QObject::connect: No such signal eror C++

Question

I'm stuck with an error and i have no idea where I am going wrong i tried running the signal with qapp aswell just to make sure it isn't the signal, and that was all fine. I have the SLOT declared as a public slot in my header file.

signal declaration:

void editPolygon(QVector <QPointF> points, QPolygonF &);

this is my connect:

  connect(this, SIGNAL(editPolygon(QVector <QPointF> &, QPolygonF &)), parent, SLOT(editPolygon(QVector <QPointF> points, CustomPolygon *poly)));

this is my slot thats emmiting the signal:

void CustomPolygon::editPolygons()
{
    QVector<QPointF> points;
    QPolygonF poly = mapToScene(polygon());

    emit editPolygon(points,poly);
}

and this is the void in mainwindow.cpp:

void MainWindow::editPolygon(QVector<QPointF> &points, CustomPolygon *poly)
{
}

thanks for you help in advance!

are you still using Qt4? if not use the modern syntax. Can you paste your signal declaration? — cppiscute
hi, ive pasted my declaration at the top, also yes I shall look into the new syntax now — jaykay
Its not to be interpreted that "new" Qt's SIGNAL/SLOT syntax can work / while old doesn't .. the issue behind the error you get (No such signal) is because Signal/Slot function signatures should exactly match — Mohammad Kanan

paxdiablo paxdiablo · Accepted Answer · 2020-04-24T22:06:42

The first thing I would do is look very hard at the discrepancy in the second parameter of your signal/slot match.

One is QPolygon& and the other is CustomPolygon*. My understanding is that these had to match. Even if one were a proper subclass of the other, I don't think you can mix references and pointers like that.

And,regarding your (slightly paraphrased) comment:

My slot has to stay as a pointer because it points to an item that is being deleted from a QGraphicsScene.

My signal cant be a pointer because there is no conversion from a QPolygonF to a QPolygonF *, and I require QPolygonF to mapToScene.

I think I understand what you're saying, and there are a couple of issues.

If your slot takes a pointer that will be freed, you have to give it a pointer, and relinquish control of the object behind it (so either start with a dynamically allocated copy you're willing to let go of, or make a dynamically allocated copy from something you don't want to let go of).

And I'm not sure why you think there's no conversion from the object to a pointer to that object. I suppose that's technically true since the class itself doesn't provide a conversion, but this is really part of the base language:

Type thing;
Type *pointerToThing = &thing;

Both those issues should be solvable with something like:

QVector<QPointF> points;
QPolygonF thisIsMyPoly = mapToScene(polygon());
QploygonF *thisIsPolyForFreeing = new QPolygonF(thisIsMyPoly);
emit editPolygon(points, thisIsPolyForFreeing);

Your signal will still have to be changed to use a pointer rather than a reference and, hopefully, that will alleviate the mismatch.

If your pointer is an actual pointer to a QPolygonF that cannot be copied (because it's the pointer to the actual item being held in the QGraphicsScene and you therefore need the original pointer to remove it), I think you should just be able to pass that pointer as-is, assuming mapToScene() is returning a reference to it rather than a copy:

QPolygonF poly = mapToScene(polygon()); // get ref.
emit editPolygon(points, &poly);        // pass ref's address.

QObject::connect: No such signal eror C++

1 Answers