How to create screenshot of QWidget?

From QWidget::Grab:

QPixmap QWidget::grab(const QRect &rectangle = QRect(QPoint(0, 0), QSize(-1, -1)))

You can use QWidget::render for this. Assuming rectangle is a QRect:

QPixmap pixmap(rectangle->size()); 
widget->render(&pixmap, QPoint(), QRegion(rectangle));