Question

I have been experimenting a bit with Objective-C and noted some, at least to me, rather strange behavior. First I define a pointer to an NSString and add it to an NSArray:

NSString *s = @"A";
NSArray *a = [NSArray arrayWithOject: s];

I then print out the value of s as well as the contents of a:

NSLog(@"%@", s);
NSLog(@"%@", myArray);

and in both cases the output is A. Now, if I change the pointer s, say

s = @"B";

then the two NSLog statements print out B and A, respectively.

That is, the pointer in my array still points to @"A". After spending years coding in Java, this is very surprising to me. Am I missing something really fundamental here?

Thanks,
Michael Knudsen

Answer 1

The easiest way to understand this is that @"A" creates a new NSString object.

In your code, you set the pointer s to point to this object, and then add the original object (not the pointer) to an array. You then change the address that the pointer points to to a new address.

If you want to change the original object, then use NSMutableString and modify the actual object (instead of changing the pointer to a new object) and they will both update as you expect.

Try:

 NSMutableString *s = @"A";
 // Add to array
 [s setString:@"B"]