TRAIN_00681.eml

NoneNoneOn Apr 16, 2010, at 3:53 PM, Hamish Allan wrote:

> Hi,
>=20
> I'm rather puzzled by some behaviour I'm seeing, as explained by the
> comment in the following short category:
>=20
> @implementation NSPersistentStore (OTAdditions)
> - (NSNumber *)autoincrementingNumberForKey:(NSString *)key
> {
>    NSNumber *number;
>    @synchronized(self)
>    {
>        NSMutableDictionary *metadata =3D [[self metadata] =
mutableCopy];
>=20
>        // need to copy and autorelease the number because the original
>        // does not remain valid outside of the @synchronized block =
(why?!)
>        number =3D [[[metadata objectForKey:key] copy] autorelease];
>=20
>        if (!number)
>            number =3D [NSNumber numberWithUnsignedLongLong:1];
>        [metadata setValue:[NSNumber
> numberWithUnsignedLongLong:([number unsignedLongLongValue] + 1)]
> forKey:key];
>        [self setMetadata:metadata];
>    }
>    return number;
> }
> @end
>=20
> The behaviour I'm seeing is as though the NSNumber returned by
> [metadata objectForKey:key] has been added to a special
> mini-autorelease pool scoped by the @synchronized block, whereas the
> autoreleased copy is added to the normal autorelease pool. At least, I
> assume the NSNumber is being deallocated, but symbolic breakpoints on
> -[NSNumber release] and -[NSNumber dealloc] remain in a pending state
> so I haven't been able confirm that.
>=20
> Can anyone tell me what's going on here?

Nothing related to @synchronized. Your problem is that you are acquiring =
an unretained pointer to an object, then doing something that releases =
the object behind your back.

    number =3D [metadata objectForKey:key];

`number` now points to some object. Importantly, -objectForKey: just =
hands back the pointer, without doing the [[result retain] autorelease] =
dance. It's possible that the only thing that retained `number` is the =
`metadata` dictionary.

    [metadata setValue:something forKey:key];

Now `metadata` has released `number`. It may be dead.

You need to be careful with -objectForKey: and -objectAtIndex: while you =
are mutating the container. Those methods avoid autorelease for =
performance, but (as you found) it's also less safe.

Your solution of copy+autorelease is correct. retain+autorelease might =
be better.=20


--=20
Greg Parker     gparker@apple.com     Runtime Wrangler


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