qobject_cast doesn't work with QWT type

[Please_Help_me_D]

Hi,

I use QWT library to plot data.
After I plotted N-curves I'm trying to get one of them with name "MyCurve"

QwtPlotCurve curve = plotForm->canvas()->findChild<QwtPlotCurve>(QString("MyCurve"), Qt::FindDirectChildrenOnly);

But the program can't compile this line of code with the errors:

• C:\Qt\5.14.1\msvc2017_64\include\QtCore\qobject.h:176: error: C2039: 'staticMetaObject': is not a member of 'QwtPlotCurve'
• C:\Qt\5.14.1\msvc2017_64\include\QtCore\qobject.h:176: ошибка: C2440: 'static_cast': cannot convert from 'QObject *' to 'T' with [T=QwtPlotCurve]

But if I write:

QObject curve = plotForm->canvas()->findChild<QObject>(QString("MyCurve"), Qt::FindDirectChildrenOnly);

it works. But then the qobject_cast<QwtPlotCurve>() throws the error :)
I googled this errors but didn't understand how to solve the problem

[KroMignon]

@Please_Help_me_D As far as I can see, QwtPlotCurve is not a QObject

So qobject_cast<QwtPlotCurve>() can not work

[J.Hilk]

@Please_Help_me_D
QwtPlotCurve is not a QObject based class. -> qobject_cast won't work.

use static_cast<QwtPlotCurve>();

that said, find child shouldn't work either. Object is probably null or invalid -> object_cast fails

[Please_Help_me_D]

@KroMignon @J-Hilk thank you
I understand that this is more QWT question but still, don't you know how to get plotted item?
In debug mode QObject curve = plotForm->canvas()->findChild<QObject>(QString("MyCurve"), Qt::FindDirectChildrenOnly); shows me:

[KroMignon]

@Please_Help_me_D said in qobject_cast doesn't work with QWT type:

I understand that this is more QWT question but still, don't you know how to get plotted item?

Sorry I think I can not help you, I never used QWT and I don't understand what you want to extract from QwtPlot.
Perhaps reading documentation could help you => https://qwt.sourceforge.io/class_qwt_plot.html

[Pl45m4]

@Please_Help_me_D said in qobject_cast doesn't work with QWT type:

how to get plotted item?

You already have the itemList. You can get a list with QwtPlotCurve-items only by passing Rtti_PlotCurve (plotForm->itemList( Rtti_PlotCurve ); ). Then you can iterate through the list and get your QwtPlotCurve(s).

https://qwt.sourceforge.io/class_qwt_plot_dict.html#a49d60b01e53a33423987aa80559449f9

There is also infoToItem()
https://qwt.sourceforge.io/class_qwt_plot.html#aa951d433d8eba6a47bfd8f0537090c82

[Astrinus]

QwtPlot inherits from QwtPlotDict, that has an itemList() method. Have you tried to iterate over it, comparing the item title() and maybe the rtti() value? Just a guess from five minutes into Qwt documentation, never used.
Furthermore, I think findChild should be used with a pointer type, even if here it is useless...

[Please_Help_me_D]

@Pl45m4 @Pl45m4 thank you very much!
I also read the documentation but sometimes it is difficult to understand the idea of some function. Only now I understood how I could use infoToItem().
The last thing now. I find all curves QwtPlotItemList itemList = plotForm->itemList( QwtPlotItem::Rtti_PlotCurve ); but they are stored in QwtPlotItemList type. How to convert QwtPlotItemList[1] to QwtPlotCurve?

[Pl45m4]

@Please_Help_me_D said in qobject_cast doesn't work with QWT type:

How to convert QwtPlotItemList[1] to QwtPlotCurve?

QwtPlotItemIterator it;
for ( it = itemList.begin(); it != itemList.end(); ++it )
{ 
  QwtPlotCurve* curve = static_cast<QwtPlotCurve *>( *it );
}

Or if you have just one item and you know that there aren't more, you can do something like this:

 if(itemList.size() > 0 && itemList[0] != nullptr)
          QwtPlotCurve* curve = static_cast<QwtPlotCurve *>( itemList[0] );

[Please_Help_me_D]

@Pl45m4 thank you a lot!