QListWidget->clear()

[Charvi]

Hi,

I want to clear my QListWidget (empty the list) before refilling and repopulating it. But when I use QlistWidget->clear() for this purpose the program terminates with a backtrace:

inside updatelist
*** glibc detected *** /home/charvi123/QtSDK/Projects/scanIt-build-desktop/scanIt: free(): invalid pointer: 0x0903f35c ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(+0x6b161)[0x617161]
/lib/tls/i686/cmov/libc.so.6(+0x6c9b8)[0x6189b8]
/lib/tls/i686/cmov/libc.so.6(cfree+0x6d)[0x61ba9d]
.
.
.

I am completely lost. How do I clear my list so that it shows the refreshed contents.

Thanks in advance,
-Charvi.

[yeye_olive]

Could you post the whole code of your program, please? The crash is most likely caused by some bug in another part of the program which merely manifests itself when you call QListWidget::clear().

[Charvi]

This happens every second time I try to free the list. The first time the list is populated nicely. What I am doing is just adding the items to the list and repopulating the list by first clearing it and then loading items from a QListWidgetItem array.

And when I don't clear the list the program just runs fine.

Is there any other method through which I could achieve the emptying of the list widget ?

[yeye_olive]

Like I said, there is not much we can do (besides guessing) until we see some code. Here is a guess; perhaps your program double-deletes the items:
- once when your QListWidgetItem array goes out of scope,
- once when you call clear(). Remember that according to the docs, QListWidget::insertItem(QListWidgetItem *item) takes ownership of item.

[Charvi]

- My QListWidgetItem array does not go out of scope

Okay here the code snippet which may be of interest.

qDebug("inside updatelist");
    quint8 qIndex;
 
    ui->listWidget->setWrapping(false);
//    ui->listWidget->clear();
 
    qDebug()<<"total items = "<<tot_items;
    for (qIndex = 0; qIndex < tot_items; ++qIndex)
    {
        qDebug("inside for loop");
        QString item_val = QString(ItemsInList[qIndex].name).append("\n").append(QString("$").append(ItemsInList[qIndex].price));
        qDebug()<<"list widget string to be populated = "<<item_val;
        item[qIndex].setText(item_val);
        QString icon = QString("/home/charvi123/QtSDK/Projects/scanIt/numbers/") + QString("%1").arg((qIndex + 1), 0, 10) + QString(".png");
        item[qIndex].setIcon(QIcon(icon));
        ui->listWidget->addItem(&item[qIndex]);
     }
    ui->listWidget->setFocus();

qDebug("inside updatelist");
    quint8 qIndex;

    ui->listWidget->setWrapping(false);
//    ui->listWidget->clear();

    qDebug()<<"total items = "<<tot_items;
    for (qIndex = 0; qIndex < tot_items; ++qIndex)
    {
        qDebug("inside for loop");
        QString item_val = QString(ItemsInList[qIndex].name).append("\n").append(QString("$").append(ItemsInList[qIndex].price));
        qDebug()<<"list widget string to be populated = "<<item_val;
        item[qIndex].setText(item_val);
        QString icon = QString("/home/charvi123/QtSDK/Projects/scanIt/numbers/") + QString("%1").arg((qIndex + 1), 0, 10) + QString(".png");
        item[qIndex].setIcon(QIcon(icon));
        ui->listWidget->addItem(&item[qIndex]);
     }
    ui->listWidget->setFocus();

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QListWidgetItem item[MAX_ITEMS];
typedef struct _Item_list_entity
{
    QString name;
    QString price;
 
}ListItemSingle;
ListItemSingle ItemsInList[ITEMS];

QListWidgetItem item[MAX_ITEMS];
typedef struct _Item_list_entity
{
    QString name;
    QString price;

}ListItemSingle;
ListItemSingle ItemsInList[ITEMS];

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[StrikeByte]

Try using this:

The QListWidget destroys your array of QListWidgetItems during the clear (so the first time it's not a problem the second time is). this code generates new items when they are needed

qDebug("inside updatelist");
    quint8 qIndex;
 
    ui->listWidget->setWrapping(false);
    // ui->listWidget->clear();
 
    qDebug()<<"total items = "<<tot_items;
    for (qIndex = 0; qIndex < tot_items; ++qIndex)
    {
    qDebug("inside for loop");
    QString item_val = QString(ItemsInList[qIndex].name).append("\n").append(QString("$").append(ItemsInList[qIndex].price));
    qDebug()<<"list widget string to be populated = "<<item_val;
    QListWidgetItem * item = new QListWidgetItem;
    item.setText(item_val);
    QString icon = QString("/home/charvi123/QtSDK/Projects/scanIt/numbers/") + QString("%1").arg((qIndex + 1), 0, 10) + QString(".png");
    item.setIcon(QIcon(icon));
    ui->listWidget->addItem(item);
    }
    ui->listWidget->setFocus();

qDebug("inside updatelist");
    quint8 qIndex;
     
    ui->listWidget->setWrapping(false);
    // ui->listWidget->clear();
     
    qDebug()<<"total items = "<<tot_items;
    for (qIndex = 0; qIndex < tot_items; ++qIndex)
    {
    qDebug("inside for loop");
    QString item_val = QString(ItemsInList[qIndex].name).append("\n").append(QString("$").append(ItemsInList[qIndex].price));
    qDebug()<<"list widget string to be populated = "<<item_val;
    QListWidgetItem * item = new QListWidgetItem;
    item.setText(item_val);
    QString icon = QString("/home/charvi123/QtSDK/Projects/scanIt/numbers/") + QString("%1").arg((qIndex + 1), 0, 10) + QString(".png");
    item.setIcon(QIcon(icon));
    ui->listWidget->addItem(item);
    }
    ui->listWidget->setFocus();

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Question why do you want to save the data twice? once in the item array and once in the listwidget
For this i would use the QStandardItemModel in combination with the QListView

[yeye_olive]

You do double-delete your items.

On line 16 of the first code snippet, you add item[qIndex] to the list widget. When calling QListWidget::clear() (or when the widget is destroyed), the item at this memory address is deleted. Since it was not allocated on the heap but (presumably) on the stack or as a global variable in line 1 of the second code snippet, the underlying code to free() reports a corruption.

This is easy to fix: allocate the items on the heap (with new) and let the list widget delete them when appropriate.

[Charvi]

Oops ..
Correct. Thanks folks.