How to access a variable from another class in Qt?

I am trying to implement in Qt a main window which has 2 widgets: one area where I draw some points and one list box where I write all the points with their respective coordinates. And I would like to implement the function "delete point" of a button on the main window, i.e. when I press the button then the point selected from the list box should disappear from my area where I am drawing. So I was thinking of doing this with signals/slots, but when I try to gain access to my list of points from my drawing area it just doesn't find any containing data. This is my code until now:

paintwidget.cpp (my main window):

PaintWidget::PaintWidget(QWidget parent) :
        QWidget(parent),
        ui(new Ui::PaintWidget)
{
    area = new RenderArea(this);
    ui->setupUi(this);
    connect(ui->displayWidget, SIGNAL(listUpdated(QList)), ui->pointsListWidget,
            SLOT(onListUpdated(QList*)));
    connect(ui->deletePoints, SIGNAL(clicked()), this, SLOT(deleteItem()));
}
void PaintWidget::deleteItem()
{
    area->deletePoint(ui->pointsListWidget->currentItem());
}

renderarea.cpp (my drawing area):

void RenderArea::mousePressEvent(QMouseEvent *e)
{
    point = e->pos();
    updateList(point);
    this->update();
}
void RenderArea::updateList(const QPoint& p)
{
    list.append(p);
    if (list.count()>1)
        lineAdded(p);
    emit listUpdated(&list);
}
void RenderArea::paintEvent(QPaintEvent * /* event */)
{
    QPainter painter(this);
    painter.setPen(QPen(Qt::black,2));
    for (int i = 0; i < list.size(); ++i)
       开发者_StackOverflow中文版 painter.drawPoint(list[i]);
    if (list.size()>1)
        for(int j = 0; j < list.size()-1; ++j)
            painter.drawLine(list[j], list[j+1]);
}
void RenderArea::deletePoint(QListWidgetItem *item)
{
    bool ok1;
    bool ok2;
    int index = item->text().indexOf(",");
    int x = item->text().left(index).toInt(&ok1, 10);
    int y = item->text().mid(index + 1).toInt(&ok2, 10);
    for (int i = 0; i < list.size(); ++i)
        //find the point with x and y as coordinates and delete it
}

listbox.cpp:

void ListBox::onListUpdated(QList *list)
{
    clear();
    for (int i = 0; i < list->size(); ++i)
        addItem(new QListWidgetItem(QString::number(list->at(i).x()) + ", " +
                                    QString::number(list->at(i).y())));
}

The list from the render area is a QList of QPoints. The problem is that in the FOR-loop the size of the list is 0 so I cannot see any of the points that it should contain. I think that I am failing to initialize it somewhere but I am not sure where.

The points are drawn with QPainter so when I delete the point from the list is there any possibility to delete them from my drawing area also?

I'm suspecting you've got two RenderArea widgets hanging around for some reason.

You're connecting ui->displayWidget's signal, but acting on the area widget for the delete.

Shouldn't you be calling ui->displayWidget->deletePoint or connecting area's signal?

As for the repaint, you should call the widget's update() method to have it repaint itself.