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c++qtsignalssignals-slots

How to pass an extra variable to a Qt slot

I would like to know how to pass a separate variable into a slot. I cant seem to get it to work. Is there some way around this?

This is my code:

QTimer * timer = new QTimer();
connect(timer,SIGNAL(timeout()),this,SLOT(method(MYVARIABLE)));
timer->start(4000);
Solution

  • If you don't want to declare MYVARIABLE in your class, but instead to have it tied to this particular signal/slot connection, you can connect the signal to a C++11 lambda using Qt5's new singal/slot syntax and then call your slot with that lambda.

    For example you could write:

    QTimer * timer = new QTimer();
connect(timer, &QTimer::timeout, [=]() {
     method(MYVARIABLE);
});
timer->start(4000);

    Another solution if you can't use C++11 and Qt5 is to use Qt's Property System to attach a variable to your QTimer*. This can be done with QObject::setProperty().

    Then in the slot you could use QObject::sender() to get your QTimer* and read the property back using QObject::property().

    However, note that it's not a very clean solution, and borderline abuse of the property system.