Application crashes while accessing Pointers and while Hovering over custom QGraphicsItems

[Lanparty]

Hello everyone, I have been working on a project for a while now in which I try to visualize data changes within a graph representing a city network. When I started this project I had never programmed anything with qt, but I had some knowledge about basic c++ (from around 2 semesters at university) and I learned a lot while working on this project. However, I don’t really know what to do right now, which is why I am posting my code and my problem in here (bare with me, I'll try to explain it as best as I can, but I might not use the correct words).

In the application I am building I am using the QGraphicsView framework. Originally I implemented a datastructure for my graph and I wanted to connect it directly to any visualization, but I decided to write a program just for the purpose of visualizing whatever's going to happen in the other program (I'll put them together once I get the visualization part running). So, it might be a little confusing reading through my code since it only serves the purpose of understanding QGraphicsView a little bit better and being able to work purposefully once I put both codes together. Right now, my program only consists of the classes Main, MainWindow, Slice (custom class derived from QGraphicsEllipseItem) and Circle (custom class derived from QGraphicsEllipseItem in my code sometimes called "Kreis" or methods called something like "addKreis" - meaning "addCircle" I might've not caught each one).

Within each object of the class Circle 3 objects of the class Slice are initialized in order to create a piechart (I know, there's a class for that in qt, but I am kind of tring to get it done on my own). Several piecharts are then connected through objects of the class QGraphicsLineItem (as you can see in the screenshot showing an earlier stage some weeks ago (ignore the second non-connected circle on the left though)).

The user is able to move each piechart around in order to get closer to the real distribution of the cities represented by each piechart.
Due to different circumstances I decided to create the class Slice (I used to use Circle for each object, meaning that a circle object always had 3 private variables of other circle objects as children), but since I did that I am stumbling over error after error, most recently these two:

ASSERT: "size == 0 || offset < 0 || size_t(offset) >= sizeof(QArrayData)" in file D:/qt/5.8/mingw53_32/include/QtCore/qarraydata.h, line 60

Or

"terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc"

(I've to say that bad_alloc has, however, become more rare – I might've caught that one when checking all member variables and their initialization again)

Concerning the first error, I thought that maybe this error is thrown due to some kind of bad usage of QString objects, but honestly, I am not sure if that is even how it works.
Right now the application does not even start, it just throws the first error and when I try to use the debugger, all I get to see is this:

(but I dont really know what I should actually do with this information and if anyone can explain to me, how this is helpful and what it is supposed to tell me except "segmentation fault" in connection to the Disassembler I'd be more than thankful.)

What I tried to do mostly includes using the Debugger in order to find the spots that cause the errors to be thrown. I looked through each member variable and its initialization again to make sure that no undefined-allocations take place and I replaced most of the pointers I used beforehand with QScopePointers in order to be able to check whether one of the pointers is NULL and of course I tried to figure it all out with the help of qt's documentation (which has been very helpful for a lot of things).

I am not really sure if I explained my problem properly, but I don’t have anyone to talk to in my life when it comes to qt and c++. Also, I would really like to avoid such errors in the futures and restructure my overall coding in order to do so. Basically I've the feeling that I cannot access any of the Slice-pointers without causing a crash and I would like to know why.

Thank you for reading through all of this (and maybe even my code, I'll try to only put in the relevant parts of it).

MainWindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QGraphicsEllipseItem>
#include <QGraphicsLineItem>
#include <QGraphicsView>
#include "Circle.h"



namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    void setView(QGraphicsView * v);


private slots:
   //not important right now, only some buttons that dont do anything


private:
    Ui::MainWindow *ui;
    QGraphicsView * view;
    QGraphicsScene * scene;
};

MainWindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "Circle.h"
#include <QtDebug>
#include <QGraphicsItem>
#include <QGraphicsScene>
#include <QApplication>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this); 
    
}

MainWindow::~MainWindow()
{
    delete ui;
}


void MainWindow::setView(QGraphicsView * v){
    this->scene = v->scene();
    this->view = ui->graphicsView;
    view->setScene(scene);
    view->setVisible(true);
    view = v;
    ui->graphicsView->setScene(v->scene());
}

