Memory management and performSelectorInBackground:

Question

Which is right? This:

NSArray* foo = [[NSArray alloc] initWithObjects:@"a", @"b", nil];
[bar performSelectorInBackground:@selector(baz:) withObject:foo];

- (void)baz:(NSArray*)foo {
    ...
    [foo release];
}

Or:

NSArray* foo = [[[NSArray alloc] initWithObjects:@"a", @"b", nil] autorelease];
[bar performSelectorInBackground:@selector(baz:) withObject:foo];

- (void)baz:(NSArray*)foo {
    NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
    ...
    [pool release];
}

I know the first one works, but Clang complains about it, so I wonder if there's a better pattern to use.

I would "just try out" the 2nd one, but with autoreleasing, who knows whether the absence of EXC_BAD_ACCESS means that you're doing it right or that you just got lucky...

Answer 1

First is wrong.

performSelectorInBackground:withObject: retains both bar and foo until task is performed. Thus, you should autorelease foo when you create it and let performSelectorInBackground:withObject take care of the rest. See documentation

Latter is correct because you autorelease foo when you create it. Autorelease pool that you create inside baz has nothing do with correctness of foo's memory management. That autorelease pool is needed for autoreleased objects inside pool allocation and release in baz, it doesn't touch foo's retain count at all.

Answer 2

IMHO this would be better:

NSArray* foo = [[NSArray alloc] initWithObjects:@"a", @"b", nil];
[bar performSelectorInBackground:@selector(baz:) withObject:foo];
[foo release];