Explain how to use pointers? - c++

Question

Sorry, I use google translator.
I am exploring how QGraphicsItem works.
But I don’t understand how the data is transmitted through the pointers to the scene.
Here is a code
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "qmyscene.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
scene = new QMyScene(this);
ui->graphicsView->setScene(scene);
scene->setSceneRect(0,0,800,600);
}
MainWindow::~MainWindow()
{
delete ui;
}
qmyscene.cpp
#include "qmyscene.h"
#include "qpixitem.h"
#include <qdebug.h>
QMyScene::QMyScene(QObject *parent) : QGraphicsScene(parent)
{
QPixItem *pix1 = new QPixItem (this,100,100);
QPixItem *pix2 = new QPixItem (this,50,50);
}
QMyScene::~QMyScene()
{
}
qpixitem.cpp
#include "qpixitem.h"
#include <QMessageBox>
QPixItem::QPixItem (QGraphicsScene *MyScene,int x,int y): QGraphicsPixmapItem()
{
QPixmap pic (":/Items/OutLet.png");
this->setPixmap(pic);
this->setPos(x, y);
this->setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemSendsGeometryChanges);
qDebug() << "create";
QGraphicsLineItem* line = MyScene->addLine(QLineF(40, 40, 80, 80));
MyScene->addItem(this);
}
QVariant QPixItem::itemChange(QGraphicsItem::GraphicsItemChange change, const QVariant &value)
{
if (change == ItemPositionChange)
{
QPointF newPos = value.toPointF();
int p1 = newPos.x();
int p2 = newPos.y();
MyScene->addLine(QLineF(40,40,p1, p2)); //error
//this->line->setLine(QLineF(40,40,p1, p2)); //error
qDebug() << "Scene::move";
}
return QGraphicsItem::itemChange(change, value);
}
qmyscene.h
#ifndef QMYSCENE_H
#define QMYSCENE_H
#include <QGraphicsScene>
#include <QDebug>
class QMyScene: public QGraphicsScene
{
Q_OBJECT
public:
explicit QMyScene(QObject *parent = 0);
~QMyScene();
private:
};
#endif // QMYSCENE_H
qpixitem.h
#ifndef QPIXITEM_H
#define QPIXITEM_H
#include <QGraphicsItem>
#include "QGraphicsScene"
#include "mainwindow.h"
#include <QGraphicsPixmapItem>
class QPixItem: public QGraphicsPixmapItem
{
public:
QPixItem (QGraphicsScene *MyScene,int x,int y);
QGraphicsScene *MyScene;
int x,y;
private:
QGraphicsLineItem* line;
QVariant itemChange(GraphicsItemChange change, const QVariant &value);
};
#endif // QPIXITEM_H
// error - the lines cause an error (closing the program due to memory access), I am sure that this is due to improperly used pointers. How to use them correctly in such code?

Answer 1

You never seem to allocate this->line.
Did you mean:
this->line = MyScene->addLine(QLineF(40, 40, 80, 80));
Instead of
QGraphicsLineItem* line = MyScene->addLine(QLineF(40, 40, 80, 80));