QT new QPainter导致的有关问题

QT new QPainter导致的问题
平台windows 7,Qt 4
.h：

#ifndef KEYBOARDEVENTTEST1_H
#define KEYBOARDEVENTTEST1_H

#include <QtGui/QWidget>
#include <QKeyEvent>
#include <QPaintEvent>
#include <QPixmap>
#include <QImage>
#include <QPainter>
#include <QPen>
#include <QPoint>
#include "ui_keyboardeventtest1.h"

class KeyboardEventTest1 : public QWidget
{
	Q_OBJECT

public:
	KeyboardEventTest1(QWidget *parent = 0, Qt::WFlags flags = 0);
	~KeyboardEventTest1();

	void drawPix();
	void keyPressEvent(QKeyEvent *);
	void paintEvent(QPaintEvent *);

private:
	Ui::KeyboardEventTest1Class ui;

private:
	QPixmap *pix;
	QImage image;
	int startX;
	int startY;
	int width;
	int height;
	int step;
};

#endif // KEYBOARDEVENTTEST1_H

.cpp

#include "keyboardeventtest1.h"
KeyboardEventTest1::KeyboardEventTest1(QWidget *parent, Qt::WFlags flags)
	:QWidget(parent)
{  
	setWindowTitle(tr("Key Event"));

	setAutoFillBackground(true);

	QPalette palette = this->palette();
	palette.setColor(QPalette::Window,Qt::white);
	setPalette(palette);

	setMinimumSize(512,256);
	setMaximumSize(512,256);

	width=size().width();
	height=size().height();
	pix = NULL;
	pix = new QPixmap(width,height);
	pix->fill(Qt::white);

	step=32;
	image.load("a.jpg");

	startX=0;
	startY=0;

	drawPix();
}

KeyboardEventTest1::~KeyboardEventTest1()
{
}

void KeyboardEventTest1::drawPix()
{
	pix->fill(Qt::white);
	QPainter painter(pix);
	QPen pen(Qt::DotLine);
	painter.setPen(pen);
	for(int i=step;i<width;)
	{
		painter.drawLine(QPoint(i,0),QPoint(i,height));
		i=i+step;
	}
	for(int j=step;j<height;)
	{
		painter.drawLine(QPoint(0,j),QPoint(width,j));
		j=j+step;
	}    
	painter.drawImage(QPoint(startX,startY),image);
}             

void KeyboardEventTest1::keyPressEvent(QKeyEvent *event)
{
	if(event->modifiers() == Qt::ControlModifier)
	{
		if(event->key() == Qt::Key_Left)
		{
			startX=(startX-1<0)?startX:startX-1;

		}
		if(event->key() == Qt::Key_Right)
		{
			startX=(startX+1+image.width ()>width)?startX:startX+1;

		}
		if(event->key() == Qt::Key_Up)
		{
			startY=(startY-1<0)?startY:startY-1;

		}	
		if(event->key() == Qt::Key_Down)
		{
			startY=(startY+1+image.height()>height)?startY:startY+1;

		}		
	}
	else
	{
		startX=startX-startX%step;
		startY=startY-startY%step;

		if(event->key() == Qt::Key_Left)
		{
			startX=(startX-step<0)?startX:startX-step;		
		}
		if(event->key() == Qt::Key_Right)
		{
			startX=(startX+step+image.width()>width)?startX:startX+step;		
		}
		if(event->key() == Qt::Key_Up)
		{
			startY=(startY-step<0)?startY:startY-step;		
		}	
		if(event->key() == Qt::Key_Down)
		{
			startY=(startY+step+image.height()>height)?startY:startY+step;
		}		
		if(event->key() == Qt::Key_Home)
		{
			startX=0;
			startY=0;
		}	
		if(event->key() == Qt::Key_End)
		{
			startX=width-image.width();
			startY=height-image.height();
		}			
	}
	drawPix();
	update();	
}              

void KeyboardEventTest1::paintEvent(QPaintEvent *)
{
	QPainter painter(this);
	painter.drawPixmap(QPoint(0,0),*pix);
}

问题出现在drawPix()函数中，
如果QPainter painter(pix);换成QPainter *painter = new QPainter(pix);的话，在屏幕第一次显示的时候，能够显示出正确的界面，但是当用户按ctrl和向左等箭头的时候，屏幕上就什么都不显示。如果使用QPainter painter(pix);就什么问题都没有，都能正常显示，想知道这是什么问题？！


------解决方案--------------------
new 完，你 delete 了吗？