[RQt-devel] another crash

[Justin Talbot]

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
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>>
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>
>
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>

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

Justin