Re: Problems with Replacing a String is a NSMutableString

[Helge Hess]

On 24.07.2008, at 10:20, Charles philip Chan wrote:
> > Keys for collections are copied
   ^^^                      ^^^^^
> > by the collection to insure they are immutable. Allowing them to be  
> > mutable would break the internal hash lookups.
> I still don't understand this
...
> | Returns an array containing all the dictionary's keys that are
                                                    ^^^^
> | associated with anObject.[1]
> `----
>
> So basically I am returning an array of the keys, and since there is
> only 1 key for the object "Album" I search for it at index zero in the
> array and return it as an NSMutableString.

Dictionary keys are never NSMutableString objects. As David says, the  
NSMutableString will get copied when its used as a dictionary key.

  NSString *key = [NSMutableString stringWithString:@"bla"];
  [yourDict setObject:@"this and that" forKey:key];
  // the yourDict has no reference to your 'key' object,
  // it made a copy

Aproximately this happens from a copy-perspective:

  NSString *key = [NSMutableString stringWithString:@"bla"];
  NSString *dictCopy;

  dictCopy = [key copy];
  [yourDict setObject:@"this and that" forKey:dictCopy];
  [dictCopy release];

As mentioned by David a mutable object can't be used as a hashtable  
key for rather obvious reasons :-) (if not, lookup hashtable in  
Wikipedia :-)
NSMutableDictionary ensures the immutability by calling -copy.

Helge
--
Helge Hess
http://helgehess.eu/