CODEWITHSUNDEEP
qtqwt
user1326050
user1326050

Reputation: 57

qt qwt QList of QwtPlotCurves

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.

Upvotes: 0

Views: 997

Answers (1)

sam-w
sam-w

Reputation: 7687

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);

Upvotes: 1

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