CODEWITHSUNDEEP
c++qtqt4signals-slots
bartolo-otrit
bartolo-otrit

Reputation: 2519

Is it possible to connect a signal to a static slot without a receiver instance?

Is it possible to connect a signal to static slot without receiver instance?

Like this: connect(&object, SIGNAL(some()), STATIC_SLOT(staticFooMember()));

There is a QApplication::closeAllWindows() function with [static slot] attribute in Qt documentation. And there is an example of using it from the documentation:

exitAct = new QAction(tr("E&xit"), this);
exitAct->setShortcuts(QKeySequence::Quit);
exitAct->setStatusTip(tr("Exit the application"));
connect(exitAct, SIGNAL(triggered()), qApp, SLOT(closeAllWindows()));

Is it allowed to do the same action but without passing an instance variable (e.g. when a class has only static functions)?

class Some : public QObject {
    Q_OBJECT
public slots:
    static void foo();
private:
    Some();
};

Maybe Frank Osterfeld is right and it is better to use singleton pattern in this case but I am still surprised why this feature has not been implemented yet.

Update:

In Qt 5 it is possible.

Upvotes: 34

Views: 26643

Answers (3)

UmNyobe
UmNyobe

Reputation: 22890

Update for QT5: Yes you can

static void someFunction() {
    qDebug() << "pressed";
}
// ... somewhere else
QObject::connect(button, &QPushButton::clicked, someFunction);

In QT4 you can't:

No it is not allowed. Rather, it is allowed to use a slot which is a static function, but to be able to connect it you need an instance.

In their example, 

connect(exitAct, SIGNAL(triggered()), qApp, SLOT(closeAllWindows()));

means than they previously called

QApplication* qApp = QApplication::instance();

Edit:

The only interface for connecting object is the function

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

How are you going to get rid of const QObject * receiver?

Check the moc files in your project, it speaks by itself.

Upvotes: 26

ManuelSchneid3r
ManuelSchneid3r

Reputation: 16091

It is. (With Qt5)

#include <QApplication>
#include <QDebug>

void foo(){
    qDebug() << "focusChanged";
}


int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    QObject::connect(&app, &QApplication::focusChanged, foo);
    return app.exec();
}

Upvotes: 5

Ashish Bansal
Ashish Bansal

Reputation: 89

In earlier versions of Qt, although you cannot do so as mentioned by @UmNyobe but you can do something like this if you really want to call that static slot :

connect(&object, SIGNAL(some()), this, SLOT(foo()));

void foo()
{
    .... //call your static function here.
}

Upvotes: 1

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