Question

I would like to make a QList of QwtPlotCurve s. The reason for this is to be able to remove them later form my QwtPlot.
I have the following code:

QList<QwtPlotCurve> myList = new QList<QwtPlotCurve>;
QwtPlotCurve* curve1 = new QwtPlotCurve();
QwtPlotCurve* curve2 = new QwtPlotCurve();
curves->append(curve1);
curves->append(curve2);

The code doesn’t compile and the compiler outputs:

error: conversion from ‘QList‘ to non-scalar type ‘QList’ requested

error: no matching function for call to ‘QList::append(QwtPlotCurve&)’
void QList::append(const T&) [with T = QwtPlotCurve]

note: candidates are:

note: void QList::append(const T&) [with T = QwtPlotCurve]

note: no known conversion for argument 1 from ‘QwtPlotCurve*’ to ‘const QwtPlotCurve&’

note: void QList::append(const QList&) [with T = QwtPlotCurve]

note: no known conversion for argument 1 from ‘QwtPlotCurve*’ to ‘const QList&’

…

I says the QwtPlotCurve should be constant, but I don’t know how to deal with it.
I don’t know neither whether storing curves in a QList and then removing it (on user demand) from the plot is the right approach.

---

After the answer of sjwarner I tried the following:

QList<QwtPlotCurve*> curves;
QwtPlotCurve* curve1 = new QwtPlotCurve();
QwtPlotCurve* curve2 = new QwtPlotCurve();
curves->append(curve1);
curves->append(curve2);

and I got the following error:

error: base operand of ‘->’ has non-pointer type ‘QList’
error: base operand of ‘->’ has non-pointer type ‘QList’

I understood this error in the following way:
curves is a QList and it should be a pointer to the QList.

If I try:

QList<QwtPlotCurve*>* curves = new QList<QwtPlotCurve*>;
QwtPlotCurve* curve1 = new QwtPlotCurve();
QwtPlotCurve* curve2 = new QwtPlotCurve();
curves->append(curve1);
curves->append(curve2);

it works fine.
I am going to look at “Implicit sharing” pointed by sjwarner to get rid of “new” operators.

Answer 1

You have two problems:

1. As commented by Kamil Klimlek above, you are declaring your QList object on the stack, and then trying to allocate it on the heap – since new returns a pointer to the type of object you are newing, so you are effectively trying to do the following:

   QList<T> = *QList<T>
   

   As an aside: it is very rare that you’ll need to new off a QList, since Qt implements implicit sharing for all of its container classes – in a nutshell, you can confidently declare all Qt containers (and plenty of other classes besides) as stack objects and pass-by-value if the contained data is needed elsewhere – Qt will handle all of the memory efficiencies and object cleanup.

   Read this for more info.

2. You are declaring a QList of objects and trying to fill it with pointers to objects. you need to decide whether you want your QList to contain copies of the data:

   QList<QwtPlotCurve> curves;
   QwtPlotCurve curve1();
   QwtPlotCurve curve2();
   curves.append(curve1);
   curves.append(curve2);
   

   Or whether you want to allocate your QwtPlotCurves on the heap and store pointers to them in the QList:

   QList<QwtPlotCurve*> curves;
   QwtPlotCurve* curve1 = new QwtPlotCurve();
   QwtPlotCurve* curve2 = new QwtPlotCurve();
   curves.append(curve1);
   curves.append(curve2);