QObject Class Reference
[QtCore module]

bool QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method, Qt::ConnectionType type = Qt::AutoConnection )   [static]

Creates a connection of the given type from the signal in the sender object to the method in the receiver object. Returns true if the connection succeeds; otherwise returns false.

You must use the SIGNAL() and SLOT() macros when specifying the signal and the method, for example:

 QLabel *label = new QLabel;
 QScrollBar *scrollBar = new QScrollBar;
 QObject::connect(scrollBar, SIGNAL(valueChanged(int)),
                  label,  SLOT(setNum(int)));

This example ensures that the label always displays the current scroll bar value. Note that the signal and slots parameters must not contain any variable names, only the type. E.g. the following would not work and return false:

 // WRONG
 QObject::connect(scrollBar, SIGNAL(valueChanged(int value)),
                  label, SLOT(setNum(int value)));

A signal can also be connected to another signal:

 class MyWidget : public QWidget
 {
     Q_OBJECT

 public:
     MyWidget();

 signals:
     void buttonClicked();

 private:
     QPushButton *myButton;
 };

 MyWidget::MyWidget()
 {
     myButton = new QPushButton(this);
     connect(myButton, SIGNAL(clicked()),
             this, SIGNAL(buttonClicked()));
 }

In this example, the MyWidget constructor relays a signal from a private member variable, and makes it available under a name that relates to MyWidget.

A signal can be connected to many slots and signals. Many signals can be connected to one slot.

If a signal is connected to several slots, the slots are activated in the same order as the order the connection was made, when the signal is emitted.

The function returns true if it successfully connects the signal to the slot. It will return false if it cannot create the connection, for example, if QObject is unable to verify the existence of either signal or method, or if their signatures aren't compatible.

By default, a signal is emitted for every connection you make; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnection type, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

The optional type parameter describes the type of connection to establish. In particular, it determines whether a particular signal is delivered to a slot immediately or queued for delivery at a later time. If the signal is queued, the parameters must be of types that are known to Qt's meta-object system, because Qt needs to copy the arguments to store them in an event behind the scenes. If you try to use a queued connection and get the error message

 QObject::connect: Cannot queue arguments of type 'MyType'
 (Make sure 'MyType' is registered using qRegisterMetaType().)

call qRegisterMetaType() to register the data type before you establish the connection.

Note: This function is thread-safe.

See also disconnect(), sender(), and qRegisterMetaType().

bool QObject::connect ( const QObject * sender, const char * signal, const char * method, Qt::ConnectionType type = Qt::AutoConnection ) const

This function overloads connect().

Connects signal from the sender object to this object's method.

Equivalent to connect(sender, signal, this, method, type).

Every connection you make emits a signal, so duplicate connections emit two signals. You can break a connection using disconnect().

Note: This function is thread-safe.

See also disconnect().

 if (QLatin1String(signal) == SIGNAL(valueChanged(int))) {
     // signal is valueChanged(int)
 }

 class MainWindow : public QMainWindow
 {
 public:
     MainWindow();

 protected:
     bool eventFilter(QObject *obj, QEvent *ev);

 private:
     QTextEdit *textEdit;
 };

 MainWindow::MainWindow()
 {
     textEdit = new QTextEdit;
     setCentralWidget(textEdit);

     textEdit->installEventFilter(this);
 }

 bool MainWindow::eventFilter(QObject *obj, QEvent *event)
 {
     if (obj == textEdit) {
         if (event->type() == QEvent::KeyPress) {
             QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);
             qDebug() << "Ate key press" << keyEvent->key();
             return true;
         } else {
             return false;
         }
     } else {
         // pass the event on to the parent class
         return QMainWindow::eventFilter(obj, event);
     }
 }

 QPushButton *button = parentWidget->findChild<QPushButton *>("button1");

 QListWidget *list = parentWidget->findChild<QListWidget *>();

 QList<QWidget *> widgets = parentWidget.findChildren<QWidget *>("widgetname");

 QList<QPushButton *> allPButtons = parentWidget.findChildren<QPushButton *>();

 QTimer *timer = new QTimer;         // QTimer inherits QObject
 timer->inherits("QTimer");          // returns true
 timer->inherits("QObject");         // returns true
 timer->inherits("QAbstractButton"); // returns false

 // QVBoxLayout inherits QObject and QLayoutItem
 QVBoxLayout *layout = new QVBoxLayout;
 layout->inherits("QObject");        // returns true
 layout->inherits("QLayoutItem");    // returns true (even though QLayoutItem is not a QObject)

 monitoredObj->installEventFilter(filterObj);

 class KeyPressEater : public QObject
 {
     Q_OBJECT
     ...

