QObject Class Reference Documentation Qt Project — Volkswagen Pointer I

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Detailed Description

The class is the base class of all Qt

QObject is the heart of the Qt Object The central feature in this is a very powerful mechanism for object communication called and slots. You can connect a signal to a with connect () and destroy the with disconnect (). To avoid ending notification loops you can block signals with (). The protected functions connectNotify () and () make it possible to track

QObjects organize themselves in trees. When you create a with another object as the object will automatically add to the parent’s children () list. The takes ownership of the object; it will automatically delete its in its destructor. You can look for an object by and optionally type using () or findChildren ().

Every object has an () and its class name can be found via the metaObject () (see QMetaObject::className You can determine whether the object’s inherits another class in the inheritance hierarchy by using the () function.

When an object is deleted, it a destroyed () signal. You can catch signal to avoid dangling to QObjects .

QObjects can receive through event () and filter the of other objects. See installEventFilter () and () for details. A convenience handler, (), can be reimplemented to catch child

Last but not least, QObject the basic timer support in Qt; see for high-level support for timers.

that the Q_OBJECT macro is for any object that implements slots or properties. You also to run the Meta Object Compiler on the file. We strongly recommend the use of macro in all subclasses of QObject of whether or not they actually use slots and properties, since to do so may lead certain functions to strange behavior.

All Qt widgets inherit QObject. The function isWidgetType () returns an object is actually a widget. It is faster than qobject_cast ( obj ) or obj (QWidget ).

Some QObject e.g. children (), return a QObjectList is a typedef for QList .


A QObject instance is to have a thread affinity . or it lives in a certain thread. a QObject receives a queued or a posted event. the slot or handler will run in the thread the object lives in.

Note: If a has no thread affinity (that is, if () returns zero), or if it lives in a that has no running event then it cannot receive signals or posted events.

By default, a QObject lives in the in which it is created. An object’s affinity can be queried using () and changed using moveToThread ().

All must live in the same as their parent. Consequently:

li () will fail if the two QObjects live in different threads. li a QObject is moved to another all its children will be automatically too. li moveToThread () will if the QObject has a parent. li If QObjects are within QThread::run (), they become children of the QThread because the QThread does not in the thread that calls ().

Note: A QObject’s member do not automatically become its children. The relationship must be set by either a pointer to the child’s constructor. or by setParent (). Without this the object’s member variables remain in the old thread when () is called.

No copy constructor or assignment

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QObject has neither a copy nor an assignment operator. This is by Actually, they are declared, but in a section with the macro (). In fact, all Qt classes derived QObject (direct or indirect) use macro to declare their constructor and assignment operator to be The reasoning is found in the discussion on vs Value on the Qt Object Model

The main consequence is that you use pointers to QObject (or to your subclass) where you might be tempted to use your QObject as a value. For example, without a constructor, you can’t use a subclass of as the value to be stored in one of the container You must store pointers.


Qt’s meta-object provides a mechanism to automatically signals and slots between subclasses and their children. As as objects are defined with object names, and slots a simple naming convention, connection can be performed at run-time by the () function.

uic generates code invokes this function to auto-connection to be performed between on forms created with Qt . More information about auto-connection with Qt Designer is in the Using a Designer UI File in Application section of the Qt Designer

Dynamic Properties

From Qt dynamic properties can be added to and from QObject instances at Dynamic properties do not need to be at compile-time, yet they provide the advantages as static properties and are using the same API — property () to read them and () to write them.

From Qt 4.3, dynamic are supported by Qt Designer. and both Qt widgets and user-created forms can be dynamic properties.


All QObject subclasses support translation features, making it to translate an application’s user into different languages.

To user-visible text translatable, it be wrapped in calls to the tr () function. is explained in detail in the Writing Code for Translation document.

Volkswagen Pointer I
Volkswagen Pointer I


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