#include <QApplication>
#include <QWidget>
#include <QTimer>
#include <QDebug>

class ChildObj : public QWidget
{
    Q_OBJECT

public:
    explicit ChildObj(QWidget* parent = nullptr) : QWidget(parent) {}
    ~ChildObj() { qDebug("ChildObj::~ChildObj()"); }
};

class ParentObj : public QWidget
{
    Q_OBJECT

public:
    explicit ParentObj(QWidget* parent = nullptr): QWidget(parent)
    {
        auto childObj = new ChildObj(this);

        // Functor-based connection: Slot is called after ParentObj is destroyed (Not expected)
        connect(childObj, &QObject::destroyed, this, &ParentObj::onChildDestroyed);

        // String-based connection: Slot is NOT called after ParentObj is destroyed (Expected)
        //connect(childObj, SIGNAL(destroyed(QObject*)), this, SLOT(onChildDestroyed(QObject*)));
    }
    ~ParentObj() { qDebug("ParentObj::~ParentObj()"); }

private slots:
    void onChildDestroyed(QObject *obj) { qDebug() << "ParentObj::onChildDestroyed():" << obj; }
};


int main(int argc, char* argv[]) {
    QApplication a(argc, argv);
    auto pObj = new ParentObj();

    QTimer::singleShot(1000, &a, [&]() {
        pObj->deleteLater();
        qApp->quit();
    });

    return a.exec();
}

#include "main.moc"