QwtPlot on ui form

**qt_gotcha** · 4th June 2010, 06:45

A simple application with a ui form, generated in eclipse. The form is opened and shown upon compiling. The form has a QWidget called widget.

If I add the basic code to draw a qwtplot on the form it crashes. The mesage is: "QWidget: Must construct a QApplication before a QPaintDevice". I thought qapplication was constructed first in the main code, so I don't get it! I have tried linking the qwtplot to "this" and "parent" but the same result. I have also tried to move the plot form the constructor of trygraph to a button press function but with the same result

In all examples of QwtPLot I can find the plot is made in the main function before "return a.exec();"
Appreciate any help or pointers to examples.

Qt Code:

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trygraph::trygraph(QWidget *parent)
    : QWidget(parent)
{
	ui.setupUi(this);
 
	double x[100], y1[100];
	QwtPlot *myPlot = new QwtPlot(QwtText("a Curves"), ui.widget);
 
	for (int i = 0; i < 100; i++)
	{
		x[i] = i;
		y1[i] = 10-0.1*i*i;
	}
 
	// add curves
	QwtPlotCurve *curve1 = new QwtPlotCurve("Curve 1");
 
	// copy the data into the curves
	curve1->setData(x,y1,100);
 
	curve1->attach(myPlot);
 
	myPlot->replot();
}
 
[the main function:]
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    trygraph w;
    w.show();
    return a.exec();
}

trygraph::trygraph(QWidget *parent)
    : QWidget(parent)
{
	ui.setupUi(this);

	double x[100], y1[100];
	QwtPlot *myPlot = new QwtPlot(QwtText("a Curves"), ui.widget);

	for (int i = 0; i < 100; i++)
	{
		x[i] = i;
		y1[i] = 10-0.1*i*i;
	}

	// add curves
	QwtPlotCurve *curve1 = new QwtPlotCurve("Curve 1");

	// copy the data into the curves
	curve1->setData(x,y1,100);

	curve1->attach(myPlot);

	myPlot->replot();
}

[the main function:]

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    trygraph w;
    w.show();
    return a.exec();
}

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