qt creator canvas and setlinewidth issue - c++

Question

I've an issue about using "canvas" with "SetLineWidth" parameter.
I guess, it is caused by creating inheritance, or it may be due to the library installations. I've already installed the qwt libraries, and have no problem with them.
I guess, it is better to write down the code here to be more specific and helpful :
#include <qwt/qwt_plot.h>
#include <qwt/qwt_plot_marker.h>
#include <qwt/qwt_plot_curve.h>
#include <qwt/qwt_legend.h>
#include <qwt/qwt_series_data.h>
#include <qwt/qwt_plot_canvas.h>
#include <qwt/qwt_plot_panner.h>
#include <qwt/qwt_plot_magnifier.h>
#include <qwt/qwt_text.h>
#include <qwt/qwt_math.h>
#include <qwt/qwt_plot_renderer.h>
#include <math.h>
#include <qprinter.h>
#include <qprintdialog.h>
#include <QMouseEvent>
#include "plot.h"
Plot::Plot(QWidget *parent, char *titel, int bufSize, bool fixedScaleX, bool fixedScaleY,
double minX, double maxX, double minY, double maxY,
bool displayScaleX, bool displayScaleY,
char* textScaleX, char* textScaleY) : QwtPlot(parent) {
mY = (QwtPlotMarker*) 0;
setAutoFillBackground(true);
setPalette(QPalette(QColor(165, 193, 228)));
QwtText title(titel);
title.setFont(QFont("Arial", 11));
setTitle(title);
if(fixedScaleX) {
setAxisScale(xBottom, minX, maxX);
}
if(fixedScaleY) {
setAxisScale(yLeft, minY, maxY);
}
if(displayScaleX) {
enableAxis(xBottom, true);
if(textScaleX[0] != '\0') {
QwtText xAxis(textScaleX);
xAxis.setFont(QFont("Arial", 11));
setAxisTitle(xBottom, xAxis);
}
} else {
enableAxis(xBottom, false);
}
if(displayScaleY) {
enableAxis(yLeft, true);
if(textScaleY[0] != '\0') {
QwtText yAxis(textScaleY);
yAxis.setFont(QFont("Arial", 11));
setAxisTitle(yLeft, yAxis);
}
} else {
enableAxis(yLeft, false);
}
// canvas
canvas()->setLineWidth(3); // Problem is here
canvas()->setFrameStyle(QFrame::Box | QFrame::Plain);//Problem is here
canvas()->setBorderRadius(15);//Problem is here
QPalette canvasPalette(Qt::black);
canvasPalette.setColor(QPalette::Foreground, QColor(133, 190, 232));
canvas()->setPalette(canvasPalette);
}
The error is that :
'class QWidget' has no member named 'setLineWidth'
'class QWidget' has no member named 'setFrameStyle'
'class QWidget' has no member named 'setBorderRadius'
The code is going on,.I mentioned above where the problem is. I tried to solve it myself, but I couldn't figure it out why the problem is caused.
If there is any suggestions, I'll be glad to know that.
Thanks in advance..

Answer 1

Your class is a subclass of QwtPlot, so canvas() is QwtPlot::canvas(), which returns a QWidget*, pointing to the member canvas. According to the docs, the default canvas is a QwtPlotCanvas.
You can cast from the returned QWidget* to QwtPlotCanvas, but you should check that the type is correct:
QwtPlotCanvas * my_canvas = dynamic_cast<QwtPlotCanvas *>(canvas());
if(my_canvas)
{
my_canvas->setLineWidth(3);
// etc
}