Question

I was recently told casting [NSNull null] to (NSString*) was “terrible”.

However, -[NSDictionary stringForKey:] method will return [NSNull null] if the key is not in the dictionary, and if I don’t do the cast the compiler yells at me.

Am I missing something?

EDIT:

My mistake… and I think I might be beginning to see the problem…

As everyone has pointed out, there is no stringForKey: method on NSDictionary, and yes, I was thinking about the user defaults when I asked the question… so I went back and looked at what I was doing… here it is:

NSString * someValue = (NSString*)[myDictionary objectForKey:@"key"];
if (someValue == (NSString*)[NSNull null]) ...

if I don’t do the cast in this case, I get the following error:

warning: comparison of distinct Objective-C types 'struct NSNull *' and 'struct NSString *' lacks a cast

Casting the value “solves” the problem, and I’d rather not write 10 lines where one will do…

Is this bad? Where will I run into problems?

ALSO:

The dictionaries are coming from a JSON library… and NULLs are valid values in this case, so perhaps it’s not that NSDictionary returns them if the key is missing, but rather that the key is, in fact, there, and the value is actually null.

Answer 1

NSDictionary does not return instances of NSNull unless you or some other source put them there. As well, NSDictionary does not respond to -stringForKey:. Are you perhaps thinking of NSUserDefaults?

If you have an instance of NSDictionary and all you want to do is get a string from it, you could try something like the following code.

NSString *MyGetString(NSDictionary *dict, id key, NSString *fallback) {
    id result = [dict objectForKey: key];

    if (!result)
        result = fallback;
    else if (![result isKindOfClass: [NSString class]])
        result = [result description];

    return result;
}

Pass a fallback string as fallback–the function will use that if no object is associated with the key key.