看Qt源码理解为什么布局类不用删除？

QWidget window;
QPushButton *button1 = new QPushButton("One");
QPushButton *button2 = new QPushButton("Two");
QPushButton *button3 = new QPushButton("Three");

QHBoxLayout *layout = new QHBoxLayout;
layout->addWidget(button1);
layout->addWidget(button2);
layout->addWidget(button3);

window.setLayout(layout);
window.show();

qDebug() << "Window children:" << window.children();

Window children: (QHBoxLayout(0x1f32ebc0), QPushButton(0x1f48ea00), QPushButton(0x1f48e9c0), QPushButton(0x1f48e580))

/* 省略部分无关代码 */
void QWidget::setLayout(QLayout *l)
{
    ...
    if (oldParent != this) {
        l->setParent(this);
        l->d_func()->reparentChildWidgets(this);
        l->invalidate();
    }
}

void QLayout::addWidget(QWidget *w)
{
    addChildWidget(w);
    addItem(QLayoutPrivate::createWidgetItem(this, w));
}

void QLayout::addChildWidget(QWidget *w)
{
    QWidget *mw = parentWidget();
    QWidget *pw = w->parentWidget();

    ...
    if (!pw && mw)
        w->setParent(mw);
    ...
}