calling QListWidgetItem setText() or setCheckState() on crashes Qt application

[CybeX]

I have an application which needs to populate a QListWidget with entries. The list widget should have 2 data entries:

1. Name
2. Checkbox

Using this as some reference and adapting it to something similar to this to enable user altering the QListWidgetItem::setCheckedState(), I get the following:

MainWindow constructor

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

    // Called AFTER ui is setup
    // Call stack for the method below is: initGui > connectSettings > connectDriveListChanges > updateDrivesList
    updateDrivesListWidget();
    //...
}

later in constructor, this is called:

void MainWindow::updateDrivesListWidget() {
    //...
	// Get name of item
	QString name = QString("Local Disk (%1)").arg(s);
	bool enabled = attachedDriveList.value(s);

	// create item for QListWidget named listDriveInfo on form
	// QListWidgetItem* item = new QListWidgetItem(name, ui->listDriveInfo); <---crashes when setting text
	QListWidgetItem* item = new QListWidgetItem(ui->listDriveInfo);

	//set checkable and state
	item->setText(name);                                                      < -----tried as alternative to crash above
	item->setCheckState(enabled ? Qt::Checked : Qt::Unchecked);
	Qt::ItemFlags localFlags = item->flags();
	localFlags = localFlags | Qt::ItemIsUserCheckable;
	item->setFlags(localFlags);

	// add item to list
	ui->listDriveInfo->addItem(item);
    //...
}

I have run also ran Run qmake, also cleaned and rebuilt the project (incase of some invalid reference to the listWidget) but it keeps crashing at this same spot.

---

Stepping through item->setText(name); callstack, I see the application crashes at this point:

qlistwidget.cpp (ref)

void QListModel::itemChanged(QListWidgetItem *item, const QVector<int> &roles)
{
    const QModelIndex idx = index(item);
    emit dataChanged(idx, idx, roles);          <-------crash when emitting signal
}

Call stack here at the emit dataChange() line

1  QListModel::itemChanged             qlistwidget.cpp 455  0x172b4ee8 
2  QListWidgetItem::setData            qlistwidget.cpp 745  0x172b5c28 
3  QListWidgetItem::setText            qlistwidget.h   179  0x4b2a7e   
4  MainWindow::updateDrivesListWidget  MainWindow.cpp  1074 0x44d953   
5  MainWindow::updateDrivesList        MainWindow.cpp  1390 0x45015b   
6  MainWindow::connectDriveListChanges MainWindow.cpp  933  0x44c83d   
7  MainWindow::connectSettings         MainWindow.cpp  466  0x448161   
8  MainWindow::initGui                 MainWindow.cpp  57   0x444b4c   
9  MainWindow::MainWindow              MainWindow.cpp  17   0x44449f   
10 qMain                               main.cpp        230  0x466faf   
11 WinMain *16                         qtmain_win.cpp  97   0x48df70   
12 main                                                     0x4e0bfd   

Debugger Content at the emit dataChange() line

idx	@0x56af450	QModelIndex
	c	0	int
	i	783195744	quintptr
	m	@0x2e9a2b18	QListModel
	r	0	int
item	@0x2eae9e60	QListWidgetItem
	[vptr]	_vptr.QListWidgetItem	 
	d	@0x2eb36c80	QListWidgetItemPrivate
	dummy	<0 items>	QVector<void*>
	itemFlags	(Qt::ItemIsSelectable | Qt::ItemIsDragEnabled | Qt::ItemIsUserCheckable | Qt::ItemIsEnabled) (0x0035)	Qt::ItemFlags
	rtti	0	int
	view	"listDriveInfo"	QListWidget
roles	<2 items>	QVector<int> &
		0	int
		2	int
this	@0x2e9a2b18	QListModel
	[QAbstractListModel]	@0x2e9a2b18	QAbstractListModel
	[d]	@0x2e9a32f0	QAbstractItemModelPrivate
	[parent]	"listDriveInfo"	QListWidget
	[children]	<0 items>	
	[properties]	<at least 0 items>	
	[methods]	<0 items>	
	[extra]		
	cachedIndexes	<0 items>	QModelIndexList
	items	<1 items>	QList<QListWidgetItem*>
	staticMetaObject	@0x17425334	QMetaObject

Any idea why it may crash at this point?

---

Update (not a solution), an extension only

from the signal emitted emit dataChanged(idx, idx, roles); in the function (mentioned above) QListModel::itemChanged(), it reaches this line (shown below) via this signal-slot connection, but I have absolutely no idea how to trace it further, nor what ever this q refers to.

void QListWidgetPrivate::_q_emitItemChanged(const QModelIndex &index)
{
    Q_Q(QListWidget);
    emit q->itemChanged(listModel()->at(index.row()));
}

q is shown here to connect the signals & slots.

