confusion with a STATEMENT used frequently

[salmanmanekia]

Hi,
May be the question ahead also comes in domain of Object Oriented ,i am confused with

class exclass :public QObject, public QGraphicsItem
{
    Q_OBJECT
 
public:
	exclass(QGraphicsItem *parent = 0);
};

class exclass :public QObject, public QGraphicsItem
{
    Q_OBJECT

public:
	exclass(QGraphicsItem *parent = 0);
};

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#include "exclass.h"
 
exclass::exclass(QGraphicsItem *parent): QGraphicsItem(parent)
{
 
}

#include "exclass.h"

exclass::exclass(QGraphicsItem *parent): QGraphicsItem(parent)
{

}

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#include "exclass.h"
 
#include <QApplication>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    exclass *mainScene = new exclass;
    return a.exec();
}

#include "exclass.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    exclass *mainScene = new exclass;
    return a.exec();
}

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I am confused with the statement

'exclass(QGraphicsItem *parent = 0); ' in the .h file and

'exclass::exclass(QGraphicsItem *parent): QGraphicsItem(parent){}' in the .cpp file

can anyone explain why the declaration as 'QGraphicsItem *parent' is necessary in the exclass constructor and why do we say it as 'QGraphicsItem *parent = 0 'in the .h file..
i avoided this question so far and just followed it but now i have to develop a QGraphicsItem parent and a child so i am confused with how to do it..