qobject_cast fails in slot

[tuli]

Sadly that isnt the problem. Yes, the Q_OBJECT macro is there.

I have created a minimal demo to reproduce the issue:

class my_wid : public QWidget
{
    Q_OBJECT
public:
    my_wid(QWidget* p=0) : QWidget(p)
    {}
 
    int a,b,c;
};
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0)
  {
        my_wid* mm = create_mywid();
        delete mm;
    }
 
    my_wid* create_mywid(QWidget* parent=0)
    {
        my_wid* w = new my_wid(parent);
        connect(w, &QObject::destroyed, this, &MainWindow::remove_mywid);
        return w;
    }
 
 
protected slots:
 
    void remove_mywid(QObject* removed)
    {
        my_wid* test = qobject_cast<my_wid*>(removed);
        qDebug() << test; // is 0 :(
    }
};

class my_wid : public QWidget
{
    Q_OBJECT
public:
    my_wid(QWidget* p=0) : QWidget(p)
    {}

    int a,b,c;
};

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0)
  {
        my_wid* mm = create_mywid();
        delete mm;
    }

    my_wid* create_mywid(QWidget* parent=0)
    {
        my_wid* w = new my_wid(parent);
        connect(w, &QObject::destroyed, this, &MainWindow::remove_mywid);
        return w;
    }

  
protected slots:

    void remove_mywid(QObject* removed)
    {
        my_wid* test = qobject_cast<my_wid*>(removed);
        qDebug() << test; // is 0 :(
    }
};

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[d_stranz]

Strange. Here's another experiment: Instead of destroying the widget in the MainWindow constructor, set up a one-shot QTimer and connect the timeout() signal to my_wid's deleteLater() slot.

This may possibly be some quirk of Qt that because the MainWindow constructor hasn't finished before you delete the my_wid instance, all of the necessary internal wiring isn't set up completely.

[tuli]

No change.
In my real-world use case the destruction of the my_widget is also user-initiated and that doesnt work either. Really strange.

QTimer::singleShot(2000, mm, &my_wid::deleteLater);

QTimer::singleShot(2000, mm, &my_wid::deleteLater);

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[d_stranz]

No idea. Do you have the header file for my_widget #included so that MainWindow sees the full class definition (and not just a forward declaration of "class my_widget")? You must, otherwise your factory method wouldn't work. I don't think the qobject_cast<> template would compile without the full definition either. The static cast would work, though, because it just forces the compiler to treat a pointer to "x" as a pointer to "y" and won't complain until you try to call some member function of "y".