses GTK (QGtkStyle).
  • In other desktop environments, it uses Fusion.

To force a specific style, you can set the QT_STYLE_OVERRIDE environment variable. Specifically, set it to gtk2 if you want to use the GTK theme (Note: you will need to install the Qt style plugins mention below to get the GTK style). Qt 5 applications also support the -style flag, which you can use to launch a Qt 5 application with a specific style.

The following styles are included in Qt 5: Fusion, Windows. Others can be installed from the official repositories:

  • Breeze — Artwork, styles and assets for the Breeze visual style for the Plasma Desktop.
https://invent.kde.org/plasma/breeze || breeze
  • Oxygen — KDE Oxygen style.
https://invent.kde.org/plasma/oxygen || oxygen
  • Lightly — Lightly is a fork of breeze theme style that aims to be visually modern and minimalistic.
https://github.com/Luwx/Lightly || lightly-gitAUR
  • QtCurve — A configurable set of widget styles for KDE and Gtk.
https://invent.kde.org/system/qtcurve || qtcurve-qt5
  • Adwaita-Qt — A style to bend Qt applications to look like they belong into GNOME Shell.
https://github.com/MartinBriza/adwaita-qt || adwaita-qt5
  • Qt style plugins — Additional style plugins for Qt 5, including GTK, Cleanlooks, Motif, Plastique.
https://code.qt.io/cgit/qt/qtstyleplugins.git || qt5-stylepluginsAUR
  • Kvantum — customizable SVG-based theme engine with a variety of built-in styles, including imitations of some popular GTK themes such as Adapta, Arc, Ambiance
https://github.com/tsujan/Kvantum/tree/master/Kvantum || kvantum

Styles in Qt 4

Qt 4 application will try to mimic the behavior of the desktop environment they are running on, unless they run into some problems or hard-coded settings.

  • In KDE Plasma, it uses the actually selected Qt style. It can be configured using KDE System Settings (systemsettings), the settings can be found in Appearance > Application Style > Widget Style.
  • In Cinnamon, GNOME, Xfce, it uses GTK (QGtkStyle).
  • In other desktop environments, it uses Windows.

For those who want to change the look and feel of Qt 4 applications, the Qt Configuration (qtconfig-qt4) GUI tool is provided by the qt4AUR package. It offers a simple interface to configure the appearance of Qt 4 applications including style, colors, fonts and some further options.

Note: If you use GTK style, then color and font settings will be ignored, and inherited from GTK 2.

Qt keeps all its configuration information in /etc/xdg/Trolltech.conf (system-wide) or ~/.config/Trolltech.conf (user-specific). The file is rather difficult to navigate because it contains a lot of information not related to appearance, but for any changes you can just add to the end of the file and overwrite any previous values (make sure to add your modification under the [Qt] header).

For example, to change the theme to QtCurve, add: 

~/.config/Trolltech.conf
...
[Qt]
style=QtCurve

The following styles are included in Qt 4: CDE, Cleanlooks, GTK, Motif, Plastique, Windows. Others can be installed from the official repositories or the AUR (most are written for KDE Plasma Desktop):

  • Breeze — Artwork, styles and assets for the Breeze visual style for the Plasma Desktop.
https://invent.kde.org/plasma/breeze || breeze-kde4AUR
  • Adwaita-Qt — A style to bend Qt applications to look like they belong into GNOME Shell.
https://github.com/MartinBriza/adwaita-qt || adwaita-qt4AUR

Qt Style Sheets

An interesting way of customizing the look and feel of a Qt application is using Style Sheets, which are just simple CSS files. Using Style Sheets, one can modify the appearance of every widget in the application.

To run an application with a different style just execute: 

$ qt_application -stylesheet style.qss

For more information on Qt Style Sheets see the official documentation or other tutorials. As an example Style Sheet see this Dolphin modification.

GTK and Qt

If you have GTK and Qt applications, their looks might not exactly blend in very well. If you wish to make your GTK styles match your Qt styles please read Uniform look for Qt and GTK applications.

