__weak example in ARC code

Question

I'm trying to understand how does __weak works in ARC code. Here is my example:

NSString *string = @"Hi!"; //1
__weak NSString *secondString = string; //2
string = @"Hello world!"; //3

NSLog(@"STR: %@", secondString); //4

I expect that NSLog shows me "nil", but it shows "Hi!". Why? This string must be dealloced at the third line.

Answer 1

Strings like that are static and will never be released. Try with manually allocated objects:

#import <Foundation/Foundation.h>

int main()
{
    NSObject* strongPtr = [NSObject new];
    NSObject* __weak weakPtr = strongPtr;
    NSLog(@"weak=%@", weakPtr);

    strongPtr = nil;
    NSLog(@"weak=%@", weakPtr);
}

The output should be

2014-01-16 15:07:49.376 a.out[34078:507] weak=<NSObject: 0x7fe391c08230>
2014-01-16 15:07:49.379 a.out[34078:507] weak=(null)

Another common reason why tests like this may appear to fail is autorelease pools — if any of APIs autorelease the object behind the scene, you may have to wait until the next hop of the event loop to see weak pointers becoming nil.