How does QT delete all controls in a layout?

1 Delete all controls in the layout

while(ui->layoutLightGroup_2->count()){
        QWidget *pWidget = ui->layoutLightGroup_2->itemAt(0)->widget();// Loop takes the first control each time , Delete it one by one 
        pWidget->setParent(NULL);
        ui->layoutLightGroup_2->removeWidget(pWidget);
        delete pWidget;
    }

  Get the number of layouts under this layout

ui->layoutLightGroup_2->count() 

2 Delete layout from layout

ui->layoutLightGroup_2->removeItem(ui->layoutLightGroup_1);