Configuration of Qt 5 applications under environments other than KDE Plasma

Unlike Qt 4, Qt 5 does not ship a qtconfig utility to configure fonts, icons or styles. Instead, it will try to use the settings from the running desktop environment. In KDE Plasma or GNOME this works well, but in other less popular desktop environments or window managers it can lead to missing icons in Qt 5 applications. One way to solve this is to fake the running desktop environment by setting XDG_CURRENT_DESKTOP=KDE or GNOME, and then using the corresponding configuration application to set the desired icon set.

Another solution is provided by the qt5ct package, which provides a Qt 5 QPA independent of the desktop environment and a configuration utility. After installing the package, run qt5ct to set an icon theme, and set the environment variable QT_QPA_PLATFORMTHEME=qt5ct so that the settings are picked up by Qt applications. Alternatively, use --platformtheme qt5ct as argument to the Qt 5 application.

qt5ct-kdeAUR provides a patched qt5ct with better integration to KDE applications, including KDE QML applications.

If the errors below are received, and some icons still do not appear in some of the applications, install oxygen and oxygen-icons: 

Icon theme "oxygen" not found.
Icon theme "oxygen" not found.
Error: standard icon theme "oxygen" not found!

Development

Supported platforms

Qt supports most platforms that are available today, even some of the more obscure ones, with more ports appearing every once in a while. For a more complete list see the Qt Wikipedia article.

Android

Tango-view-refresh-red.pngThis article or section is out of date.Tango-view-refresh-red.png

Reason: One of the mentioned SDK versions has been removed. What does "some tools have been removed" mean? Is this still relevant? The comments about OpenJDK also need to be updated. (Discuss in Talk:Qt)

First of all, you need an Android SDK and NDK. Install SDK android-sdkAUR (some tools have been removed) or android-sdk-25.2.5AUR[broken link: package not found] and NDK android-ndkAUR from AUR or using Android Studio.

SDK requires OpenJDK in version <= 8. At this moment there is OpenJDK in version 7 and 8 available, but to avoid build problems stick to the latter.

Next you are going to need Qt 5 for Android. You can install it from AUR as described below or build it yourself, you can find build instructions on Qt wiki page.

In case of problems you may want to visit known issues.

Or you can use a Qt Installer, for a full list see Official Qt installers.

Tools

The following are official Qt tools:

  • Qt Creator — A cross-platform IDE tailored for Qt that supports all of its features.
https://doc.qt.io/qtcreator/ || qtcreator
  • Qt Linguist — A set of tools that speed the translation and internationalization of Qt applications.
https://doc.qt.io/qt-5/qtlinguist-index.html || Qt 5: qt5-tools, Qt 4: qt4AUR
  • Qt Assistant — A configurable and redistributable documentation reader for Qt qch files.
https://doc.qt.io/qt-5/qtassistant-index.html || Qt 5: qt5-tools, Qt 4: qt4AUR
  • Qt Designer — A powerful cross-platform GUI layout and forms builder for Qt widgets.
https://doc.qt.io/qt-5/qtdesigner-manual.html || Qt 5: qt5-tools, Qt 4: qt4AUR
  • Qt Quick Designer — A visual editor for QML files which supports WYSIWYG. It allows you to rapidly design and build Qt Quick applications and components from scratch.
https://doc.qt.io/qtcreator/creator-using-qt-quick-designer.html || qtcreator
  • qmlscene — A tool for loading QML documents that makes it easy to quickly develop and debug QML applications.
https://doc.qt.io/qt-5/qtquick-qmlscene.html || Qt 5: qt5-declarative, Qt 4 QML Viewer: qt4AUR
  • qmake — A tool that helps simplify the build process for development project across different platforms, similar to cmake, but with fewer options and tailored for Qt applications.
https://doc.qt.io/qt-5/qmake-manual.html || Qt 5: qt5-base, Qt 4: qt4AUR
  • uic — A tool that reads *.ui XML files and generates the corresponding C++ files.
https://doc.qt.io/qt-5/uic.html || Qt 5: qt5-base, Qt 4: qt4AUR
  • rcc — A tool that is used to embed resources (such as pictures) into a Qt application during the build process. It works by generating a C++ source file containing data specified in a Qt resource (.qrc) file.
https://doc.qt.io/qt-5/rcc.html || Qt 5: qt5-base, Qt 4: qt4AUR
  • moc — A tool that handles Qt's C++ extensions (the signals and slots mechanism, the run-time type information, and the dynamic property system, etc.).
https://doc.qt.io/qt-5/moc.html || Qt 5: qt5-base, Qt 4: qt4AUR

Bindings

Qt has bindings for all of the more popular languages, see https://wiki.qt.io/Language_Bindings for a full list.