Stack Trace here

1  QListWidgetPrivate::_q_emitItemChanged qlistwidget.cpp            1198 0x171948ac 
2  QListWidget::qt_static_metacall        moc_qlistwidget.cpp        196  0x17198264 
3  QMetaObject::activate                  qobject.cpp                3809 0x6bb282a7 
4  QMetaObject::activate                  qobject.cpp                3660 0x6bb2851b 
5  QAbstractItemModel::dataChanged        moc_qabstractitemmodel.cpp 557  0x6bac9e3f 
6  QListModel::itemChanged                qlistwidget.cpp            455  0x17194eff 
7  QListWidgetItem::setData               qlistwidget.cpp            745  0x17195c28 
8  QListWidgetItem::setText               qlistwidget.h              179  0x4b2a7e   
9  MainWindow::updateDrivesListWidget     MainWindow.cpp             1074 0x44d953   
10 MainWindow::updateDrivesList           MainWindow.cpp             1390 0x45015b   
11 MainWindow::connectDriveListChanges    MainWindow.cpp             933  0x44c83d   
12 MainWindow::connectSettings            MainWindow.cpp             466  0x448161   
13 MainWindow::initGui                    MainWindow.cpp             57   0x444b4c   
14 MainWindow::MainWindow                 MainWindow.cpp             17   0x44449f   
15 qMain                                  main.cpp                   230  0x466faf   
16 WinMain *16                            qtmain_win.cpp             97   0x48df70   
17 main                                                                   0x4e0bfd   

Debugger Output here:

Locals		
	index	@0x56af450	QModelIndex &
		c	0	int
		i	783486936	quintptr
		m	@0x2e9e3280	QListModel
			[QAbstractListModel]	@0x2e9e3280	QAbstractListModel
			[d]	@0x2e9e3a58	QAbstractItemModelPrivate
			[parent]	"listDriveInfo"	QListWidget
			[children]	<0 items>	
			[properties]	<at least 0 items>	
			[methods]	<0 items>	
			[extra]		
			cachedIndexes	<0 items>	QModelIndexList
			items	<1 items>	QList<QListWidgetItem*>
			staticMetaObject	@0x17305334	QMetaObject
		r	0	int
Inspector		
Expressions		
Return Value		
Tooltip		
	q	<not accessible>

[VRonin]

I suspect the problem is to something connecting to that signal.

Try not passing ui->listDriveInfo to the item constructor (this prevents the signal being sent when you change the data)

[CybeX]

@VRonin

Doing a few (rather simple) tests, I found the following (which confirms your suspicion)

Works: (but no check box) - this is expected to work

// create item
          QListWidgetItem* item = new QListWidgetItem(name, ui->listDriveInfo);

//          item->setText(name);
//          item->setFlags(item->flags() | Qt::ItemIsUserCheckable);
//          item->setCheckState(enabled ? Qt::Checked : Qt::Unchecked);
          // add item to list
          ui->listDriveInfo->addItem(item);

this crashes on setFlags() call

          // create item
          QListWidgetItem* item = new QListWidgetItem(name, ui->listDriveInfo);

//          item->setText(name);
          //set checkable and state
          Qt::ItemFlags localFlags = item->flags();
          item->setFlags(localFlags | Qt::ItemIsUserCheckable);
          item->setCheckState(enabled ? Qt::Checked : Qt::Unchecked);

          // add item to list
          ui->listDriveInfo->addItem(item);

but these 2 both work (without setting owning QListWidget)

      // create item
      QListWidgetItem* item = new QListWidgetItem(name);

//          item->setText(name);
      //set checkable and state
      Qt::ItemFlags localFlags = item->flags();
      item->setFlags(localFlags | Qt::ItemIsUserCheckable);
      item->setCheckState(enabled ? Qt::Checked : Qt::Unchecked);

      // add item to list
      ui->listDriveInfo->addItem(item);

and

      // create item
      QListWidgetItem* item = new QListWidgetItem();

//         item->setText(name);
      //set checkable and state
      Qt::ItemFlags localFlags = item->flags();
      item->setFlags(localFlags | Qt::ItemIsUserCheckable);
      item->setCheckState(enabled ? Qt::Checked : Qt::Unchecked);

      // add item to list
      ui->listDriveInfo->addItem(item);

Testing:

I create a new application, did exactly the same (add a string and a checkbox) and that work 100%. Create a form, throw on a
QListWidget and run it with this code.

     QListWidgetItem* item = new QListWidgetItem("New", ui->listWidget);
     item->setFlags(item->flags() | Qt::ItemIsUserCheckable);
     item->setCheckState(Qt::Unchecked);
     QListWidgetItem* item1 = new QListWidgetItem("New 1", ui->listWidget);
     item1->setFlags(item->flags() | Qt::ItemIsUserCheckable);
     item1->setCheckState(Qt::Unchecked);
     QListWidgetItem* item2 = new QListWidgetItem("New 2", ui->listWidget);
     item2->setFlags(item->flags() | Qt::ItemIsUserCheckable);
     item2->setCheckState(Qt::Unchecked);

     ui->listWidget->addItem(item);
     ui->listWidget->addItem(item1);
     ui->listWidget->addItem(item2);

     connect(ui->listWidget, &QListWidget::itemClicked, this, [this](QListWidgetItem * i) {
          // Get drive info from driveList
          int pos = ui->listWidget->row(i);

          // check state & set state
          bool active = i->checkState() == Qt::Checked;
          qDebug() << (active ? "enabled" : "disabled");
     });

So my question is, bug (on whose side) or some setting on my side? Thoughts on what to look for?