Why a Qwidget extended class does not receive parent ?

[tonnot]

Is a very basic QT question :
At adressbook example I see :

at h : class NewAddressTab : public QWidget ()
at cpp :   NewAddressTab::NewAddressTab(QWidget *parent){...}

at h : class NewAddressTab : public QWidget ()
at cpp :   NewAddressTab::NewAddressTab(QWidget *parent){...}

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Why it is not neccesary to pass parent ?
Thanks