[Development] Help debugging "impossible condition"

Thiago Macieira thiago.macieira at intel.com
Mon May 14 13:46:34 CEST 2012

On segunda-feira, 14 de maio de 2012 13.16.46, Olivier Goffart wrote:
> On Monday 14 May 2012 11:42:33 Thiago Macieira wrote:
> > On segunda-feira, 14 de maio de 2012 11.36.04, Thiago Macieira wrote:
> > > Since no one has a suggestion on how to fix the hack, I'll close the
> > > task
> > > and abandon the patch that cleans up QEvent.
> I think you should keep the change, just remove the assert.

I can't do that. It would mean that the d pointer could be overwriting 

Besides, it's not the assertion in qgraphicsview.cpp that is the problem. It's 
the one I added to ~QEvent that checks that the d pointer is still null.

> > > I'll add a ### to QEvent saying that the d pointer cannot be used
> > > because
> > > of a hack in graphics view.
> d pointer can be used if the graphics view hack is taken in account at the
> time.  (example: if there is ever a QEventPrivate, we can change
> QGraphicsView as this point to use it)

Then we should keep the assertion so that the code fails clearly if the 
private pointer shows up.

> > Actually, I have a better idea: I'll just add an extra pointer to QEvent
> > for the sole reason of letting QGraphicsView use it.
> > 
> > If someone thinks that adding 4 or 8 bytes to EVERY EVENT is a bad idea,
> > please help me come up with a better alternative.
> I think it is a bad idea.
> My proposed alternative is in the gerrit comments:
>  - Keep your change that adds the appropriate copy contstructor, but take in
> account the graphicsview hack instead of asserting)
>  - Maybe even void *extra_ptr is a better name than 'd', as long as its
> usage is internal.

I can reset the pointer back to zero after it's done using. But someone will 
have to fix QGraphicsView later if we start using the QEvent private.

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
     Intel Sweden AB - Registration Number: 556189-6027
     Knarrarnäsgatan 15, 164 40 Kista, Stockholm, Sweden
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