fastest key cat?

NSUInteger lengthA = [a length];
NSUInteger lengthB = [b length];
NSUInteger keyLength = lengthA + lengthB + 1;
assert(keyLength * sizeof(unichar) < SOME_REASONABLE_MAXIMUM_FOR_STACK_ALLOCATION);
unichar cat[keyLength];
[a getCharacters:cat range:NSMakeRange(0, lengthA)];
cat[lengthA] = '.';
[b getCharacters:cat + lengthA + 1 range:NSMakeRange(0, lengthB)];
NSString *key = [[NSString alloc] initWithCharactersNoCopy:cat length:keyLength freeWhenDone:NO];
... [dict objectForKey:key];
[key release];