Problem with zoomer

Qwt 6.0.1 / Qt 4.4.1 / VC++ 9.0 / Win XP

I have written a graphics module for the SigLib library (Digital Signal Library from Numerix-DSP) and have encountered a problem with the zooming function. I get a memory access violation in my routine handling the return to the zoom base:
Unhandled exception at 0x1007463a (qwtd.dll) in SigLibGraph.exe: 0xC0000005: Access violation reading location 0xcdcdcded.
Following the fragment of code where the access violation occured:
uint QwtPlotZoomer::zoomRectIndex() const
{
return d_data->zoomRectIndex;
}

d_data seems to have been not initialized, where should I do it?

Code:

---

// main.cpp
// AW - 14.03.2012
 
#include <QApplication>				// Qt library
#include <qwt_plot.h>				// Qwt Plot library
#include <qwt_plot_curve.h>
#include <qwt_math.h>
#include <siglib.h>					// SigLib DSP library 
#include "SigLibGraph.h"			// Qt/Qwt SigLib Graphics library
 
int main( int argc, char * argv[] )
{
	QApplication app( argc,argv );
 
	const unsigned int FFT_LENGTH = 512;
	double *pXData = new double[FFT_LENGTH];
	double *pResult = new double[FFT_LENGTH];
	double *pResult2 = new double[FFT_LENGTH];
 
 
	// fill data arrays
	for ( unsigned int idx = 0; idx < FFT_LENGTH; idx++ ) {
		pXData[idx] = double( idx );
		pResult[idx] = sin( M_PI * 0.02 * double( idx ));
		pResult2[idx] = cos( M_PI * 0.02 * double( idx ));
	}
 
	// plot object
	Plot2D *myPlot = new Plot2D( "function plot", "time", "amplitude" );
	myPlot->attachCurve( pXData, pResult, FFT_LENGTH, "sine" );
	myPlot->attachCurve( pXData, pResult2, FFT_LENGTH, "cosine", Qt::cyan );
	myPlot->show();
 
	return app.exec();
}

---

Code:

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/***************************************************************	
	SigLibGraph.h
 AW - 31.03.2012
***************************************************************/
 
#ifndef SIGLIBGRAPH_H
#define SIGLIBGRAPH_H
 
#include <siglib.h>					// SigLib DSP library 
#include <qwt_plot.h>				// Qwt library
#include <qwt_plot_grid.h>
#include <qwt_legend.h>
#include <qwt_plot_zoomer.h>
 
 
class Plot2D: public QwtPlot
{
	Q_OBJECT
 
public:	
	Plot2D( QString plotTitle = "",
				QString xLabel = "x-axis",
				QString yLabel = "y-axis",
				QWidget *parent = NULL ); 
 
	void attachCurve( double *xData,
							double *yData,
							uint dataCount, 
							QString curveLabel = "",
							QColor curveColor = Qt::yellow );
 
	void setXAxisScale( double firstValue, double lastValue );
	void setYAxisScale( double firstValue, double lastValue );
	void setXLogAxis();
	void setYLogAxis();
	void setBackgroundColor( QColor backgroundColor );
 
public Q_SLOTS:
    void autoRescale( QRectF );
 
protected:
    virtual void resizeEvent( QResizeEvent * );
 
private:
	QwtPlotGrid *grid;
	QwtLegend *legend;
	QwtPlotZoomer *zoomer;
};
 
#endif	// SIGLIBGRAPH_H

---

Code:

---

/************************************************************
	SigLibGraph.cpp
	AW - 31.03.2012
************************************************************/
 
#include <QApplication>
#include <QtDebug>
#include <qpainter.h>
#include <qwt_plot.h>
#include <qwt_scale_engine.h>
#include <qwt_symbol.h>
#include <qwt_plot_grid.h>
#include <qwt_plot_curve.h>
#include <qwt_legend.h>
#include <qwt_text.h>
#include <qwt_picker.h>
#include <qwt_picker_machine.h>
#include <qwt_plot_zoomer.h>
#include <qwt_plot_renderer.h>
#include <qwt_event_pattern.h>
#include <qwt_text.h>
#include "SigLibGraph.h"
 
 
Plot2D::Plot2D( QString plotTitle,
					QString xLabel,
					QString yLabel,
					QWidget *parent ): 
					QwtPlot( parent )
{
	// plot object
	setTitle( plotTitle );
	setCanvasBackground( QColor( Qt::darkBlue ));
 
