Assign And Weak

I want to look a difference between assign and weak.So I run this code below:



@interface Test : NSObject

@property(nonatomic, strong) NSString *str;
@property(nonatomic, assign) NSString *assignString;
@property(nonatomic, weak) NSString *weakString;

@end

@implementation Test

- (id)init
{
self =[super init];
if (self)
{
self.str = @"i'm test string";

self.assignString = self.str;
self.weakString = self.str;

self.str = nil;

NSLog(@"dealloc \nstr = %p\n assignstr = %p\n weakStr = %p\n", self.str, self.assignString, self.weakString);

NSLog(@"str = %@ \nassignStr = %@\n weakString = %@\n", self.str, self.assignString, self.weakString);
}

return self;
}

@end


I think it should output like this:




str = 0x0



assignString = 0x0



weakString = 0x0



str = (null)



assignString = (null)



weakString = (null)




But I get this output:




2015-06-17 11:22:04.676 AssignWeakDiff[4696:1897735]



str = 0x0



assignstr = 0x100002078



weakStr = 0x100002078



str = (null)



assignStr = i'm test string



weakString = i'm test string




It's there something wrong with my code?



Answers

As CRD said, strings have all sorts of optimizations that alter their memory management. Repeat this exercise with your own custom NSObject subclass and you should see traditional object lifecycle behaviors.


Your expected output for the assign property is incorrect. You should expect that to have a dangling pointer to the deallocated object. The assign reference is not set to nil automatically when the object is deallocated. The weak reference will, but the assign reference will not.




Thus, if you have properties like so:



@property (nonatomic, strong) MyObject *strongObj;
@property (nonatomic, assign) MyObject *assignObj;
@property (nonatomic, weak) MyObject *weakObj;


And then do:



self.strongObj = [[MyObject alloc] init];
self.assignObj = self.strongObj;
self.weakObj = self.strongObj;

NSLog(@"%@ %@ %@", self.strongObj, self.assignObj, self.weakObj);

self.strongObj = nil;

NSLog(@"%@ %@ %@", self.strongObj, self.assignObj, self.weakObj);


At the second NSLog statement, the strong and weak references will be nil, but the assign reference will not.