The following examples display a small 'Hello world!' message in a window.

C++ 

hello.cpp
#include <QApplication>
#include <QLabel>

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    QLabel hello("Hello world!");

    hello.show();
    return app.exec();
}

QML

hello.qml
import QtQuick 2.3

Rectangle {
    id: page
    width: 400; height: 100
    color: "lightgray"

    Text {
        id: helloText
        text: "Hello world!"
        anchors.horizontalCenter: page.horizontalCenter
        anchors.verticalCenter: page.verticalCenter
        font.pointSize: 24; font.bold: true
    }
}

Python (PyQt) 

hello-pyqt.py
import sys
from PyQt5 import QtWidgets

app = QtWidgets.QApplication(sys.argv)
label = QtWidgets.QLabel("Hello world!")

label.show()
sys.exit(app.exec_())

Python (PySide2) 

hello-pyside2.py
import sys
from PySide2.QtWidgets import QApplication, QLabel

app = QApplication(sys.argv)
label = QLabel("Hello world!")

label.show()
sys.exit(app.exec_())

C#

See QtSharp.

Troubleshooting

Disable/Change Qt journal logging behaviour

When using KDE and/or any other Qt desktop environment debug info may be frequently be logged in the systemd journal.

Set QT_LOGGING_RULES as environment variable to change this behaviour, e.g. to completely disable logging: 

/etc/environment
QT_LOGGING_RULES='*=false'

To disable only debug logging, use QT_LOGGING_RULES="*.debug=false".

Icon theme is not applied

Since Qt 5.1 SVG support has moved into a module. Since qt5-base does not depend on qt5-svg it may happen that the qt5-base is installed but not qt5-svg. This results in deceptive icon theme behaviour. Since SVG is not supported the icons are silently skipped and the icon theme may seem to be unused. Installing qt5-svg explicitly solves the problem.

Theme not applied to root applications

As the user theme file ($XDG_CONFIG_HOME/Trolltech.conf), are not read by other accounts, the selected theme will not apply to X applications run as root. Possible solutions include:

  • Create symlinks, e.g 
    # ln -s /home/[username]/.config/Trolltech.conf /etc/xdg/Trolltech.conf
  • Configure system-wide theme file: /etc/xdg/Trolltech.conf
  • Adjust the theme as root

Qt 4 style not respected

If pure Qt 4 (non-KDE) applications do not stick with your selected Qt 4 style, then you will probably need to tell Qt 4 how to find KDE's styles (Oxygen, Phase etc.). You just need to set the environment variable QT_PLUGIN_PATH. E.g.: 

QT_PLUGIN_PATH=$HOME/.kde4/lib/kde4/plugins/:/usr/lib/kde4/plugins/

qtconfig-qt4 should then be able to find your kde styles and everything should look nice again!

Alternatively, you can symlink the Qt 4 styles directory to the KDE4 styles one: 

# ln -s /usr/lib/{kde,qt}4/plugins/styles/theme_name

All Qt 5-based applications fail to run after Qt 5 update

If you get an error similar to 

Qt FATAL: Cannot mix incompatible Qt library (version 0x50900) with this library (version 0x50901)

then you are most likely using a Qt 5 platform theme or style plugin which has not been recompiled against the latest version of Qt 5. These usually use Qt private headers which means they depend on an exact version of Qt and not just a matching soname. Figure out which theme/style you are using by checking the QT_STYLE_OVERRIDE and QT_QPA_PLATFORMTHEME environment variables, and rebuild the AUR package that provides it.

QXcbConnection: XCB error: 2 (BadValue)

Create a file with such content [1]: 

/etc/xdg/QtProject/qtlogging.ini
[Rules]
qt.qpa.xcb.xcberror=false

Graphics misaligned or scaled improperly

See HiDPI#Qt 5.

See also