Inconsistent object deallocation with ARC?

Question

I was playing around with memory (de)allocation stuff on a simple command line app for Mac OSX 10.7 built using Xcode Version 4.2.1 with ARC enabled, and the default build setti

Answer 1

This does seem strange. You're right (in your comments) about the 2nd bit of code just being because the memory hasn't been re-used yet. But the 3rd bit of code is stranger. Here is a more simplified test case which shows this strange problem:

#import 

@interface SomeClass : NSObject 
@end

@implementation SomeClass
- (void)foo {
}
@end

int main (int argc, const char * argv[]) {
    @autoreleasepool {
        SomeClass *objPtr1 = [[SomeClass alloc] init];
        __weak SomeClass *weakRef = objPtr1;

//        [weakRef foo];
        [weakRef foo];

        objPtr1 = nil;

        NSLog(@"%p", weakRef);

        return 0;
    }
}

With that line commented out the output is:

$ clang -fobjc-arc -framework Foundation test.m -o test -O3 && ./test
2012-02-12 00:39:42.769 test[6684:707] 0x0

With that line uncommented out the output is:

$ clang -fobjc-arc -framework Foundation test.m -o test -O3 && ./test
2012-02-12 00:42:04.346 test[6688:707] 0x100f13f50

This seems deeply odd and looks entirely like a bug to me. I don't actually know what the answer is but thought I'd post this as an answer to get the ball rolling on figuring out what's going on.

Update:

If you build this at O0 then it seems that weakRef is zeroed only if there are no calls to foo. A single call to foo will mean that it won't get zeroed.