11.2 writeToURL:atomically

Tested my Cocoa prowess as suggested on p246, by coming up with this code:

- (BOOL)writeToURL:(NSURL *)aURL atomically:(BOOL)flag
{
return [mainDictionary writeToURL:aURL atomically:YES];
}

and I get the following error in the log:
LookItUp[24804:903] dataOfType:error: is a subclass responsibility but has not been overridden.

I copied this signature from the NSDictionary Class Reference. However, the author’s sample code works even though it doesn’t use the NSDictionary Class Reference writeToURL:atomically signature. I guess I need to subclass the dataofType, like the author’s sample code. I’m confused as to why that wasn’t in the NSDictionary signature, if it is required.

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