Circle.h: (without getter and setter)

#ifndef Circle_H
#define Circle_H
#include <QGraphicsEllipseItem>
#include <QPair>
#include <QPaintEvent>
#include <QScopedPointer>
#include <QString>

class Slice;

class Circle : public QGraphicsEllipseItem
{
public:
    Circle (const QRectF &rect, QString name, Circle * parent = nullptr);

    void addLine(QGraphicsLineItem *line, bool isPoint1);

    QVariant itemChange(GraphicsItemChange change, const QVariant &value);

    void moveLineToCenter(QPointF newPos);

    void hoverEnterEvent(QGraphicsSceneHoverEvent *);
    void hoverLeaveEvent (QGraphicsSceneHoverEvent *);

    void mouseMoveEvent(QGraphicsSceneMouseEvent * event);

    void checkPointers();

    QList <QScopedPointer <Slice> > getSlices();

    void updateSlice(QString category, qreal data);

    int getNumberOfSlices() const;


private:
    QGraphicsLineItem *line;
    QGraphicsTextItem * text; //a label that specifies the object when the cursor hovers over it 

//for each link a bool variable (isP1) is stored within the list of links
    QPair <bool, QGraphicsLineItem *> currPair;
    QList <QPair<bool, QGraphicsLineItem *>> links;

    QString name;
    qreal population;
    int numberOfSlices;

    bool isP1;
    bool hover;

    QScopedPointer <Slice> s_slice;
    QScopedPointer <Slice> i_slice;
    QScopedPointer <Slice> r_slice;
};

#endif // Circle_H

Circle.cpp: (without getter/setter)

#include "Circle.h"
#include <QBrush>
#include <QPainter>
#include <QDebug>
#include <QGraphicsScene>
#include <QRgb>

#include <QPainter>
#include <QPaintDevice>
#include <Slice.h>
#include <QScopedPointer>
#include <Slice.h>

#include <QApplication>

Circle::Circle(const QRectF &rect, QString name, Circle * parent) : QGraphicsEllipseItem (rect),
    name (name),
    population (100),
    s_slice (new Slice (this->rect(), int(this->population), int(this->population), QString("Susceptible"), this)),
    i_slice (new Slice(this->rect(), int(this->population), 0, QString("Infected"), this)),
    r_slice (new Slice (this->rect(), int(this->population), 0, QString("Removed"), this))
{
    
    this->setBrush(QBrush(Qt::darkBlue, Qt::SolidPattern));

    setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemSendsScenePositionChanges
             | QGraphicsItem::ItemIsSelectable | QGraphicsItem::ItemSendsGeometryChanges
             | QGraphicsItem::ItemClipsChildrenToShape);

    this->setAcceptHoverEvents(true);
    hover = false; 
    isP1 = false;

    this->line = new QGraphicsLineItem();

    this->text = new QGraphicsTextItem();

    this->numberOfSlices = 0;
	
	/* each city is basically just a circle with a thick outline and no filling color
*/
        this->setZValue(1);


        QBrush testBrush (Qt::black);

        //different colors for different cities (not yet implemented)
        QPen testPen (Qt::black);
        testPen.setWidth(10);
        this->setPen(testPen);
        this->setBrush(Qt::NoBrush);

        //at first only s_slice visible == 100% population
        s_slice->setPos(this->pos());
        s_slice->setZValue(0);
        s_slice->setParentItem(this);
        this->numberOfSlices ++;

        //I-Slice
        i_slice->setPos(this->pos());
        i_slice->setZValue(0);
        i_slice->setParentItem(this);
        this->numberOfSlices ++;

        //R-Slice
        r_slice->setPos(this->pos());
        r_slice->setZValue(0);
        r_slice->setParentItem(this);
        this->numberOfSlices ++;

        this->slices.append(s_slice);
        this->slices.append(i_slice);
        this->slices.append(r_slice);
}



void Circle::addLine(QGraphicsLineItem *newLine, bool isPoint1){
    currPair.first = isPoint1;
    currPair.second = newLine;

    if (this->links.isEmpty()){
        this->line = newLine;
        isP1 = isPoint1;
        //hochgenommen, damit nicht doppelt eingefügt wird.
        this->links.append(currPair);

