App crashing on dropevent in QtabWidget

Narutoblaze

I am trying to drag tab from one QtabWidget to another QtabWidget i have implemented my code based on this python solution " https://forum.qt.io/topic/67542/drag-tabs-between-qtabwidgets/5 " everything is working as expected but my app is crashing when i drop my tab in QtabWidget, my app is crashing exactly inside dropEvent(QDropEvent) addTab() and insertTab() i have no idea what is causing this any help is appreciated.

class CustomTab : public QTabWidget
{
public:
    CustomTab();

protected:
    void mouseMoveEvent(QMouseEvent *e) override;
    void dragEnterEvent(QDragEnterEvent *e) override;
    void dragLeaveEvent(QDragLeaveEvent *e) override;
    void dropEvent(QDropEvent *e) override;
};

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

    QSplitter *splitter = new QSplitter;
    CustomTab *tab1 = new CustomTab;
    CustomTab *tab2 = new CustomTab;

    tab1->addTab(new QWidget(), "TAB ONE 1");
    tab1->addTab(new QWidget(), "TAB ONE 2");

    tab2->addTab(new QWidget(), "TAB TWO 1");
    tab2->addTab(new QWidget(), "TAB TWO 2");

    splitter->addWidget(tab1);
    splitter->addWidget(tab2);

    QGridLayout *gL = new QGridLayout;
    gL->addWidget(splitter);

    ui->centralwidget->setLayout(gL);
}

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

// Custom Tab
CustomTab::CustomTab()
{
    setAcceptDrops(true);
    setMovable(true);
    setMouseTracking(true);
    setTabsClosable(true);

    tabBar()->setMouseTracking(true);
}

void CustomTab::mouseMoveEvent(QMouseEvent *e)
{
    if (e->buttons() != Qt::RightButton)
        return;

    QPoint global_pos = mapToGlobal(e->pos());
    QTabBar *tabbar = tabBar();

    QPoint posInTab = tabbar->mapFromGlobal(global_pos);

    int indexTab = tabbar->tabAt(e->pos());
    QRect tabrect = tabbar->tabRect(indexTab);

    QPixmap pix = QPixmap(tabrect.size());
    tabbar->render(&pix, QPoint(), QRegion(tabrect));
    QMimeData mimedata = QMimeData();

    QDrag drag = QDrag(tabbar);
    drag.setMimeData(&mimedata);
    drag.setPixmap(pix);

    QCursor cursor = QCursor(Qt::OpenHandCursor);
    drag.setHotSpot(e->pos() - posInTab);
    drag.setDragCursor(cursor.pixmap(), Qt::MoveAction);
    drag.exec(Qt::MoveAction);
}

void CustomTab::dragEnterEvent(QDragEnterEvent *e)
{
    e->accept();
    if (e->source()->parent() != this)
        return;
}

void CustomTab::dragLeaveEvent(QDragLeaveEvent *e)
{
    e->accept();
}

void CustomTab::dropEvent(QDropEvent *e)
{
    if (e->source()->parent() == this)
        return;

    e->setDropAction(Qt::MoveAction);
    e->accept();
    int counter = count();

    QTabWidget *parentWidget = qobject_cast<QTabWidget *>(e->source()->parent());

// crashing from here :
    if (counter == 0) {
        addTab(parentWidget->currentWidget(), parentWidget->tabText(parentWidget->currentIndex()));
    } else {
        insertTab(counter + 1,
                  parentWidget->currentWidget(),
                  parentWidget->tabText(parentWidget->currentIndex()));
    }
}

Abderrahmene_Rayene

Hi,

I made an example based on the code you provided and ran it through a debugger, it crashed in qdrag.cpp, when it tried to delete your QDrag data after leaving mouseMoveEvent scope.

I allocated QMimeData on the heap and it worked, I was able to drag and drop tabs.

in mouseMoveEvent:
QMimeData *mimedata = new QMimeData();

See if this is it.

Christian Ehrlicher

@Narutoblaze said in App crashing on dropevent in QtabWidget:

// crashing from here :

Then you should look at the debugger to see what's wrong there. I would guess parentWidget is a nullptr.

mpergand

@Narutoblaze said in App crashing on dropevent in QtabWidget:

QTabWidget *parentWidget = qobject_cast<QTabWidget *>(e->source()->parent());

parentWidget == null ?

Narutoblaze

@Christian-Ehrlicher @mpergand I have debugged this portion of the code and found it is not null.

qDebug() << parentWidget();
qDebug() << parentWidget()->tabText(parentWidget()->currentIndex());

I got the address of parentWidget & the text of the parentWidget "values received provided below"

output :

0xc00009e230
TAB ONE 1

Abderrahmene_Rayene

Hi,

I made an example based on the code you provided and ran it through a debugger, it crashed in qdrag.cpp, when it tried to delete your QDrag data after leaving mouseMoveEvent scope.

I allocated QMimeData on the heap and it worked, I was able to drag and drop tabs.

in mouseMoveEvent:
QMimeData *mimedata = new QMimeData();

See if this is it.

JonB

@Narutoblaze said in App crashing on dropevent in QtabWidget:

drag.setMimeData(&mimedata);

void QDrag::setMimeData(QMimeData *data) states:

Ownership of the data is transferred to the QDrag object.

Whenever you read about a function taking "ownership" of a pointer that means Qt infrastructure will delete the object you passed when the now-parent object later gets destroyed. That can only be performed against an object allocated on the heap (new) not on the stack (local variable whose address you take with &, like your code).

Consequently @Abderrahmene_Rayene's code will address this correctly by creating your QMimeData object on the heap. Do not delete it yourself once you have passed to setMimeData()!

Narutoblaze

@JonB if i don't delete the mimedata created on heap it will cause memory leak right ?

jsulm

@Narutoblaze Please read again what @JonB wrote. Especially what he copied from Qt documentation...