[SOLVED] how are these functions called if I didn't call them?

how are these functions called if I didn't call them?

I'm new to QT. How are these functions automatically called if I didn't call them?

Maybe somewhere inside the parent class there are connections that somehow connect them and launch them?

I know there will be a default constructor here, but how are these functions called, if the default constructor is empty

class CustomItem : public QGraphicsItem{

public:

QRectF boundingRect() const override{
    return QRectF(0, 0, 80, 80);
}

void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override{
    Q_UNUSED(option);
    Q_UNUSED(widget);

    painter->fillRect(boundingRect(),Qt::red);
}

}item;

Solution

Public methods can be called by anyone who has a pointer or reference to objects of your class.

Presumably objects of this type are added to a QGraphicsScene, which among other things will call boundingRect to determine how much space your CustomItem occupies, and paint when it needs to draw the part of the scene with that item.