    }else{
        //davor links.append(this->line); links.append(newLine);
        links.append(currPair);
    }
}


QVariant Circle::itemChange(GraphicsItemChange change, const QVariant &value){

    if (change == ItemPositionChange && scene()){

        //value is the new pos
        QPointF newPos = value.toPointF();
        prepareGeometryChange();
        moveLineToCenter(newPos);
    }
    return QGraphicsItem::itemChange(change,value);
}


void Circle::moveLineToCenter(QPointF newPos){
    //Converts the ellipse pos (top-left)
    //to its center-pos

    int xOffset = int(rect().x() + rect().width()/2);
    int yOffset = int(rect().y() + rect().height()/2);

    QPointF newCenterPoint = QPointF(newPos.x()+ xOffset, newPos.y() + yOffset);


    //Move Line to center
    for (int i = 0; i < links.size(); i++){
        currPair = links.at(i);

        QPointF p1 = currPair.first ? newCenterPoint : currPair.second->line().p1();
        QPointF p2 = currPair.first ? currPair.second->line().p2() : newCenterPoint;

        links.at(i).second->setLine(QLineF(p1,p2));
    }

    QPointF textPoint = newCenterPoint;
    textPoint.setX(this->x() + 100);
    text->setPos(textPoint);
}

void Circle::mouseMoveEvent(QGraphicsSceneMouseEvent * event){
    QGraphicsItem::mouseMoveEvent(event);

 //CHECK BOUNDS still missing

}

//probably causes crash
void Circle::hoverEnterEvent(QGraphicsSceneHoverEvent *){

    text->setPlainText(this->getName());

    int xOffset = int(rect().x() + rect().width()/2);
    int yOffset = int(rect().y() + rect().height()/2);

    QPointF newCenterPoint = QPointF(this->pos().x()+ xOffset, this->pos().y() + yOffset);
    QPointF textPoint = newCenterPoint;

    textPoint.setX(this->x() + 100);

    text->setPos(textPoint);
    text->setVisible(true);
    hover = true;
}

//probably causes crash 
void Circle::hoverLeaveEvent(QGraphicsSceneHoverEvent *){
    if (this->isSelected())
        setVisible(true);
    else if (text) text->setVisible(false);
    hover = false;

}

void Circle::addSlice(QScopedPointer <Slice>  s)
{
    this->slices.append(s);
    
    //s->setParentItem(this); should no longer be necessary

    s->setVisible(true);
    
    return;
}

QList <QScopedPointer<Slice>> Circle::getSlices()
{
    QList <QScopedPointer <Slice>> sliceList;
    sliceList.append(this->s_slice);
    sliceList.append(this->i_slice);
    sliceList.append(this->r_slice);

    return sliceList;
}




//causes crash
void Circle::checkPointers()
{

    if (!s_slice.isNull())

        qDebug() << "S-Slice existiert mit den Werten: " << s_slice.data()->getName()
                 << s_slice.data()->getPPopulation();

    if (!i_slice.isNull())
        qDebug() << "i-Slice existiert : " << i_slice.data()->getName() << i_slice.data()->getPPopulation() ;

}

Slice.h (without getter/setter except getName() and getSpanAngle()

#ifndef SLICE_H
#define SLICE_H
#include <QGraphicsEllipseItem>
#include <Circle.h>
#include <QString>


class Slice : public QGraphicsEllipseItem
{
public:
           // parent rect, population part, population category, parent object
    Slice (QRectF p_rect, int totalPop, int pop_value, QString category, Circle* parent);


    //updates data & correlating spanAngle
    void updateData(QString category, qreal angle);

    QVariant itemChange(GraphicsItemChange change, const QVariant &value);

    void hoverEnterEvent(QGraphicsSceneHoverEvent *);
    void hoverLeaveEvent (QGraphicsSceneHoverEvent *);

    QString getName() const;

    int getSpanAngle(int value);


private:
    QScopedPointer <QGraphicsTextItem> text;
    qreal angle;
    QString category;

    qreal populationPart;
    int totalPopulation;
    int initialPopulationvalue;
};

