Mutable objects in set

MutableObjInSet.m

#import <Foundation/Foundation.h>

void logIt(NSSet *set) {
    NSLog(@"set: %@", set);
    for (id object in set) {
        NSLog(@"element with hash: %lu", (unsigned long)[object hash]);
    }
}

int main(int argc, char *argv[]) {
    NSMutableString *a = [@"a" mutableCopy];
    NSMutableString *b = [@"b" mutableCopy];
    NSSet *set = [NSSet setWithObjects:a, b, nil];
    
    logIt(set);
    
    [b setString:@"a"];
    
    logIt(set);
}

Output

2014-04-19 17:14:15.877 Untitled 2[98114:507] set: {(
    a,
    b
)}
2014-04-19 17:14:15.879 Untitled 2[98114:507] element with hash: 1062
2014-04-19 17:14:15.879 Untitled 2[98114:507] element with hash: 1065
2014-04-19 17:14:15.880 Untitled 2[98114:507] set: {(
    a,
    a
)}
2014-04-19 17:14:15.880 Untitled 2[98114:507] element with hash: 1062
2014-04-19 17:14:15.880 Untitled 2[98114:507] element with hash: 1062