Segfault when calling setAxisScaleEngine more than 2 times

I have a program where I want to be able to switch between a logarithmic and linear scale. It works fine for the first couple of times, but then fails with a segmentation fault. Any pointers on what I might be doing wrong before submitting a bug report.

Here is a sample code to reproduce the bug

Code:

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#include "TestQwtPlot.h"
#include "TestQwtPlot.moc"
 
#include <QtGui/QMenu>
#include <QtGui/QMenuBar>
#include <QtGui/QAction>
#include <qwt/qwt_plot.h>
#include <qwt/qwt_scale_engine.h>
#include <qwt/qwt_data.h>
#include <qwt/qwt_plot_curve.h>
 
TestQwtPlot::TestQwtPlot()
{
  plot = new QwtPlot(this);
  linScale = new QwtLinearScaleEngine();
  logScale = new QwtLog10ScaleEngine();
  double x[5] = {1,2,3,4,5};
  QwtArrayData qwtArray ( x, x, 5 );
  QwtPlotCurve *curve1 = new QwtPlotCurve ( "Curve 1" );
  curve1->setData ( qwtArray );
  curve1->attach( plot );
  plot->replot();
  QAction* log = new QAction(this);
  QAction* lin = new QAction(this);
  log->setText( "Logarithmic scale" );
  lin->setText( "Linear scale" );
  setCentralWidget(plot);
    QAction* a = new QAction(this);
    a->setText( "Quit" );
    connect(a, SIGNAL(triggered()), SLOT(close()) );
    connect(lin, SIGNAL(triggered()), SLOT(on_lin_triggered()));
    connect(log, SIGNAL(triggered()), SLOT(on_log_triggered()));
    QMenu * m = menuBar()->addMenu( "File" );
    m->addAction( a );
    m->addAction( log );
    m->addAction( lin );
}
 
TestQwtPlot::~TestQwtPlot()
{}
 
void TestQwtPlot::on_lin_triggered(){
  plot->setAxisScaleEngine(QwtPlot::yLeft, linScale);
  plot->setAxisScaleEngine(QwtPlot::xBottom, linScale);
  plot->replot();
}
 
void TestQwtPlot::on_log_triggered(){
  plot->setAxisScaleEngine(QwtPlot::yLeft, logScale);
  plot->setAxisScaleEngine(QwtPlot::xBottom, logScale);
  plot->replot();
}

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and the header

Code:

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#ifndef TestQwtPlot_H
#define TestQwtPlot_H
 
#include <QtGui/QMainWindow>
#include <qwt/qwt_plot.h>
#include <qwt/qwt_scale_engine.h>
 
class TestQwtPlot : public QMainWindow
{
Q_OBJECT
public:
    TestQwtPlot();
    virtual ~TestQwtPlot();
  private:
    QwtPlot *plot;
    QwtLinearScaleEngine *linScale;
    QwtLog10ScaleEngine *logScale;
  private slots:
    void on_log_triggered();
    void on_lin_triggered();
};
 
#endif // TestQwtPlot_H

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and finally the backtrace

Code:

