On Feb 14, 2013, at 8:30 AM, Uli Kusterer wrote:
> On Feb 14, 2013, at 3:57 AM, Ken Thomases <k...@codeweavers.com> wrote:
>> Your question prompted me to try to design an analog of NSKeyedArchiver,
>> NSCode, and NSCoding that would generate the new object graph on the fly as
>> it went instead of producing a data object. The idea is that the copier
>> (the analog of the archiver/coder) would know which objects had already been
>> copied and so would avoid over-duplicating them in the new graph. However,
>> that ends up being hard because each object has to copy its related object
>> before it's complete enough to be registered with the copier. So, it isn't
>> successful in avoiding the potential for infinite recursion.
>
> What NSKeyedArchiver probably does is have a dictionary that maps the
> original object pointer values to the copied objects. So instead of just
> straight-out copying an object, it does:
>
> NSString* theKey = [NSString stringWithFormat: @"%p", theOriginal];
> id theCopy = [objectCopies objectForKey: theKey];
> if( !theCopy )
> {
> theCopy = [theOriginal copy];
> [objectCopies setObject: theCopy forKey: theKey];
> }
>
> That way, every object only gets copied once, and the copy re-used in other
> spots. That may be part of why it is slower. (NB - you could probably use an
> NSValue +valueWithUnretainedPointer: or whatever as the key, I just quickly
> typed this untested code into the e-mail)
That's what I considered but it doesn't work. You can't add the copy into the
archiver's database of already-copied objects until it's complete. But, for
the case where the graph has cycles, making the complete copy will try to make
copies of all its related objects first. When the cycle loops back to the same
original, that original still does not have a copy in the database, so it tries
to make another copy. Infinite recursion.
Regards,
Ken
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