Qt collections crash with SIGSEGV

[asmfreak]

Hello!
I'm writing a class using Qt collections classes (i.e. QVector, QMultiHash, QMultiMap). The first class I tried to use was QMultiHash. After getting crash similar to described below, I tried using QMultiMap, got same crash and now tried QVector class.

Back to subject. The class i'm writing looks like this:

//graph.h
class Graph :public QObject, public QGraphicsItemGroup
{
    Q_OBJECT
    QVector< QVector <Node*> > nodes;
public:
    Graph(QObject* parent=0);
    ~Graph();
signals:
    void maxLevel(int);
public slots:
    Node* addNode(const int level=0);
private slots:
    void removeNode(QObject *arg);
private:
    void calculateGeometry();
};

//graph.h
class Graph :public QObject, public QGraphicsItemGroup
{
    Q_OBJECT
    QVector< QVector <Node*> > nodes;
public:
    Graph(QObject* parent=0);
    ~Graph();
signals:
    void maxLevel(int);
public slots:
    Node* addNode(const int level=0);
private slots:
    void removeNode(QObject *arg);
private:
    void calculateGeometry();
};

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And the crash happens in this function.

//graph.cpp
Node* Graph::addNode(const int level)
{
    Node* ret=NULL;
    if(level<0||level>nodes.count())return ret;                              //crash here
    if(level==nodes.count()) nodes.insert(level,QVector<Node*>());
    Node* node=new Node();
    ret=node;
    node->setText(trUtf8("Level %1").arg(level));
    connect(node,SIGNAL(destroyed(QObject*)),this,SLOT(removeNode(QObject*)));
    addToGroup(node);
    nodes[level].append(node);
    calculateGeometry();
    emit maxLevel(nodes.count());
    return ret;
}

//graph.cpp
Node* Graph::addNode(const int level)
{
    Node* ret=NULL;
    if(level<0||level>nodes.count())return ret;                              //crash here
    if(level==nodes.count()) nodes.insert(level,QVector<Node*>());
    Node* node=new Node();
    ret=node;
    node->setText(trUtf8("Level %1").arg(level));
    connect(node,SIGNAL(destroyed(QObject*)),this,SLOT(removeNode(QObject*)));
    addToGroup(node);
    nodes[level].append(node);
    calculateGeometry();
    emit maxLevel(nodes.count());
    return ret;
}

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A pointer to an object of this class is used inside of a class, derived from QMainWindow.
Being called from it's constructor it works just fine.

MainWindow::MainWindow(QWidget *parent):
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    //........
    Graph* g=new Graph();
    g->addNode();
    //........
}

MainWindow::MainWindow(QWidget *parent):
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    //........
    Graph* g=new Graph();
    g->addNode();
    //........
}

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But when addNode() is called later (not inside constructor, via signal/slot) it crashes on the string I specified. In debug mode it stops on this line inside QVector:

//qvector.h
//........
    inline int count() const { return d->size; }
//........

//qvector.h
//........
    inline int count() const { return d->size; }
//........

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As I said, the same thing happened with QMultiHash and QMultiMap. I'm using Qt 4.8.1 with QtCreator 2.4.1 under Linux Mint 13 (Maya) 64 bit (kernel 3.2.0-23-generic.

Is this an issue of my code or Qt? (I tend to think that this is my code's fault, but I can't find source of the problem. Please help! BTW, The code provided compiles without a single error.

[ChrisW67]

The indicated line cannot crash. The call to Vector::count() is effectively a one-liner returning an int member variable. If this were broken Qt would be fundamentally unable to start: QVector is used widely.

Run a debug build of your program in your debugger. When it crashes copy and paste the stack backtrace and the value of the pointer you are calling add Node() through and post them here. My guess is that you are trying to access addNode() through a null or invalid pointer to Graph.

[asmfreak]

the first call to addNode() fuction from MainWindow's constructor:

0	Graph::addNode	graph.cpp	43	0x4100b8	
1	MainWindow::MainWindow	mainwindow.cpp	28	0x407894	
2	main	main.cpp	7	0x4074de

0	Graph::addNode	graph.cpp	43	0x4100b8	
1	MainWindow::MainWindow	mainwindow.cpp	28	0x407894	
2	main	main.cpp	7	0x4074de

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A line before crash:

level	0	int
	node	29 @0x1d	Node
	ret	0x0	Node *
	this	4271749 @0x412e85	Graph
		QObject	 	QObject
			[vptr]	1142560928759121224	
			d_ptr	 @0x894cfa894c00401f	QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>>	
		QGraphicsItemGroup		QGraphicsItemGroup
			QGraphicsItem		QGraphicsItem
				[vptr]	15858580846535066870	
				d_ptr	 @0x75eb394801c38348	QScopedPointer<QGraphicsItemPrivate, QScopedPointerDeleter<QGraphicsItemPrivate>>
		nodes	<unavailable synchronous data>	QVector<QVector<Node*>>

level	0	int
	node	29 @0x1d	Node
	ret	0x0	Node *
	this	4271749 @0x412e85	Graph
		QObject	 	QObject
			[vptr]	1142560928759121224	
			d_ptr	 @0x894cfa894c00401f	QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>>	
		QGraphicsItemGroup		QGraphicsItemGroup
			QGraphicsItem		QGraphicsItem
				[vptr]	15858580846535066870	
				d_ptr	 @0x75eb394801c38348	QScopedPointer<QGraphicsItemPrivate, QScopedPointerDeleter<QGraphicsItemPrivate>>
		nodes	<unavailable synchronous data>	QVector<QVector<Node*>>

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And crash. QtCreator shows a message with "App is stopped due to system message. SIGSEGV Sigmentation fault."

