Qwt proper pointer management

[zvoopz]

Dear community, I have started to learn cpp just a few month ago and thanks to other threads of forum, I become more familiar with Qt and Qwt as well. I want to ask you how to manage Qwt`s pointers properly to avoid memory leaks.
Consider a snippet of code where I try manually to delete my QwtPlotCurve* pointer that makes my app crash. This might happen due to attempt of double delete because the statement “detachItems(QwtPlotItem::Rtti_PlotCurve, true)” has already deleted it. I just want to ensure that using it is valid way to avoid memory leaks. Does this statement also delete curve-associated pointers like QwtPlotMarker*, QwtText*, etc if they would exist or I need to watch them myself? Just set pointer to nullptr is enough here to dangling pointer?

void MainWindow::on_pushButton_clicked()
{
    ui->stackedWidget->setCurrentIndex(1);
    QwtPlot *plotWidget = findChild<QwtPlot *>("plotWidget");
    curve = new QwtPlotCurve();		//curve is class member declared in header
    qDebug() << "Curve created, its address:" << curve;
    curve->setPen(Qt::blue);
    int y;
    QVector<double> xData, yData;	//vectors to plot curve data
    for (int i=-100; i<=100; i++)
    {
        y=i*qSin(i);
        xData.append(i);	//fill vectors
        yData.append(y);	//fill vectors
    }
int size = xData.size();
    curve->setSamples(xData.data(), yData.data(), size);
    curve->attach(plotWidget);
    plotWidget->setAxisAutoScale(QwtPlot::xBottom);
    plotWidget->setAxisAutoScale(QwtPlot::yLeft);
    Replot to update the plot with the curve
    plotWidget->replot();
}
void MainWindow::on_pushButton_2_clicked()
{
    ui->stackedWidget->setCurrentIndex(0);
    ui->plotWidget->detachItems(QwtPlotItem::Rtti_PlotCurve, true);
    //delete curve;     ------ this line crashes my app if uncommented
    curve = nullptr;
    if(curve==nullptr)
    { qDebug()<<"curve is nullptr";}
}

void MainWindow::on_pushButton_clicked()
{
    ui->stackedWidget->setCurrentIndex(1);
    QwtPlot *plotWidget = findChild<QwtPlot *>("plotWidget");
    curve = new QwtPlotCurve();		//curve is class member declared in header
    qDebug() << "Curve created, its address:" << curve;
    curve->setPen(Qt::blue);
    int y;
    QVector<double> xData, yData;	//vectors to plot curve data
    for (int i=-100; i<=100; i++)
    {
        y=i*qSin(i);
        xData.append(i);	//fill vectors
        yData.append(y);	//fill vectors
    }
int size = xData.size();
    curve->setSamples(xData.data(), yData.data(), size);
    curve->attach(plotWidget);
    plotWidget->setAxisAutoScale(QwtPlot::xBottom);
    plotWidget->setAxisAutoScale(QwtPlot::yLeft);
    Replot to update the plot with the curve
    plotWidget->replot();
}
void MainWindow::on_pushButton_2_clicked()
{
    ui->stackedWidget->setCurrentIndex(0);
    ui->plotWidget->detachItems(QwtPlotItem::Rtti_PlotCurve, true);
    //delete curve;     ------ this line crashes my app if uncommented
    curve = nullptr;
    if(curve==nullptr)
    { qDebug()<<"curve is nullptr";}
}

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[d_stranz]

If "curve" is derived from QObject, then it is owned by a parent QObject. When you are inside a slot, this ownership is not usually cleaned up until after the slot exits and control returns to the Qt event loop. So when you "delete" a QObject, your code can crash because the parent QObject still thinks it has control of the pointer. Do something like this:

curve->deleteLater();
curve = nullptr;

curve->deleteLater();
curve = nullptr;

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And be sure to check the value of curve elsewhere in your code to make sure you aren't trying to use a null pointer. deleteLater() tells Qt to delete the object after it has returned control to the event loop, not immediately the way "delete" does.