A bug ghost!

[Duino]

I just meet a bug ghost in my code.That means if I try to find what happen inside,the code runs well.But if I don't, bugs happen.
The code is trying to draw a histogram of an image using OpenCV functions and show it in qml.Here is the code.

C++ code

"histogram.h"
#include <QtQuick/QQuickPaintedItem>
#include <QImage>
#include <QPainter>

#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>

using namespace cv;

class Histogram : public QQuickPaintedItem{

Q_OBJECT
Q_PROPERTY(QList<int> hist READ hist WRITE setHist)

public:

Histogram(QQuickItem *parent = 0);
~Histogram();
void paint(QPainter *painter);
QList<int> hist()const { return m_hist; }
void setHist(const QList<int> arg);

private:

QList<int> m_hist;
QImage histPainter;

};

"histogram.cpp"

#include "histogram.h"
Histogram::Histogram(QQuickItem *parent):
QQuickPaintedItem(parent)
{
}
Histogram::~Histogram()
{
}
void Histogram::paint(QPainter *painter)
{

if(histPainter.isNull()){
    qDebug() << "histPainter is NULL!";
    return;
}
painter->drawImage(QPoint(0,0), histPainter);

}
void Histogram::setHist(const QList<int> arg)
{

if(arg.length() <=1)
{
    qDebug() << "no Hist from QML!";
    return;
}
m_hist = arg;
Mat histImage = Mat::ones(200, 320, CV_8UC3)*255;
histImage = Scalar::all(200);
int histSize = 320;
int binW = cvRound((double)histImage.cols/histSize);
for( int i = 0; i < histSize; i++ )
    rectangle( histImage, Point(i*binW, histImage.rows),
               Point((i+1)*binW, histImage.rows - m_hist.at(i)),
               Scalar::all(0), -1, 8, 0 );
histPainter  = QImage((unsigned char*)histImage.data,
        histImage.cols, histImage.rows,
        histImage.step,
        QImage::Format_RGB888);

//try to find what happened......
//histPainter.save("dumin.jpg");
//histPainter.load("dumin.jpg");

update();

}

QML code

    Histogram{
        id: histogram
        width: 320; height: 200
    }

If I use" histPainter.save("dumin.jpg"); histPainter.load("dumin.jpg");",it can show the histogram well.But if I comment the code out, it just shows something strange and not stable,just like a ghost.

How should I catch this bug ghost???
Thank you for reading my question! :)

[JKSH]

@Duino said:

I just meet a bug ghost in my code.That means if I try to find what happen inside,the code runs well.But if I don't, bugs happen.

It's not a ghost. It's a Heisenbug :)

How should I catch this bug ghost???

Simplify your program, do unit testing. and use a tool like Valgrind.