clear widget in a QGraphicsScene : crash
c++, qt4
Solution
The reason your program crashes is that the QGraphicsScene::clear() method deletes the QButton (and its associated data structures) in the middle of a method call that uses those very data structures. Then, immediately after clear() returns, the calling method tries to access the now-deleted data (because it wasn't expecting to be deleted in the middle of its routine), and bang -- a crash. Your problem is an example of a re-entrancy problem.
The easiest way to avoid tripping over your shoelaces like that is to make your signal/slot connection be a QueuedConnection rather than an AutoConnection:
QObject::connect(button, SIGNAL(clicked()), scene, SLOT(clear()), Qt::QueuedConnection);
That way the clear() method call won't be executed until after the button's mouse-press-handling routine has returned, and therefore clear() will be called from a context where it is safe to delete the button.
Problem
I have a QGraphicsScene with a QPushButton inside, and clearing this scene will crash my application. Is there a correct way to clear a scene with a QWidget ? The following code crashes when clicking on the button: ``` #include <QApplication> #include <QGraphicsView> #include <QGraphicsScene> #include <QGraphicsProxyWidget> #include <QPushButton> int main(int argc, char *argv[]) { QApplication app(argc, argv); QGraphicsScene *scene = new QGraphicsScene(); QGraphicsView *view = new QGraphicsView(); view->setScene(scene); view->show(); QPushButton *button = new QPushButton("button"); QObject::connect(button, SIGNAL(clicked()), scene, SLOT(clear())); QGraphicsProxyWidget *proxy = scene->addWidget(button); return app.exec(); } ```