Question

The documentation for NSDictionary says that the method objectForKey returns nil if the NSDictionary does not contain your key; however, isn’t nil the same as zero? If so, how do I know if the return value means that the dictionary contains the key mapped to zero or if that key is just non-existant?

Answer 1

NSDictionaries (and, indeed, all Cocoa collection objects) can only contain Objective-C objects, not C primitives like int. Therefore, were you to store 0 in a dictionary, you’d do it like this:

[myDictionary setObject:[NSNumber numberWithInt:0] forKey:@"myKey"];

and therefore, retrieving it would go like this:

NSNumber* resultObj = [myDictionary objectForKey:@"myKey"];
if (resultObj == nil)
{
    //Key didn't exist
}
else
{
    int result = [resultObj intValue];
    //Now do your work.
}

This concept of ‘wrapping’ a C primitive in an object to store it in a collection, then ‘unwrapping’ it on the other end is very common in Cocoa. (You might also look into NSValue if you’re trying to work with non-number values.)