[Solved] QMainWindow subclass not receiving winEvents unless QMessageBox is called

[fruchtquark]

Hello all,

I'm new to Qt (4.8.1 for Desktop + MSVC2010) and also just starting to get a handle on C++, and I'm trying to interact with a 3rd party driver DLL that sends Events to a HWDN (which I attempt to set via MainWindow::effectiveWinId() ). This has been working, though apparently only by chance, as inside my MainWindow, before setting said HWND, I called created a QMessageBox.
My MainWindow is a regular UI-Designer subclassed QMainWindow:

namespace Ui {
    class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    virtual bool winEvent( MSG* message, long* result );
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
....
    void installmessages();
 
private slots:
     void on_freezeButton_clicked();
 
};

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    virtual bool winEvent( MSG* message, long* result );
    ~MainWindow();
    
private:
    Ui::MainWindow *ui;
....
    void installmessages();

private slots:
     void on_freezeButton_clicked();

};

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The code in MainWindow::installmessages();

qDebug() << "before";
    qDebug() << MainWindow::effectiveWinId() ;
 
// *
    qDebug() << "messagebox";
    QMessageBox::information(   this,
                                tr("Messagebox title"),
                                tr("WinID %1").arg( (long) MainWindow::effectiveWinId() )
                             );
 
// */
    qDebug() << "after";
    qDebug() << MainWindow::effectiveWinId() ;
 
....
 
[call to proprietary library with MainWindow::effectiveWinId() as parameter]

qDebug() << "before";
    qDebug() << MainWindow::effectiveWinId() ;

// *
    qDebug() << "messagebox";
    QMessageBox::information(   this,
                                tr("Messagebox title"),
                                tr("WinID %1").arg( (long) MainWindow::effectiveWinId() )
                             );

// */
    qDebug() << "after";
    qDebug() << MainWindow::effectiveWinId() ;

....

[call to proprietary library with MainWindow::effectiveWinId() as parameter]

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results in this output

before 
0x0 
messagebox  [the output is "WinID 0"]
after 
0x17046a

before 
0x0 
messagebox  [the output is "WinID 0"]
after 
0x17046a

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So with message box, my program works since I get a valid ID and thus receive messages.
Why is the WinID valid only when the MessageBox is run?

[high_flyer]

in which method are your qDebug() outputs?
And show the rest, the code (where the debug outputs are) only shows that you are outputing debug messages - show the rest of the code there.

[fruchtquark]

Apparently windows / widgets don't get a system window ID by default and are called non-native or alien.
While setAttribute(Qt::WA_NativeWindow); did not have an effect, winId did: http://qt-project.org/doc/qt-4.8/qwidget.html#winId

I'm not sure if this is a workaround or a permanent solution, so any comments are still welcome..