Question

I’ve been working through Apple’s “Your Second IOS App” tutorial” and I’ve noticed that every time I use an NSDateFormatter for producing a date string I am expected to cast the input date object.

e.g. (Page – under “To implement the configureView method”)

BirdSighting *theSighting = self.sighting;

if (theSighting) {
    self.birdNameLabel.text = [theSighting name];
    self.locationLabel.text = [theSighting location];

    self.dateLabel.text = [formatter stringFromDate:(NSDate *)theSighting.date]; // Here
}

However I know that in the BirdSighting class that the date property is always an NSDate object.

So I was wondering why the tutorial always casts the input, is there some sort of objective-c convention or framework convetion that recommends that you do this? and if so why?

Answer 1

It’s not necessary. The cast is simply for clarity, as the property is already defined as a NSDate, here:

@property (nonatomic, strong) NSDate *date; 

The only situation where a cast would be necessary in that form would either be in non-arc when dealing with core foundation, like this:

CFDateRef asCFDate;
NSDate *asNSDate = (NSDate *) asCFDate;

However, in ARC, you would use a __bridge cast instead.