Parent child relationship in Qt

[TheIndependentAquarius]

Every widget has a parent which helps in cases of deletion, i.e. when the parent gets deleted, the child gets automatically thrown out to /dev/null.

Now I see QWidget *parent being passed to the user defined class's constructor here, in this link: http://doc.qt.nokia.com/4.7/tutorial...ook-part1.html

class AddressBook : public QWidget
 {
     Q_OBJECT
 
 public:
     AddressBook(QWidget *parent = 0);
 
 private:
     QLineEdit *nameLine;
     QTextEdit *addressText;
 };

class AddressBook : public QWidget
 {
     Q_OBJECT

 public:
     AddressBook(QWidget *parent = 0);

 private:
     QLineEdit *nameLine;
     QTextEdit *addressText;
 };

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What does this indicate? In what way is this useful?