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void Widget::showAllHiddenTopLevelWidgets()
{
    int count=0;
    foreach (QWidget *widget, QApplication::topLevelWidgets())
    {
        if (widget->isWindow())
           count++;
    }
    qDebug()<<count;
}

HWND ff=findWindow("1.exe");
QWidget *wgt= QWidget::find(ff);
wgt->setParent(this->parentWidget());

typedef WId
Platform dependent window identifier.

QWidget *wgt= QWidget::find(ws);

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;

    HWND test = FindWindowA(0,"Калькулятор");
    SetParent(test, w.winId());
    w.show();
    return a.exec();
}