#endif // SLICE_H

Slice.cpp (see above)

#include "Slice.h"
#include <QDebug>
#include <QBrush>
#include <QScopedPointer>
#include <QString>

Slice::Slice(QRectF p_rect, int totalPop, int pop_value, QString category, Circle * parent) :
    text (new QGraphicsTextItem(this)),
    category (category),
    populationPart (qreal(pop_value))
{
    this->setRect(p_rect);
    this->populationPart = pop_value;
    this->initialPopulationvalue = pop_value; //in order to be able to access initial value
    this->totalPopulation = totalPop;

    this->setSpanAngle(getSpanAngle(pop_value));

    this->category = category;

    this->setParentItem(parent);
    this->setPos(parent->pos());


    this->angle = this->spanAngle();
    this->setAcceptHoverEvents(true);

    text.data()->setPlainText(category);
    text.data()->setVisible(false);

    // "SUSCEPTIBLE"
    if (category == "Susceptible")
        this->setBrush(Qt::green);

    // "INFECTED"
    if (category == "Infected")
        this->setBrush(QBrush(Qt::darkRed));

    // "RECOVERED/REMOVED"
    if (category == "Removed")
        this->setBrush(Qt::darkGray);
    qDebug() << "Ein Objekt der Klasse Slice wurde erstellt."
             << "Daten: "
             << this->getName()
             << this->getPPopulation();

}

QString Slice::getName() const
{
    return this->category;
}

int Slice::getSpanAngle(int value)
{
    int population = totalPopulation;
    if (value < 0)
        return 0;

    //ueberdenken!
    if (value > population)
        return 5760;

    return int((5760.0/100.0) * (value/(population/100.0)));
}

void Slice::updateData(QString category, qreal data){

    if (this->category != category)
        return;
    else {
        this->populationPart = int(data);
        this->angle = getSpanAngle(int(data));
        this->setSpanAngle(int(angle));

    }
    //not yet done 

}


QVariant Slice::itemChange(GraphicsItemChange change, const QVariant &value){
    if (change == QGraphicsItem::ItemSelectedChange){

        prepareGeometryChange();
        return QGraphicsItem::itemChange(change,value);
    }

    prepareGeometryChange();
    return QGraphicsItem::itemChange(change,value);
}


void Slice::hoverEnterEvent(QGraphicsSceneHoverEvent *)
{
    if (text.isNull())
        return;

	//getPercentage() only returns correlating percentage in relation to total population
    QString percentageAsString = QString::number(int(this->getPercentage()));

    text.data()->setPlainText(this->category + " " + percentageAsString + "% ");

    int xOffset = int(rect().x() + rect().width()/2);
    int yOffset = int(rect().y() + rect().height()/2);

    QPointF newCenterPoint = QPointF(this->pos().x()+ xOffset, this->pos().y() + yOffset);
    QPointF textPoint = newCenterPoint;

    textPoint.setX(this->x() + 100);

    text.data()->setPos(textPoint);

    text.data()->setVisible(true);
}

void Slice::hoverLeaveEvent(QGraphicsSceneHoverEvent *)
{
    if (text.isNull())
        return;

    text.data()->setVisible(false);
}

Main.cpp

#include "mainwindow.h"
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsItem>
#include <QGraphicsEllipseItem>
#include <QGraphicsRectItem>
#include <QDebug>
#include <QLine>
#include <QList>
#include <Circle.h>
#include <Slice.h>

QT_CHARTS_USE_NAMESPACE


int main(int argc, char *argv[])
{
    QApplication a (argc, argv);

    MainWindow w;

    QGraphicsView view;

    QGraphicsScene * Q = new QGraphicsScene(0,0,600,600, &view);
    view.setRenderHint(QPainter::Antialiasing);

    QString name = "Gruen";

    Circle * ellipse1 = new Circle(QRectF(0, 0,100,100), name, nullptr);

    ellipse1->checkPointers();

    ellipse1->movePos(QPointF(200, 200));

    Q->addItem(ellipse1);

    name = "Rot";