0	QVector<QVector<Node*> >::count	qvector.h	259	0x41066f	
1	Graph::addNode	graph.cpp	33	0x40fef4	
2	MainWindow::addNode	mainwindow.cpp	197	0x4098db	
3	MainWindow::qt_static_metacall	moc_mainwindow.cpp	124	0x411f6d	
4	QMetaObject::activate	qobject.cpp	3547	0x7ffff6dc9281	
5	QAbstractButton::clicked	moc_qabstractbutton.cpp	220	0x7ffff7967c72	
6	QAbstractButtonPrivate::emitClicked	qabstractbutton.cpp	548	0x7ffff76a5a4e	
7	QAbstractButtonPrivate::click	qabstractbutton.cpp	541	0x7ffff76a6d8b	
8	QAbstractButton::mouseReleaseEvent	qabstractbutton.cpp	1123	0x7ffff76a6ffc	
9	QWidget::event	qwidget.cpp	8362	0x7ffff7328144	
10	notify_helper	qapplication.cpp	4559	0x7ffff72d7894	
11	QApplicationPrivate::notify_helper	qapplication.cpp	4531	0x7ffff72d7894	
12	QApplication::notify	qapplication.cpp	4102	0x7ffff72dd0bf	
13	QCoreApplication::notifyInternal	qcoreapplication.cpp	876	0x7ffff6db4e9c	
14	sendEvent	qcoreapplication.h	231	0x7ffff72d8862	
15	QApplicationPrivate::sendMouseEvent	qapplication.cpp	3170	0x7ffff72d8862	
16	QETWidget::translateMouseEvent	qapplication_x11.cpp	4617	0x7ffff7357bf5	
17	QApplication::x11ProcessEvent	qapplication_x11.cpp	3732	0x7ffff7356bae	
18	x11EventSourceDispatch	qguieventdispatcher_glib.cpp	146	0x7ffff73800d2	
19	g_main_context_dispatch	/lib/x86_64-linux-gnu/libglib-2.0.so.0	0	0x7ffff5750d53	
20	??	/lib/x86_64-linux-gnu/libglib-2.0.so.0	0	0x7ffff57510a0	
...	<Более>

0	QVector<QVector<Node*> >::count	qvector.h	259	0x41066f	
1	Graph::addNode	graph.cpp	33	0x40fef4	
2	MainWindow::addNode	mainwindow.cpp	197	0x4098db	
3	MainWindow::qt_static_metacall	moc_mainwindow.cpp	124	0x411f6d	
4	QMetaObject::activate	qobject.cpp	3547	0x7ffff6dc9281	
5	QAbstractButton::clicked	moc_qabstractbutton.cpp	220	0x7ffff7967c72	
6	QAbstractButtonPrivate::emitClicked	qabstractbutton.cpp	548	0x7ffff76a5a4e	
7	QAbstractButtonPrivate::click	qabstractbutton.cpp	541	0x7ffff76a6d8b	
8	QAbstractButton::mouseReleaseEvent	qabstractbutton.cpp	1123	0x7ffff76a6ffc	
9	QWidget::event	qwidget.cpp	8362	0x7ffff7328144	
10	notify_helper	qapplication.cpp	4559	0x7ffff72d7894	
11	QApplicationPrivate::notify_helper	qapplication.cpp	4531	0x7ffff72d7894	
12	QApplication::notify	qapplication.cpp	4102	0x7ffff72dd0bf	
13	QCoreApplication::notifyInternal	qcoreapplication.cpp	876	0x7ffff6db4e9c	
14	sendEvent	qcoreapplication.h	231	0x7ffff72d8862	
15	QApplicationPrivate::sendMouseEvent	qapplication.cpp	3170	0x7ffff72d8862	
16	QETWidget::translateMouseEvent	qapplication_x11.cpp	4617	0x7ffff7357bf5	
17	QApplication::x11ProcessEvent	qapplication_x11.cpp	3732	0x7ffff7356bae	
18	x11EventSourceDispatch	qguieventdispatcher_glib.cpp	146	0x7ffff73800d2	
19	g_main_context_dispatch	/lib/x86_64-linux-gnu/libglib-2.0.so.0	0	0x7ffff5750d53	
20	??	/lib/x86_64-linux-gnu/libglib-2.0.so.0	0	0x7ffff57510a0	
...	<Более>

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Added this.d to watch window:

this.d		QVectorData
		alloc		int
		capacity	<unavailable synchronous data>	
		ref	<unavailable synchronous data>	QBasicAtomicInt
		reserved	<unavailable synchronous data>	
		sharable	<unavailable synchronous data>	
		size		int

this.d		QVectorData
		alloc		int
		capacity	<unavailable synchronous data>	
		ref	<unavailable synchronous data>	QBasicAtomicInt
		reserved	<unavailable synchronous data>	
		sharable	<unavailable synchronous data>	
		size		int

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One thing I can say for sure - I'm not doing anything with Graph* variable except calling deleteLater() on it in destructor of MainWindow.

