Qt can't connect a subclass of QGraphicsView signal (not inherited) to SLOT

Question

I defined class MyGraphicsView, a subclass of QGraphicsView. Than, I add a signal test() in MyGraphicsView. In my MainWindow class, I have MyGraphicsView* myView and I connect like:

connect(myView, SIGNAL( test() ) , this, SLOT( zoom() )) ;

But I got:

    QObject::connect: No such signal QGraphicsView::test() in ..\Proto_version_2\mainwindow.cpp:73

Adiana

Can we see your code? – Sodium

Did you add Q_OBJECT in your MyGraphicsView declaration? – Impacted

Answer 1

In order to use slots and signals in a class, it must be derived from either QObject or a QObject derived class and your class must include the Q_OBJECT macro

class MyClass : public QGraphicsView
{
    Q_OBJECT // Without this macro, signals and slots will not work

    public:
        MyClass(QObject* parent);
};

The Q_OBJECT macro allows classes to use QT's C++ extensions. As the documentation states: -

The Meta-Object Compiler, moc, is the program that handles Qt's C++ extensions. The moc tool reads a C++ header file. If it finds one or more class declarations that contain the Q_OBJECT macro, it produces a C++ source file containing the meta-object code for those classes. Among other things, meta-object code is required for the signals and slots mechanism, the run-time type information, and the dynamic property system.

Note, however that Qt 5 provides an additional connect call, which warns if the Q_OBJECT is missing: -

connect(myView, QMainView::test, myClassObj, MyClass::zoom);

In this case, the 2nd and 4th argument are pointers to the functions. In addition, run-time checking of the connect call is performed. You can read more about this here.

Melissamelisse

moc is always needed whether you use the old or the new syntax. The Qt 5 connect will not even compile if you attempt connection to an object without the Q_OBJECT macro. – Downhaul

@KubaOber, Oops my mistake. I was writing that in a rush. I've corrected the answer. – Melissamelisse