[RQt-devel] another crash

[Michael Lawrence]

On Thu, Nov 19, 2009 at 9:22 AM, Justin Talbot <justintalbot at gmail.com>wrote:

> On Thu, Nov 19, 2009 at 8:54 AM, Michael Lawrence <lawremi at gmail.com>
> wrote:
> >
> >
> > On Thu, Nov 19, 2009 at 4:15 AM, Deepayan Sarkar <
> deepayan.sarkar at gmail.com>
> > wrote:
> >>
> >> On Wed, Nov 18, 2009 at 2:49 PM, Deepayan Sarkar
> >> <deepayan.sarkar at gmail.com> wrote:
> >> > On Wed, Nov 18, 2009 at 1:46 PM, Deepayan Sarkar
> >> > <deepayan.sarkar at gmail.com> wrote:
> >> >> Any idea what's going wrong here?
> >> >>
> >> >> ### Just code:
> >> >>
> >> >> library(qtbase)
> >> >>
> >> >> gscene <- Qt$QGraphicsScene()
> >> >> rtxt <- gscene$addText("some text")
> >> >> rtxt
> >> >> (rtxt$toPlainText()) ## fine
> >> >
> >> > And as a confirmation that the <QList> thing is not a problem, I get:
> >> >
> >> >> item0 <- gscene$itemAt(0, 0)
> >> >> item0
> >> > <environment: 0x9e21428>
> >> > attr(,"class")
> >> > [1] "QGraphicsTextItem"   "QObject"             "QGraphicsItem"
> >> > [4] "UserDefinedDatabase" "environment"
> >> >> item0$toPlainText()
> >> >
> >> >  *** caught segfault ***
> >>
> >> Other QGraphicsItem-s seem fine, so it must be the multiple
> >> inheritance (QObject+QGraphicsItem) problem of  QGraphicsTextItem
> >> again. Given that rtxt works but item0 doesn't, maybe it's a coercion
> >> problem somewhere in the wrapping stage?
> >>
> >
> > Everything method invocation fails with item0. The weird thing is that
> the
> > environments are different. Every C++ pointer should have the same
> > environment, and this holds true for the other QGraphicsItem types. This
> > indicates that the pointer being returned by itemAt() and items() is
> > incorrect.
> >
> > Just checked it, and it looks like the pointer retrieved by e.g. itemAt()
> > points 8 bytes higher than the one from addText(). This happens at the
> Smoke
> > level, but looking at their wrappers reveals nothing. Yes, they are
> casting
> > to different types, (QGraphicsTextItem *) vs (QGraphicsItem *), but that
> > shouldn't change the value of the pointer, should it? The same error
> happens
> > for addWidget() and its QGraphicsProxyWidget return value. So yes it
> seems
> > like a multiple inheritance issue, but I don't understand how a simple
> cast
> > could cause this. Maybe it's just another C++ surprise?
> >
> >>
> >> -Deepayan
> >> _______________________________________________
> >> Qtinterfaces-devel mailing list
> >> Qtinterfaces-devel at lists.r-forge.r-project.org
> >>
> >>
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/qtinterfaces-devel
> >
> >
> > _______________________________________________
> > Qtinterfaces-devel mailing list
> > Qtinterfaces-devel at lists.r-forge.r-project.org
> >
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/qtinterfaces-devel
> >
> >
>
> I haven't looked at this problem, but casting can change the pointer's
> value in C++ when there's multiple inheritance. This is due to C++'s
> virtual function table mechanism for inheritance. For more info see
> section 4.4 of http://www-plan.cs.colorado.edu/diwan/class-papers/mi.pdf
>
>
Alrighty then. Right now, we're using the introspection information of the
QGraphicsItem to resolve the class from the QGraphicsItem returned by
itemAt() down to QGraphicsTextItem (so that we can use methods like
toPlainText()).  But the pointer is still of QGraphicsItem type, so must be
cast appropriately via smoke->cast(). I'll fix this tonight.


> Justin
>
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