UPD: I tried using that QVector in other fuctions. It works cool. The only function that seem to work falsely is addNode().

[amleto]

How many Qt installs do you have? Are you mixing release and debug libraries? Please show compiler & linker output to confirm what you say.

Thanks

[asmfreak]

I guess that I need to send all of the sources.
In this ->demo.tar.gz<- archive files:
1) build.log - log from QtCreator after rebuild
2) 'dpkg-query --list | grep qt4.log' - log from console from dpkg.
everything else - sources.

Thanks in advance.

[amleto]

look at how you set up (or rather do not setup) MainWindow::g

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include <QGraphicsScene>
#include "graph.h"
#include "node.h"
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
    QStateMachine *saveStates;
    QGraphicsScene* scene;
    Graph* g; ///////////
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
 
    void createStates();
    void createDialogs();
signals:
    void accept_file();
    void accept_save();
   // <SNIP>
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QGraphicsScene>
#include "graph.h"
#include "node.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT
    QStateMachine *saveStates;
    QGraphicsScene* scene;
    Graph* g; ///////////
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    
private:
    Ui::MainWindow *ui;

    void createStates();
    void createDialogs();
signals:
    void accept_file();
    void accept_save();
   // <SNIP>
};

#endif // MAINWINDOW_H

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#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    scene=new QGraphicsScene();
    Graph* g=new Graph; ///////////  really?
    g->setPos(0,0);
    g->getGeometry();
    scene->addItem(g);
    Node* node=new Node;
    node->setPos(20,20);
    scene->addItem(node);
    node=new Node;
    node->setPos(80,80);
    scene->addItem(node);
    node=new Node;
    node->setPos(180,180);
    scene->addItem(node);
    ui->graphicsView->setScene(scene);
    connect(ui->horizontalSlider,SIGNAL(valueChanged(int)),this,SLOT(setScale(int)));
    ui->horizontalSlider->setValue(75);
    connect(ui->pushButton,SIGNAL(clicked()),this,SLOT(adNode()));
    connect(g,SIGNAL(maxLevel(int)),this,SLOT(setMaxDial(int)));
    Node* test=g->addNode(0);
    g->addNode(0);
    g->addNode(1);
    g->addNode(2);
    saveStates=new QStateMachine;
    this->createStates();
}
 
void MainWindow::adNode()
{
    this->g->addNode(ui->dial->value()); ////// oops!
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    scene=new QGraphicsScene();
    Graph* g=new Graph; ///////////  really?
    g->setPos(0,0);
    g->getGeometry();
    scene->addItem(g);
    Node* node=new Node;
    node->setPos(20,20);
    scene->addItem(node);
    node=new Node;
    node->setPos(80,80);
    scene->addItem(node);
    node=new Node;
    node->setPos(180,180);
    scene->addItem(node);
    ui->graphicsView->setScene(scene);
    connect(ui->horizontalSlider,SIGNAL(valueChanged(int)),this,SLOT(setScale(int)));
    ui->horizontalSlider->setValue(75);
    connect(ui->pushButton,SIGNAL(clicked()),this,SLOT(adNode()));
    connect(g,SIGNAL(maxLevel(int)),this,SLOT(setMaxDial(int)));
    Node* test=g->addNode(0);
    g->addNode(0);
    g->addNode(1);
    g->addNode(2);
    saveStates=new QStateMachine;
    this->createStates();
}

void MainWindow::adNode()
{
    this->g->addNode(ui->dial->value()); ////// oops!
}

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[asmfreak]

Oh, what an idiot I am!
Thank you for your help!!
I guess this thread can be closed, since the problem is solved.

[ChrisW67]

I missed it in your code too.

I always add member variables, particularly pointers, to the class initialiser list at the same time as I add them to the declaration. If your code had read:

MainWindow(QWidget *p = 0): g(0) { ... }

MainWindow(QWidget *p = 0): g(0) { ... }

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Then the invalid pointer would have been more obvious both as a 0x0 value in the debugger and, depending on compiler/toolchain, a more specific message when it crashed. Uninitialised it contains whatever garbage is in the memory and is far harder to see that it is not valid.

// or this
MainWindow(QWidget *p = 0): g(new Graph(this)) { ... }

// or this
MainWindow(QWidget *p = 0): g(new Graph(this)) { ... }

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This provides a valid initial value and would have removed the need for the initialisation that tripped you up.