    Circle *ellipse2 = new Circle(QRectF(0, 0,100, 100), name, nullptr);

    ellipse2->moveBy(300,300);
    Q->addItem(ellipse2);

    QGraphicsLineItem * line = Q->addLine(QLineF(ellipse1->boundingRect().center().toPoint(),
                                                 ellipse2->boundingRect().center().toPoint()),
                                          QPen(Qt::blue, 3));
    line->setZValue(-1);

    ellipse1->setBrush(QBrush(Qt::green));
    ellipse2->setBrush(QBrush(Qt::red));

    //hinzufuegen der Verbindungen
    ellipse1->addLine(line, true);
    ellipse2->addLine(line, false);

    ellipse1->moveLineToCenter(ellipse1->pos());
    ellipse2->moveLineToCenter(ellipse2->pos());

    Q->addItem(ellipse1->textItem());
    Q->addItem(ellipse2->textItem());

    name = "Blau";
    Circle * ellipse3 = new Circle(QRectF(0, 0, 100, 100), name, nullptr);

    ellipse3->moveBy(500,200);

    line = Q->addLine(QLineF(ellipse1->boundingRect().center().toPoint(),
                                                 ellipse3->boundingRect().center().toPoint()),
                                          QPen(Qt::blue, 3));

    //moves line behind slices and circles 
    line->setZValue(-1);

    ellipse1->addLine(line, true);
    ellipse3->addLine(line, false);

    ellipse1->moveLineToCenter(ellipse1->pos());
    ellipse3->moveLineToCenter(ellipse3->pos());

    line = Q->addLine(QLineF(ellipse2->boundingRect().center().toPoint(),
                                                 ellipse3->boundingRect().center().toPoint()),
                                          QPen(Qt::blue, 3));

    line->setZValue(-1);
    ellipse2->addLine(line, true);
    ellipse3->addLine(line, false);

    ellipse2->moveLineToCenter(ellipse2->pos());
    ellipse3->moveLineToCenter(ellipse3->pos());


    Q->addItem(ellipse3);
    Q->addItem(ellipse3->textItem());

	//only appends a list in MainWindow filled with Circle objects 
    w.addKreis(ellipse1);
    w.addKreis(ellipse2);
    w.addKreis(ellipse3);

    view.setWindowTitle("My network");
    view.setScene(Q);
    w.setView(&view);

    w.show();

    return a.exec();

}

This is the current version and I did not know what is needed in terms of evaluation and actually providing you with enough information about my problem. I guess I'm passing a pointer somewhere (maybe several times) and I am doing that in a bad way. I've the feeling that I'm doing something fundamentally wrong but I dont get what it is and why it happens. Thank you again for reading through all of this. Also I hope that this is the right place in the forum to post it. If it is not, I would kindly ask it to be moved elsewhere and I apologize for the inconvenience.

[mrjj]

Hi and welcome to the forums
When does the crash happen?
When you call some function or when you close app ?

[Lanparty]

Hi there! Thank you so much.

The crash currently happens when I start the application (i think due to ellipse1->checkPointers();) or when I hover over one of the circles with my cursor.

[mrjj]

@Lanparty
Hi
So if you put break point at
void Slice::hoverEnterEvent(QGraphicsSceneHoverEvent *)
and single step over the code.
Can you then pinpoint which line it does crash in?

[Lanparty]

@mrjj Hey, if i put the breakpoint where you implied it does not even get to the breakpoint and I only get the segfault (see above in screenshot) again.

Even after commenting // ellipse1->checkPointers(); out it still behaves the same way while debugging.

However when starting the application it starts now without crashing immediately (after taking out / ellipse1->checkPointers();), it only crashes once I hover over the objects.

But I'll try to find the position where it crashes and I'll update this message or post another reply once I know.

[mrjj]

@Lanparty
Ok so its before hoverEnterEvent. thats odd.
Also, there really is nothing of your code in call stack ?

[SGaist]

Hi,

The stack trace you posted seems to be from a release build. Can you share the stack trace from your debug build ?

