CODEWITHSUNDEEP
c++qtdelegates
tom
tom

Reputation: 1344

Qt Custom Delegate Crash with no error code

I try to follow an example in the book "Foundations of Qt Development" to create a custom delegate.

The goal is to create a table with two columns. The first is just the row number. The second column is some arbitrary number but show in terms of a bar style. How the program just crashed after I run it.

Here is my code:

The MainWindow Class

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

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

    Table = new QTableView(this);
    Model = new QStandardItemModel(this);
    dataInit(Model);
    Table->setModel(Model);


    setCentralWidget(Table);

    // If I comment out these two lines
    // the program works well
    // A table view with number shows
    BarDelegate delegate;
    Table->setItemDelegateForColumn(1, &delegate);
}

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



void MainWindow::dataInit(QStandardItemModel* Model)
{
    // Fill the Model with data
    for(int r = 1; r < 11; ++r)
    {
        QStandardItem* item = new QStandardItem(QString("0%1").arg(r));
        item->setEditable(false);
        Model->setItem(r - 1, 0, item);

        Model->setItem(r - 1, 1, new QStandardItem(QString(QString::number(r*17%100))));
    }
}

The custom delegate class

#include "bardelegate.h"

BarDelegate::BarDelegate(QObject *parent)
{

}


QSize BarDelegate::sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    return QSize(30, 15);
}

void BarDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    if (option.state & QStyle::State_Selected)
        painter->fillRect(option.rect, option.palette.highlight());

    int value = index.model()->data(index, Qt::DisplayRole).toInt();
    double factor = (double) value/100.0;

    painter->save();

    if(factor > 1)
    {
        painter->setBrush(Qt::red);
        factor = 1;
    }
    else
        painter->setBrush(QColor(0, (int)(factor*255), 255-(int)(factor*255)));

    painter->setPen(Qt::black);
    painter->drawRect(option.rect.x()+2, option.rect.y()+2, (int)(factor*(option.rect.width()-5)), (int)(factor*(option.rect.height()-5)));
    painter->restore();
}

What is the problem?

Upvotes: 1

Views: 541

Answers (1)

vahancho
vahancho

Reputation: 21220

The problem might be in these two lines:

BarDelegate delegate;
Table->setItemDelegateForColumn(1, &delegate);

You allocate delegate in the stack and pass its address to the setItemDelegateForColumn function. However delegate is deleted as soon as the execution leaves the scope of MainWindow contructor. Thus your table view gets an invalidated delegate. To fix this you need to use a pointer to your delegate. I.e. declare BarDelegate delegate; as MainWindow class member and:

delegate = new BarDelegate(this);
Table->setItemDelegateForColumn(1, delegate);

Upvotes: 4

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