tutorial6: when and why do I have to add *parent as a class parameter and when not?

[dsyleixa]

hi,
as tp tutorial6:
when do I have to add *parent as a class parameter and when not?

e.g., not:
.cpp

Bullet::Bullet(): QObject(), QGraphicsRectItem(){
    setRect(0,0,10,50);
    QTimer * timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(move()));
    timer->start(50);
}

header:

class Bullet: public QObject, public QGraphicsRectItem{
    Q_OBJECT
public:
    Bullet(QGraphicsItem * parent=0);  // parent only in header
public slots:
    void move();
};

but e.g., yes:
.cpp

Enemy::Enemy(QGraphicsItem *parent): QObject(), QGraphicsRectItem(parent){ // <<<<<<<<<<<<<<<<<<<<<<<<<<<
    int random_number = rand() % 700;
    setPos(random_number,0);
    setRect(0,0,100,100);
    QTimer * timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(move()));
    timer->start(50);
}

header:

class Enemy: public QObject, public QGraphicsRectItem{
    Q_OBJECT
public:
    Enemy(QGraphicsItem * parent=0);  // parent in header like above
public slots:
    void move();
};

i.e., why can't I just declare
Enemy::Enemy(): QObject(), QGraphicsRectItem(){ ...}[/code]`
similar to the Bullet file?

(ref.: https://github.com/MeLikeyCode/QtGameTutorial/tree/master/tutorial6)

(edit,
BTW, in previous tutorials https://github.com/MeLikeyCode/QtGameTutorial/tree/master/tutorial1 ... https://github.com/MeLikeyCode/QtGameTutorial/tree/master/tutorial5 this "parents" thing has not been added at all for either class)