[Qt] Родительские/дочерние widget-ы и отношения между ними

Qt::Window

0x00000001

Indicates that the widget is a window, usually with a window system frame and a title bar, irrespective of whether the widget has a parent or not.

--

QDialog and QMainWindow widgets are by default windows, even if a parent widget is specified in the constructor. This behavior is specified by the Qt::Window flag.

надо было создавать в куче .

куча тут ни при чём, проблема во флаге Qt::Window :

When a widget accepts the close event, it is hidden (and destroyed if 
it was created with the Qt::WA_DeleteOnClose flag)

#include <QApplication>
#include <QWidget>
#include <QLabel>

class S : public QLabel
{
    public:
        S(QWidget *p) : QLabel("Label", p)
        {
            qDebug("+S");
        }

        ~S()
        {
            qDebug("-S");
        }
};

int main(int argc, char ** argv)
{
    QApplication app(argc,argv);

    QWidget wdg1;

    wdg1.resize(200,200);
    wdg1.setWindowTitle("Qt parent window");

    S wdg2(&wdg1);

    wdg1.show();

    int r = app.exec();

    qDebug("Event loop stopped");

    return r;
}

# make
# ./q
+S
Event loop stopped
-S

Спасибо всем, вроде понятно стало.