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#0  0x0000000000000021 in ?? ()
#1  0x00007ffff6b943b2 in QwtPlot::setAxisScaleEngine(int, QwtScaleEngine*) () from /usr/lib64/libqwt.so.5
#2  0x0000000000403838 in TestQwtPlot::on_lin_triggered (this=0x7fffffffd200) at /home/gudlaugu/projects/TestQwtPlot/TestQwtPlot.cpp:44
#3  0x00000000004049d8 in TestQwtPlot::qt_metacall (this=0x7fffffffd200, _c=QMetaObject::InvokeMetaMethod, _id=1, _a=0x7fffffffa9f0) at /home/gudlaugu/projects/TestQwtPlot/build/TestQwtPlot.moc:69
#4  0x00007ffff7affee2 in QMetaObject::activate (sender=0x887370, from_signal_index=<value optimized out>, to_signal_index=6, argv=0x0) at kernel/qobject.cpp:3113
#5  0x00007ffff6f86a37 in QAction::triggered (this=0x749d10, _t1=false) at .moc/release-shared/moc_qaction.cpp:236
#6  0x00007ffff6f87eb0 in QAction::activate (this=0x887370, event=<value optimized out>) at kernel/qaction.cpp:1167
#7  0x00007ffff7370ddd in QMenuPrivate::activateCausedStack (this=0x82ba90, causedStack=@0x7fffffffab10, action=0x887370, action_e=QAction::Trigger, self=true) at widgets/qmenu.cpp:967
#8  0x00007ffff7376dde in QMenuPrivate::activateAction (this=0x82ba90, action=0x887370, action_e=QAction::Trigger, self=true) at widgets/qmenu.cpp:1060
#9  0x00007ffff6fdda3d in QWidget::event (this=0x72f5a0, event=0x7fffffffb2d0) at kernel/qwidget.cpp:7554
#10 0x00007ffff737962b in QMenu::event (this=0x72f5a0, e=0x7fffffffb2d0) at widgets/qmenu.cpp:2358
#11 0x00007ffff6f8ca4d in QApplicationPrivate::notify_helper (this=0x6072e0, receiver=0x72f5a0, e=0x7fffffffb2d0) at kernel/qapplication.cpp:4065
#12 0x00007ffff6f9543a in QApplication::notify (this=<value optimized out>, receiver=0x72f5a0, e=0x7fffffffb2d0) at kernel/qapplication.cpp:3767
#13 0x00007ffff7aeb15c in QCoreApplication::notifyInternal (this=0x7fffffffd240, receiver=0x72f5a0, event=0x7fffffffb2d0) at kernel/qcoreapplication.cpp:610
#14 0x00007ffff6f94688 in QApplicationPrivate::sendMouseEvent (receiver=0x72f5a0, event=0x7fffffffb2d0, alienWidget=0x0, nativeWidget=0x72f5a0, buttonDown=<value optimized out>, lastMouseReceiver=@0x7ffff799ff30)
    at ../../src/corelib/kernel/qcoreapplication.h:216
#15 0x00007ffff6ffe644 in QETWidget::translateMouseEvent (this=0x72f5a0, event=<value optimized out>) at kernel/qapplication_x11.cpp:4343
#16 0x00007ffff6ffd4f5 in QApplication::x11ProcessEvent (this=0x7fffffffd240, event=0x7fffffffce50) at kernel/qapplication_x11.cpp:3550
#17 0x00007ffff7024854 in x11EventSourceDispatch (s=0x610340, callback=0, user_data=0x0) at kernel/qguieventdispatcher_glib.cpp:146
#18 0x00007ffff57970fb in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#19 0x00007ffff579a8cd in ?? () from /usr/lib64/libglib-2.0.so.0
#20 0x00007ffff579aa8b in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#21 0x00007ffff7b1456c in QEventDispatcherGlib::processEvents (this=0x60c430, flags=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:407
#22 0x00007ffff7023fdf in QGuiEventDispatcherGlib::processEvents (this=0x749d10, flags=<value optimized out>) at kernel/qguieventdispatcher_glib.cpp:202
#23 0x00007ffff7ae99e2 in QEventLoop::processEvents (this=<value optimized out>, flags={i = -11968}) at kernel/qeventloop.cpp:149
#24 0x00007ffff7ae9db4 in QEventLoop::exec (this=0x7fffffffd180, flags={i = -11888}) at kernel/qeventloop.cpp:201
#25 0x00007ffff7aec0a4 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#26 0x0000000000404cb7 in main (argc=1, argv=0x7fffffffd338) at /home/gudlaugu/projects/TestQwtPlot/main.cpp:10

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Replying to myself so others don't fall into this trap. The problem is that setAxisScaleEngine deletes the previously set scale engine. So my allocated engine is deleted in the second call to setAxisScaleEngine, causing the segfault. Solution is to create the scale engine before the call to setAxisScaleEngine and never delete it.

This is not a logical behavior and I have submitted a bug report. So this might change in subsequent releases, currently using 5.2.

I presume QwtPlot takes ownership of the scale engine after the call to setAxisScaleEngine. This sort of thing is standard behaviour with Qt, so I would say it is logical in this context.

Having said that, the Qt documentation is very explicit about when ownership of an object is and is not transferred. The Qwt documentation should be updated to be very explicit about when this does and does not occur.

Jord,
So what should one do to address or circumvent the behaviour? In my case, the call works only in Mac OS and not in Linux, the second call to either change the scale between Linear and Log, or change the Min and Max values is called as show here:

Code:

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calibPlot->setAxisScale(QwtPlot::xBottom, xMinVal.toInt(&ok,10),axisXEnd->text().toInt(&ok,10) );

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When called to change the Scale:

Code:

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void CentralPage::xAxisScaleChange( const QString &xAxisScaleType)
{
	if (xAxisScaleType == "[X]Linear") { 
		calibPlot->setAxisScaleEngine(QwtPlot::xBottom, new QwtLinearScaleEngine ());
		calibPlot->replot();
	} else {
		calibPlot->setAxisScaleEngine(QwtPlot::xBottom, new QwtLog10ScaleEngine());
		calibPlot->replot();
	}
}

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