Qt program crash when I delete qwtplot array.

[LeonSu]

OS: Ubuntu 18.02
GCC version 7.4.0
QMake Version 3.1
Qt Version 5.9.5
QWT Version 6.1.4

Hello Everyone

I created a qwtplot array, and set a point on origin at them.

When I delete the qwtplot array before close windows, the Qt is crashed.

The Code is show in the following

mainwindows.h

#include <QMainWindow>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_grid.h>
#include <qwt_plot_canvas.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_grid.h>
#include <qwt_symbol.h>
#include <QPolygonF>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
private:
    Ui::MainWindow *ui;
    QwtPlot *qwtpt;
    QPolygonF *polygon;
    QwtPlotCurve *curve;
    QwtPlotGrid *grid;
    QwtSymbol *symbol;
};

mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    int numpoints = 10;
    qwtpt = new QwtPlot[numpoints];
    polygon = new QPolygonF[numpoints];
    curve = new QwtPlotCurve[numpoints];
    grid = new QwtPlotGrid[numpoints];
    symbol = new QwtSymbol[numpoints];
    QString style = "color: white; ";
    QPolygonF initialpoint;
    initialpoint << QPointF(0.0f, 0.0f);
    int k;
    for(k = 0; k < numpoints; ++k){
        //plot
        qwtpt[k].setParent(ui->scrollArea);
        qwtpt[k].setGeometry(10, 20 + 181 * k, 760, 161);
        qwtpt[k].setAxisTitle(QwtPlot::xBottom, "points");
        qwtpt[k].setAxisTitle(QwtPlot::yLeft, "efficience %");
        qwtpt[k].setAxisScale(QwtPlot::xBottom, 0.0, 132);
        qwtpt[k].setAxisScale(QwtPlot::yLeft, 0.0, 100);
        qwtpt[k].setStyleSheet(style);
        //symbol
        symbol[k].setPen(QPen(Qt::red, 2));
        symbol[k].setStyle(QwtSymbol::Ellipse);
        symbol[k].setBrush(QBrush(Qt::yellow));
        symbol[k].setSize(QSize(8,8));
        //curve
        curve[k].setPen(Qt::blue, 4);
        curve[k].setRenderHint(QwtPlotItem::RenderAntialiased, true);
        curve[k].setSymbol(&symbol[k]);
        curve[k].attach(&qwtpt[k]);
        curve[k].setSamples(initialpoint);
        //grid
        grid[k].attach(&qwtpt[k]);
    }
}
MainWindow::~MainWindow()
{
    delete [] qwtpt;	//  crash on this line
    delete [] polygon;
    delete [] curve;
    delete [] grid;
    delete [] symbol;
    delete ui;
}

if I do not to delete the qwtpt array, the Qt debug show the crash on function ~MainWindow()

if I exchange between qwtpt and curve, the crash is happened on “delete [] curve”

     delete [] curve;	//  crash on this line
    delete [] polygon;
    delete [] qwtpt;
    delete [] grid;
    delete [] symbol;
    delete ui;

Is anyone have idea, or some suggestion for this issues?

Thanks in advance.

[SGaist]

Hi,

You are deleting objects that are properly parented and/or owned. Therefore you are triggering a double delete hence your crash.

[LeonSu]

I change method to create the qwtplot object

from

QwtPlot *qwtpt = new QwtPlot[numpoints];

to

QVector<QwtPlot *> qwtplot;
qwtplot.resize(numpoints );
for(int k = 0; k < numpoints ; ++k)
    qwtplot = new QwtPlot (this);

So I don't need to delete qwtplot array;
The Program is worked fine;

But
The QwtPlotCurve object is crashed when I set setSymbol before delete it;
Maybe this is a bug.

[Christian Ehrlicher]

@LeonSu said in Qt program crash when I delete qwtplot array.:

So I don't need to delete qwtplot array;

Using a static c array or a QVector does not change anything if the object must be deleted manually or not.

The QwtPlotCurve object is crashed when I set setSymbol before delete it;

A bug on your side I would guess - please show us your current code.

[LeonSu]

Thank you for your reply. Christian Ehrlicher

The test code as following

mainwindow.h

#include <QMainWindow>

#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_grid.h>
#include <qwt_plot_canvas.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_grid.h>
#include <qwt_symbol.h>
#include <QPolygonF>
#include <QVector>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

    const int numpoints = 10;

    QVector<QwtPlot *> qwtpt;
    QPolygonF *polygon;
    QwtPlotCurve *curve;
    QwtPlotGrid *grid;
    QwtSymbol *symbol;
};

mainwindow.cpp

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

    polygon = new QPolygonF[numpoints];
    curve = new QwtPlotCurve[numpoints];
    grid = new QwtPlotGrid[numpoints];
    symbol = new QwtSymbol[numpoints];

    QString style = "color: white; ";

    int k;
    for(k = 0; k < numpoints; ++k)
        polygon[k] << QPointF(0.0f, 0.0f);

    qwtpt.resize(numpoints);

    for(k = 0; k < numpoints; ++k){
        //plot
        qwtpt[k] = new QwtPlot(ui->scrollArea);
        qwtpt[k]->setGeometry(10, 20 + 181 * k, 760, 161);
        qwtpt[k]->setAxisTitle(QwtPlot::xBottom, "points");
        qwtpt[k]->setAxisTitle(QwtPlot::yLeft, "efficience %");
        qwtpt[k]->setAxisScale(QwtPlot::xBottom, 0.0, 132);
        qwtpt[k]->setAxisScale(QwtPlot::yLeft, 0.0, 100);
        qwtpt[k]->setStyleSheet(style);

        //symbol
        symbol[k].setPen(QPen(Qt::red, 2));
        symbol[k].setStyle(QwtSymbol::Ellipse);
        symbol[k].setBrush(QBrush(Qt::yellow));
        symbol[k].setSize(QSize(8,8));

        //curve
        curve[k].setPen(Qt::blue, 4);
        curve[k].setRenderHint(QwtPlotItem::RenderAntialiased, true);
        curve[k].attach(qwtpt[k]);
        curve[k].setSamples(polygon[k]);
        curve[k].setSymbol(&symbol[k]);

        //grid
        grid[k].attach(qwtpt[k]);
    }
}

MainWindow::~MainWindow()
{

    delete [] symbol;
    delete [] polygon;
    delete [] grid;
    delete [] curve; //crash on this line
    delete ui;
}

if I marked "curve[k].setSymbol(&symbol[k]); " in MainWindow, the program will work fine.

//curve[k].setSymbol(&symbol[k]);

[Christian Ehrlicher]

Even if I can't find it directly in the documentation (you may ask the qwt maintainer for this), QwtPlot takes ownership of all QwtPlotItems which are attached via attach(). Therefore you're working on a dangling pointer.