Crash on QwtPlotZoomer in QT5 - qt

Question

I am trying to use a QwtPlotZoomer in QT5, but I get a seg fault when its constructor runs.
class Qwt_widget : public QwtPlot
{
Q_OBJECT
public:
Qwt_widget(QWidget* parent = 0) :
QwtPlot(parent),
m_canvas()
{
QwtPlotZoomer zoomer(&m_canvas); // Crashes here
}
private:
QwtPlotCanvas m_canvas;
};
The above widget is added to a simple main window so that it will be created (full code at https://github.com/chrisburnham/Qwt_qt5_crash). Also here are the QWT lines that I have added to the QT creators default pro file:
INCLUDEPATH += /usr/include/qwt
LIBS += -lqwt-qt5
I am running on Ubuntu 18.04 and installed Qt5 with libqt5-default (5.9.5) and QWT with libqwt-qt5-dev (6.1.3). I have checked all my libraries with LDD and looked at the headers I am using and they are all coming from the packages that were installed with libqwt-qt5-dev. I have previously used QwtPlotZoomers in this way in Qt4 (with the Qt4 version of QWT) and as far as I can tell from the documentation it should still work here.

Answer 1

You need to assign the QwtPlotCanvas to the QwtPlot before you ask QwtPlotZoomer to work on it. Note that QwtPlot::setCanvas() takes ownership, so do not use a member variable for the canvas.
But since you are using a default QwtPlotCanvas and not some derived class you could probably just use the canvas QwtPlot comes with:
QwtPlotZoomer zoomer(this->canvas()); // Should not crash here