error: static assertion failed: Signal and slot arguments are not compatible

Iftekhar

I am getting this error with connect function

QTimer timer;
       QObject::connect(&timer, &QTimer::timeout,this, &MainWindow::draw);
       timer.start(1000);t

both timeout and draw are predefined functions. How can I solve this? Even If I use a different function like sprite = new Sprite(this);

QTimer timer;
   QObject::connect(&timer, &QTimer::timeout,sprite, &Sprite::drawNextBall);
   timer.start(1000);

void Sprite::drawNextBall(QPainter &painter)
{
    qDebug() <<"Sprite::drawNextBall() called";
    painter.drawEllipse(x, y, 15, 15);
}

Chris Kawa

Hi, welcome to the forum.

As the error says - the signal and slot arguments need to match as the argument is passed from one to the other. In this case QTimer::timeout does not have any arguments and Sprite::drawNextBall takes a QPainter. There's no way Qt can automatically create a painter for you that you'd like. You have to do it yourself.

So to solve it you need a function that doesn't take any parameters. Inside you can create a painter that you want and call drawNextBall with it. Alternatively you can replace the slot in that connect statement with a parameterless lambda and do the same inside it - create a painter and call the thing you want with it.

Also, with painting you don't usually call draw functions directly, but rather schedule a paint event and let Qt call paintEvent at the right time with the right arguments. So something like

QObject::connect(&timer, &QTimer::timeout, this, &MainWindow::update);

update() is a function that schedules a paint event, so you can override paintEvent method and do your drawing there.

Christian Ehrlicher

@Chris-Kawa said in error: static assertion failed: Signal and slot arguments are not compatible:

QObject::connect(&timer, &QTimer::timeout, this, &MainWindow::update);

Nearly, sadly QWidget::update() has some overloads so

QObject::connect(&timer, &QTimer::timeout, this, qOverload<>(&MainWindow::update));

Iftekhar

@Christian-Ehrlicher said in error: static assertion failed: Signal and slot arguments are not compatible:

QObject::connect(&timer, &QTimer::timeout, this, qOverload:<>:of(&MainWindow::update));

I get an error if I use the above connect function

mainwindow.cpp:15:54: error: use of undeclared identifier 'qOverload'

Christian Ehrlicher

Then your compiler is to old - use QOverload

https://doc.qt.io/qt-5/qtglobal.html#qOverload

Iftekhar

@Chris-Kawa
I have already overridden the paintEvent() in the Mainwindow class.
Please refer to this post and provide some help
https://forum.qt.io/topic/135315/ball-creation-at-regular-intervals-falling-ball-game/2