When and how to properly destroy QMenu context menu? - c++

Question

I allow custom context menu to appear over a table. This is how the menu is generated, using a generic function that accepts target widget and coordinates:
#include <QMenu>
void MainWindow::makeContextMenu(const QPoint& pos, QWidget* target)
{
QMenu *menu = new QMenu(this);
menu->addAction(new QAction("Action 1", menu));
menu->addAction(new QAction("Action 2", menu));
menu->addAction(new QAction("Action 3", menu));
// Notify window about clicking
QObject::connect(menu, &QMenu::triggered, this, &MainWindow::menuClicked);
// If this is a scroll area, map coordinates to real app coordinates
if(QAbstractScrollArea* area = dynamic_cast<QAbstractScrollArea*>(target))
menu->popup(area->viewport()->mapToGlobal(pos));
else
menu->popup(pos);
}
The problem is that the QMenu* menu never gets destroyed and removed from memory. It persists as MainWindow's child even after it's hidden.
What should I do? Can I set the menu to delete itself? Or should I reuse the same instance of menu or maybe save it into same pointer?

Answer 1

It doesn't have to be so complicated. That's it already:
menu->setAttribute(Qt::WA_DeleteOnClose);
That way when the QMenu is closed, the class is deleted as soon as the event loop is entered again. And it doesn't matter if an action was triggered or the popup was just closed.
To prove my answer, you can test it yourself by checking when the menu is created and if the 'deleted' message is triggered with the same address:
qDebug() << "created" << (qintptr)menu;
connect(menu, &QMenu::destroyed,
this, [menu]() { qDebug() << "deleted" << (qintptr)menu; });

Answer 2

From your code, it seems like menu should be deleted after this event has taken place?
// Notify window about clicking
QObject::connect(menu, &QMenu::triggered, this, &MainWindow::menuClicked);
Can I set the menu to delete itself?
Yes, you can have the object delete itself like this:
// Notify window about clicking
QObject::connect(menu, &QMenu::triggered, this, &MainWindow::menuClicked);
QObject::connect(menu, &QMenu::triggered, menu, &QMenu::deleteLater);
If you are worried about the order of those slots being called, see this
Or should I reuse the same instance of menu or maybe save it into same
pointer?
Well, you can do something like
//Your constructor
MainWindow::MainWindow(....)
{
menu = nullptr;
....
}
//Make context Menu
void MainWindow::makeContextMenu(const QPoint& pos, QWidget* target)
{
if(menu)
delete menu;
menu = new QMenu(this);
....
}
As for the MainWindow::~MainWindow() destructor, it will take care of the menu's clean up. Since MainWindow (which is a QObject derived class) automatically deletes all children
Lastly, you can simply have the menu as a member of MainWindow, and whenever you need to have fresh actions for menu, you can use QMenu::clear to delete all existing actions.
//Your constructor
MainWindow::MainWindow(....)
{
menu = new QMenu(this);
....
}
void MainWindow::makeContextMenu(const QPoint& pos, QWidget* target)
{
menu->clear();
//QMenu *menu = new QMenu(this);
menu->addAction(new QAction("Action 1", menu));
menu->addAction(new QAction("Action 2", menu));
menu->addAction(new QAction("Action 3", menu));
// Notify window about clicking
QObject::connect(menu, &QMenu::triggered, this, &MainWindow::menuClicked);
// If this is a scroll area, map coordinates to real app coordinates
if(QAbstractScrollArea* area = dynamic_cast<QAbstractScrollArea*>(target))
menu->popup(area->viewport()->mapToGlobal(pos));
else
menu->popup(pos);
}

Answer 3

It is possible to delete QMenu when it's hidden. I designed event filter class for that purpose:
#ifndef DELETEONHIDEFILTER_H
#define DELETEONHIDEFILTER_H
#include <QObject>
#include <QEvent>
class DeleteOnHideFilter : public QObject
{
Q_OBJECT
public:
explicit DeleteOnHideFilter(QObject *parent = 0) : QObject(parent) {}
protected slots:
bool eventFilter(QObject *obj, QEvent *event) override {
if(event->type() == QEvent::Hide) {
obj->deleteLater();
}
return false;
}
};
#endif // DELETEONHIDEFILTER_H
It can be used for other objects as well.