 protected:
     bool eventFilter(QObject *obj, QEvent *event);
 };

 bool KeyPressEater::eventFilter(QObject *obj, QEvent *event)
 {
     if (event->type() == QEvent::KeyPress) {
         QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
         qDebug("Ate key press %d", keyEvent->key());
         return true;
     } else {
         // standard event processing
         return QObject::eventFilter(obj, event);
     }
 }

 KeyPressEater *keyPressEater = new KeyPressEater(this);
 QPushButton *pushButton = new QPushButton(this);
 QListView *listView = new QListView(this);

 pushButton->installEventFilter(keyPressEater);
 listView->installEventFilter(keyPressEater);

 QObject *obj = new QPushButton;
 obj->metaObject()->className();             // returns "QPushButton"

 QPushButton::staticMetaObject.className();  // returns "QPushButton"

 myObject->moveToThread(QApplication::instance()->thread());

 if (receivers(SIGNAL(valueChanged(QByteArray))) > 0) {
     QByteArray data;
     get_the_value(&data);       // expensive operation
     emit valueChanged(data);
 }

 class MyObject : public QObject
 {
     Q_OBJECT

 public:
     MyObject(QObject *parent = 0);

 protected:
     void timerEvent(QTimerEvent *event);
 };

 MyObject::MyObject(QObject *parent)
     : QObject(parent)
 {
     startTimer(50);     // 50-millisecond timer
     startTimer(1000);   // 1-second timer
     startTimer(60000);  // 1-minute timer
 }

 void MyObject::timerEvent(QTimerEvent *event)
 {
     qDebug() << "Timer ID:" << event->timerId();
 }

 void MainWindow::createMenus()
 {
     fileMenu = menuBar()->addMenu(tr("&File"));
     ...

 MyWindow::MyWindow()
 {
     QLabel *senderLabel = new QLabel(tr("Name:"));
     QLabel *recipientLabel = new QLabel(tr("Name:", "recipient"));
     ...

 label->setText(tr("F\374r \310lise"));

 QPushButton::staticMetaObject.className();  // returns "QPushButton"

 QObject *obj = new QPushButton;
 obj->metaObject()->className();             // returns "QPushButton"

 QObject *obj = new QTimer;          // QTimer inherits QObject

 QTimer *timer = qobject_cast<QTimer *>(obj);
 // timer == (QObject *)obj

 QAbstractButton *button = qobject_cast<QAbstractButton *>(obj);
 // button == 0

 class MyClass : public QObject
 {
     Q_OBJECT
     Q_CLASSINFO("Author", "Pierre Gendron")
     Q_CLASSINFO("URL", "http://www.my-organization.qc.ca")

 public:
     ...
 };

 class MyClass : public QObject
 {

   private:
     Q_DISABLE_COPY(MyClass)
 };

 class MyClass : public QObject
 {

   private:
      MyClass(const MyClass &);
      MyClass &operator=(const MyClass &);
 };

   QWidget w = QWidget();

 class MyClass : public QObject
 {
     Q_OBJECT
     Q_ENUMS(Priority)

 public:
     MyClass(QObject *parent = 0);
     ~MyClass();

     enum Priority { High, Low, VeryHigh, VeryLow };
     void setPriority(Priority priority);
     Priority priority() const;
 };

 class QLibrary : public QObject
 {
     ...
     Q_FLAGS(LoadHint LoadHints)
     ...

     ...
 public:
     enum LoadHint {
         ResolveAllSymbolsHint = 0x01,
         ExportExternalSymbolsHint = 0x02,
         LoadArchiveMemberHint = 0x04
     };
     Q_DECLARE_FLAGS(LoadHints, LoadHint)
     ...

 class BasicToolsPlugin : public QObject,
                          public BrushInterface,
                          public ShapeInterface,
                          public FilterInterface
 {
     Q_OBJECT
     Q_INTERFACES(BrushInterface ShapeInterface FilterInterface)

 public:
     ...
 };

 class Window : public QWidget
 {
     Q_OBJECT

 public:
     Window();
     void normalMethod();
     Q_INVOKABLE void invokableMethod();
 };

 #include <QObject>

 class Counter : public QObject
 {
     Q_OBJECT

 public:
     Counter() { m_value = 0; }

     int value() const { return m_value; }

 public slots:
     void setValue(int value);

 signals:
     void valueChanged(int newValue);

 private:
     int m_value;
 };

 Q_PROPERTY(type name
            READ getFunction
            [WRITE setFunction]
            [RESET resetFunction]
            [NOTIFY notifySignal]
            [DESIGNABLE bool]
            [SCRIPTABLE bool]
            [STORED bool]
            [USER bool]
            [CONSTANT]
            [FINAL])

 Q_PROPERTY(QString title READ title WRITE setTitle USER true)