QT notes


  • QtApplication::exec -> starts main event loop defined in QtCoreApplication
  • Qt framework is divided into modules. Each module is a library. QtCore, QtGUI and QtWidgets are used by default.
  • QtCoreApplication designed for inheritance (to add some global data to the application, or override some functions). This way developer can avoid using singleton anti-pattern.
  • when using multiple inheritance QObject must be the first one. Developer must avoid multiple inheritance resulting in multiple iheritance from QObject or any other Qt base class.
  • Qt extends C++ with meta info (aka Class in Java). This meta is used by MOC (MetaObjectCompiler). MOC “replaces” Qt macro, e.g. Q_OBJECT with required code in a separate file.
  • resources are defined in special XML file (with .qrc extension) and can be accessed in Runtime using the following path: “:/folder/filename.ext”, e.g. picture->setPixmap(QPixmap(“:/images/open.png”))
  • qInstallMessageHandler(func) attaches handler for QMessages, aka gDebug etc. Can be used to redirect messages to file.
  • QtGlobal has a number of usefull methods and macros
  • QLibraryInfo contains info about current Qt setup (path to plugins, library version etc)
  • QApplication::setPalette() defines palette for the whole application. Good practice is to use 3-7 colors in a asingle application
  • by default Qt supports double buffering for painting. QWidget::setAttribute(Qt::WA_PaintOnScreen) disables this.


  • on Windows add console option to CONFIG variable in the .pro file to see console output


  • defines signal/slot; timer; hierarchy (simplifies memory managmet: parent object destroys its children); signals filter; meta info (aka Class in Java); qobject_cast()
  • to inherit from QObject developer must add Q_OBJECT marco just after class declaration (no semi-colon!!!)
  • Q_PROPERTY macro defines object property visible to QScript and UIDesigner. At minumum must provide name and read function: Q_PROPERTY(bool myProp READ getMyProp)
  • parent of the object is defined in the constructor (ptr-to-QObject) or using setParent(). parent() and children() returns info about parent and children correspondignly.
  • setObjectName() and objectName() can be very useful for debugging
  • child of the object can be found using findChild<QObject>(const char), where argument is the name of the object or a regex
  • rule of thumb – create all objects on the heap, via new. Only objects without children may be created on the stack (short live objects)
  • dumpObjectInfo() and dumpObjectTree() prints corresponding info in stdout
  • QObject::metaObject() aka this.getClass() in Java
  • QObject::inherits() aka instanceof
  • QObject is not copyable nor assignable. These are defined as private.


  • to define signal put a method under signals: section of the class declaration (similar to public, private etc)
  • signal does not have definition – only declaration. MOC generates all required code.
  • signal always returns void
  • to send a signal use emit keyword
  • slots can be private, protected and public, e.g. public slots:
  • slots do not have arguments with default values
  • avoid virtual slots (barzo slow)
  • slot method provides acces to the sender via sender(). sender() returns ptr to QObject that has emitted the signal
  • signal/slots are connected via QObject::connect method
  • conncetion can be of three types: Direct, Queued and Auto. Direct – aka method call; Queued – signal is transformed into event and queued; Auto – if in the same thread -> Direct, otherwise Queued.
  • connections of the object are removed automatically when object is removed but can also be removed via QObject::disconnect method
  • QSignalMapper maps signal onto some function. It can define parameters, e.g. QString: mapper->setMapping(QObject, “Value”)


  • events are low level signals e.g. QMouseEvent provides info about mouse location and/or which button was pressed
  • each event is handled by a single method (virtual protected) from which developer can emit a signal
  • QEvent is the base class for all events defined in Qt
  • if QEvent::ignore() is set event handling will be delegated to its succesors, if any
  • events are handled after QApplication::exec() is called
  • by default mouseMoveEvent() is called only if mouse is moving when one of its button is pressed. QWidget::setMouseTracking(true) enables mouse movement events without buttons
  • custom events can be sent using QCoreApplication::sendEvent() or QCoreApplication::postEvent()
  • custom events can be recieved with QObject::event() or QObject::customEvent()
  • it is possible to override event method, but this can blow up the code base. Better approach is to override specific event handler function and change event only in case specific method does not exist
  • QCoreApplication::processsEvents() ensures that all queued up till now events will be processed. It is a good practice to insert this after heavy operations so that application does not freeze totally
  • QCoreApplication::processsEvents() ensures that all queued up till now events will be processed. It is a good practice to insert this after heavy operations so that application does not freeze totally
  • to define event filter inherit from QObject and override eventFilter() so that if it returns true event won’t further processed, otherwise event will be passed to the destination object
  • to attach filter object use QObject::installEventFilter(filter). This way filter functionality can be easily shared
  • QCoreApplication::installFilter() installs filter for evey widget in the app. May affect event delivery perfomance
  • QCoreApplication::notify() defines the way how events are delivered. Ovveride this one for full control over the process

