Re: Trying to understand how "explicit sharing" works with QwtPlotCurve::setSamples()

[K. F. <kfr...@gm...>]

Hi Uwe!

Thanks for your clarifications.

On Tue, Oct 25, 2011 at 2:49 AM, Uwe Rathmann <Uwe...@ti...> wrote:
> On 10/25/2011 01:48 AM, K. Frank wrote:
>> I update the "explicitly shared" QVectors that contain the data for a
>> QwtPlotCurve.  Calling QwtPlot::replot() does not reliably update the
>> plot.  Calling QwtPlotCurve::setSamples() does.
> When QwtPlot::autoReplot() is enabled QwtPlotCurve::setSamples() implicitely starts a replot - otherwise nothing is repainted. Thus your statement above doesn't make much sense.

Yes.  The behavior I saw didn't make sense to me either.  Also, the
behavior changed when I moved from 6.0.0 to 6.0.1.

>> The way I understand this -- perhaps incorrectly -- is that the QwtPlotCurve
>> is given references to xData and yData, and does not make a private copy
>> of that data.  So I expect that when I update xData and yData and call
>> QwtPlot::replot() I should see the changed data in the plot.
> No, Qwt doesn't have a reference - it has copies of the vectors, that are implicitely shared with the vectors in your application. But as soon as you append a point to your vectors you create deep copies.

Okay, thanks.  That clears things up.  (Also, the behavior I see in 6.0.1 is
consistent with this explanation.)

I do have one follow-up question:

The documentation for the QVector overload of QwtPlotCurve::setSamples():

   void  setSamples (const QVector< double > &xData, const
QVector<double > &yData)

uses the phrase "explicitly shared":

   Initialize data with x- and y-arrays (explicitly shared)

This kind of confused me.  Given that QVector makes copies of the
QVectors, I might have said "copies and doesn't share."  How should
I understand the phrase "explicitly shared" in this context?

>> ...
>> I don't want QwtPlotCurve to have
>> to re-copy all the data into its own private buffers every time I add a single
>> point.  So giving QwtPlotCurve a reference to a data structure that I can
>> then add points to externally of QwtPlotCurve seems to be the way to go.)
>>
> See: http://qwt.sourceforge.net/class_qwt_series_data.html#_details.
>
> QwtPlotCurve is completely independent from how the application organizes its samples. All you have to do is to write a couple of lines, so that QwtPlotCurve is able to iterate.

Well, what I did is I used the QwtPlotCurve::setRawSamples() member function.
In this case, even thought the data vectors are growing, I know in advance an
upper bound to their length.  So I allocate some c-style arrays of
doubles of that
maximum length on the heap, and keep my own currentLength variable.  As I add
data to the arrays, I increment currentLength, and re-call
setRawSamples with the
same arrays (pointers to doubles) and the updated value of curentLength.

This seems to work properly, and if I understand correctly, qwt is not
copying the
contents of the arrays under the hood.

> Uwe

Thanks for your help.


K. Frank