[Lanparty]

@mrjj @SGaist

Hi! Thanks again for your replies and your interest in my current problem.
I'm not quite sure if I did this the correct way, but I think I figured out how to get what you asked and I hope that this information is what you've been looking for.

This should be the call stack from my debug build (at least I hope it is :'D):

1  ntdll!RtlGetCurrentServiceSessionId                                                   0x77ae4cd9 
2  ??                                                                                    0xb80afd3f 
3  ??                                                                                    0x1ef069b0 
4  ntdll!RtlFreeHeap                                                                     0x77ae3812 
5  ntdll!RtlpNtSetValueKey                                                               0x77b82ea1 
6  ntdll!RtlGetCurrentServiceSessionId                                                   0x77ae3c45 
7  ??                                                                                    0x1ef069b8 
8  ntdll!RtlFreeHeap                                                                     0x77ae3812 
9  msvcrt!free                                                                           0x76a070b5 
10 ??                                                                                    0x1f0000   
11 QWindowsFontDatabase::releaseHandle                     qwindowsfontdatabase.cpp 1601 0x62a928b6 
12 QtFontStyle::~QtFontStyle                               qfontdatabase.cpp        234  0x1372329  
13 QtFontFoundry::~QtFontFoundry                           qfontdatabase.cpp        303  0x138530c  
14 QtFontFamily::~QtFontFamily                             qfontdatabase.cpp        365  0x1375138  
15 QFontDatabasePrivate::free                              qfontdatabase.cpp        482  0x13a8915  
16 QFontDatabasePrivate::~QFontDatabasePrivate             qfontdatabase.cpp        470  0x13a89af  
17 (anonymous namespace)::Q_QGS_privateDb::Holder::~Holder qfontdatabase.cpp        720  0x1121bed  
18 __tcf_0                                                 qfontdatabase.cpp        720  0x1121c0a  
19 _CRT_INIT *12                                                                         0x10511b4  
20 __DllMainCRTStartup                                                                   0x1051325  
21 DllMainCRTStartup *12                                                                 0x1051433  
22 ntdll!RtlCheckTokenCapability                                                         0x77b0a896 
23 ntdll!RtlHashUnicodeString                                                            0x77ae01a6 
24 ntdll!LdrShutdownProcess                                                              0x77ad458d 
25 ??                                                                                               

I hope this helps. Enjoy your weekend and thanks again!

[mrjj]

@Lanparty
Hi
Super.
Is that from when it crashes ?

[Lanparty]

@mrjj yeah, well I ran the debugger and this is what happened afterwards (including the application crashing)

[Lanparty]

@Lanparty said in Application crashes while accessing Pointers and while Hovering over custom QGraphicsItems:

void Slice::hoverEnterEvent(QGraphicsSceneHoverEvent *)
{
    if (text.isNull())
        return;

    QString percentageAsString = QString::number(int(this->getPercentage()));

    text.data()->setPlainText(this->category + " " + percentageAsString + "% ");

    int xOffset = int(rect().x() + rect().width()/2);
    int yOffset = int(rect().y() + rect().height()/2);

    QPointF newCenterPoint = QPointF(this->pos().x()+ xOffset, this->pos().y() + yOffset);
    QPointF textPoint = newCenterPoint;

    textPoint.setX(this->x() + 100);

    text.data()->setPos(textPoint);

    text.data()->setVisible(true);
}

so, I actually managed to find out where my application crashed. The problem was this line:

 text.data()->setPlainText(this->category + " " + percentageAsString + "% ");

once I checked the expected parameters it kind of made sense. I'll try to find another way to achieve the string I originally wanted to be displayed.

[SGaist]

Do you mean you were missing the text.isNull() check ?

[Lanparty]

@SGaist No, that's the first thing I did inside the method. However, I simply assumed that I could use the method setPlainText() as I pleased, but it was expecting other arguments and crashed instead (I guess because the QString text would've been longer than the original one.

btw: how do I change the status of my post to "solved"?

[mrjj]

@Lanparty said in Application crashes while accessing Pointers and while Hovering over custom QGraphicsItems:

btw: how do I change the status of my post to "solved"?

First post, Topic Tools button to the side.