Tulip (aka STL)

  • sequence containers: QVector; QList; QLinkedList; QStack; QQueue.
  • associative containers: QSet; QMap<K,T>; QMultiMap<K,T>; QHash<K,T>; QMultiHash
  • value() is not defined for QSet
  • prefer empty() over size() to check emptiness
  • QXXIterator have same semantics as Java Iterators, aka next(), hasNext(). It is an object (not pointer) that points to siblings of the element not the element.
  • QXXMutableIterator allow changes to the underlying object: remove(), insert() and setValue()
  • Qt creates a copy of a container when enetering foreach
  • QByteArray can be compressed and uncompressed uaing global functions qCompress and qUncompress(). QByteArray::toBase64, QByteArray::fromBase64
  • QBitArray aka BitSet
  • QList::at() returns const ref to an element
  • prefer simple value when specifying associative container’s size
  • operator[] creates a new element if it does not exists, i.e. map["MyKey"] returns newly created value, if it does not exists
  • QMapXX sorts by key while QHashXX uses hash-table
  • to be eligible for QHash object must have operator== and qHash() defined
  • QHash::squeeze reduces hash-table size


  • qEqual – compares two containers. Elements must have operator==


  • contains text data in Unicode (2 bytes per symbol)
  • str.setNum(3.14) is the same as QString::number(3.14) and is the smae as QTextStream(&str) << 3.14
  • when created without value (e.g. on the stack) QString points to shared_null


  • contains value of an arbitrary type. QVariant::type returns actual type’s id (int); QVariant::typeName returns actual type as string
  • to extract value use toXX() or value(). This creates a copy of the value.


  • 254 methods, 53 fields…
  • QWidget::geometry() returns QRect (position + size)
  • QWidget::setAutoFillBackground(true) makes widget visible, e.g. fill widget with blue color
  • QStackedWidget shows one widget at a time
  • widgets can be organized using QLayout. setSpacing() and setMargin() are used for fine tuning
  • QLayout assigns root widget (defined by setLayout()) as a parent to newly added widgets
  • tab order be default by the order widgets were added to the layout
  • QSplitter is a layout with a movable border between widgets
  • QLabel can actually show graphics (setPixmap()) or even multimedia (setMovie()). Hold on a sec – it renders HTML!!!
  • QLabel can be grouped with any other widget using setBuddy()
  • Using & defines shortcut, e.g. N&ame defines shortcut — <Alt>+a
  • To define sort order in QListWidget its items must inherit from QListWidgetItem (QTreeWidgetItem for trre like lists) and override operator<()
  • QWidget::palette() returns QPalette which defines colors for three widget states: Active, inactive and Disabled


  • QGuiApplication::setOverrideCursor(Qt::cursor) overrides cursor for the whole application, e.g. loading cursor when application loads something; QGuiApplication::restoreOverrideCursor() – restores default behavior

MV, aka Interview

  • QAbstractProxyModel is a convenient abstraction for sortinf, filtering and other manipulations with model
  • to define selection model, aka data binding in webix, use QAbstarctItemView::selectionModel()
  • to check whether requested index is valid (QModelIndex index = pModel->index(2, 5, QModelIndex());) use QModelIndex::isValid(). Index may be invalid in case these is no data at the position in the model specified by the index.
  • each data element in the model may have its won role, aka tooltip, decoration etc

Arthur (drawing engine)

  • Qt provides double buffering out-of-the-box transparantely for a developer
  • QPainter, QPaintEngine, QPaintDevice are base classes in Arthur.
  • QPainter is used to draw
  • QPainter::drawText(x, y, str) draws str text. To define font used for drawing text one uses QPainter::setFont()
  • QPaintDevice is an abstraction i.e. something to draw on, aka QWidget, QImage etc
  • in general drawing is performed in QPaintEvent handler. For instance if user movesa window and it overlaps another window, overlapped window will exeucte QWidget::paintEvet()
  • QPainter::setRenderHint(QPainter::Antialiasing, true) enables anti-aliasing
  • (0; 0) pixel’s center is (0.5; 0.5)
  • QPicture provides a context with history. This history can be saved into a file and then reused.
  • Qt provides 4 predefined graphic effects: blur; colorization; drop shadow and opacity
  • to implement custom graphics effect one inherits from QGraphicsEffect and overrides draw()
  • QImage is non-context-independent class for drawing, aka all drawing happens in memory
  • to enanable Qt to render specified image format a corresponding dll (so) must supplied with the application. These files must reside in /imageformats folder
  • ::load() can be used to load image at any time (not just at construction)
  • QPixmap is context-dependent class and must be used for drawing on display etc