[Kde-bindings] Moving the Qyoto C# bindings from playground to kdebindings

[Arno Rehn]

Am Donnerstag, 12. April 2007 schrieb Richard Dale:
> On Thursday 12 April 2007, Phil Thompson wrote:
> > On Wednesday 11 April 2007 11:27 pm, Arno Rehn wrote:
> > > Am Mittwoch, 11. April 2007 schrieb Richard Dale:
> > > > On Wednesday 11 April 2007, Richard Dale wrote:
> > > > > On Wednesday 11 April 2007, Arno Rehn wrote:
> > > > > > I found out about a little bug in the GetHashCode() stuff of the
> > > > > > Qyoto classes. When you call the method of certain classes, e.g.
> > > > > > QTextBlock, the whole app segfaults with no further output. I
> > > > > > would just return the real pointer to the underlying C++ object,
> > > > > > this would always be unique and would work, wouldn't it?
> > > > >
> > > > > Yes, that's exactly what I was intending it to do. But I was
> > > > > wondering about 64 bit pointers though, if the hash is an int.
> > > >
> > > > Just been looking in the Qt libs, and there's a qHash() function in
> > > > the QHash class that takes a ulong and returns a uint:
> > > >
> > > > inline uint qHash(ulong key)
> > > > {
> > > >     if (sizeof(ulong) > sizeof(uint)) {
> > > >         return uint((key >> (8 * sizeof(uint) - 1)) ^ key);
> > > >     } else {
> > > >         return uint(key);
> > > >     }
> > > > }
> > > >
> > > > So maybe calling that on the C++ pointer would be good enough.
> > >
> > > Yes, that looks good.
> >
> > Note that this isn't reliable - you can have any number of different
> > (nested) data structures at the same address. I think I've only ever seen
> > it happen once in Qt (Qt v2.x I think) but it was a nasty bug to track
> > down.
>
> That sounds nasty. I'm not actually sure what the C# runtime does with the
> hash values though. 
As far as I know only hash tables depend on the hash values generated here.

> Another problem is that I'm not sure if a ulong is always the same length
> as a pointer, and so I really think we should have our own version of the
> 'fairly reliable' hash like this:
>
> int QyotoHash(void * key)
> {
>     if (sizeof(void*) > sizeof(int)) {
>         return (int) ((key >> (8 * sizeof(int) - 1)) ^ key);
>     } else {
>         return (int) key;
>     }
> }
Looks good. I didn't know you can apply operators like ^ on pointers.

-- 
Arno Rehn
arno at arnorehn.de