Subclassing NSString?

[Frederic]

Hi,

I need to create NSString which are unique in memory. (I actually need
that for performance reasons).
So I decided to subclass NSString and create a class method
+(id)keyWithString:(NSString*)keyName which 
looks first in a hashtable to ensure it returns a singleton.
But when invoking [[MyNSStringSubClass] alloc] initWithString:keyName],
it fails saying:
"subclass ASKey should override
initWithCharactersNoCopy:length:freeWhenDone:"

Is this really necessary and how should I do that?

Thanks in advance,
Frederic