	// axis
	setAxisTitle( QwtPlot::xBottom, xLabel );
	setAxisTitle( QwtPlot::yLeft, yLabel );
 
	// grid 
	QwtPlotGrid* grid = new QwtPlotGrid;
	grid->enableXMin( true );
	grid->setMajPen( QPen( Qt::white, 0, Qt::DotLine ));
	grid->setMinPen( QPen( Qt::gray, 0 , Qt::DotLine ));
	grid->attach( this );
 
	// legend
	QwtLegend* legend = new QwtLegend;
	insertLegend( legend, QwtPlot::RightLegend );
 
	//zoomer
	QwtPlotZoomer* zoomer = new QwtPlotZoomer( QwtPlot::xBottom, 
															QwtPlot::yLeft, 
															this->canvas() );
	zoomer->setRubberBand( QwtPicker::RectRubberBand );
	zoomer->setRubberBandPen( QColor( Qt::green ));
	zoomer->setTrackerMode( QwtPicker::ActiveOnly );
	zoomer->setTrackerPen( QColor( Qt::white ));
	// RightButton: zoom out by one step
	zoomer->setMousePattern( QwtEventPattern::MouseSelect3,
										Qt::RightButton );
	// Ctrl+RightButton: zoom out to full size
	zoomer->setMousePattern( QwtEventPattern::MouseSelect2,
										Qt::RightButton, 
										Qt::ControlModifier );
	// handle zoom base
	connect( zoomer, 
				SIGNAL( zoomed( QRectF )), 
				SLOT( autoRescale( QRectF )));
 
	// resize Widget
	resize( 600, 400 );
 
	show();
}
 
void Plot2D::autoRescale( QRectF )
{
	qDebug("Plot2D::autoRescale called!");
	if( zoomer->zoomRectIndex() == 0 )				// autorescale only if you are at base... 
	{	
		this->setAxisAutoScale( QwtPlot::xBottom );
		this->setAxisAutoScale( QwtPlot::yLeft );
		this->updateAxes();
		zoomer->setZoomBase( false );
	}
}
 
void Plot2D::attachCurve( 	double *xData,
									double *yData,
									uint dataCount, 
									QString curveLabel,
									QColor curveColor )
{
	// define the curve object
	QwtPlotCurve* curve = new QwtPlotCurve;
	curve->setSamples( xData, yData, dataCount );
	curve->setPen( QPen( curveColor ));
	curve->setRenderHint( QwtPlotItem::RenderAntialiased );
	curve->setTitle( curveLabel );
	// qDebug() << "curveLabel.isEmpty = " << curveLabel.isEmpty();
	if ( curveLabel.isEmpty() )
		curve->setItemAttribute( QwtPlotItem::Legend, false );
	else 
		curve->setLegendAttribute( QwtPlotCurve::LegendShowLine );	
	curve->attach( this );
}
 
void Plot2D::setXAxisScale( double firstValue, double lastValue )
{
	setAxisScale( xBottom, firstValue, lastValue );
}
 
void Plot2D::setYAxisScale( double firstValue, double lastValue )
{
	setAxisScale( yLeft, firstValue, lastValue );
}
 
void Plot2D::setXLogAxis()
{
	setAxisScaleEngine( QwtPlot::xBottom, new QwtLog10ScaleEngine );
}
 
void Plot2D::setYLogAxis()
{
	setAxisScaleEngine( QwtPlot::yLeft, new QwtLog10ScaleEngine );
}
 
void Plot2D::setBackgroundColor( QColor backgroundColor )
{
	setCanvasBackground( backgroundColor );
}
 
void Plot2D::resizeEvent( QResizeEvent *event )
{
    QwtPlot::resizeEvent( event );
}

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In line #51 of your last snippet of code you are creating a local variable called "zoomer" which shadows the variable you have declared as the class member. Thus you end up initializing the local variable instead of the class member. Same goes with other variables, by the way.

thank you for your help, Wysota!

I have been asked which changes I made to resolve the issue.

In SigLibGraph.cpp, instead of

Code:

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QwtPlotZoomer* zoomer = new QwtPlotZoomer( QwtPlot::xBottom, QwtPlot::yLeft, this->canvas() );

---

just do

Code:

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zoomer = new QwtPlotZoomer( QwtPlot::xBottom, QwtPlot::yLeft, this->canvas() );

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Apply the same procedure to grid and legend!