Question

Let’s say I have this code:

NSString *inspDate = @"20120515";
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyyMMdd"];
NSDate *inspectionDate;
inspectionDate = [dateFormatter dateFromString:inspDate];

When I check to see that it works (NSDate contains the correctly formatted data) it does… but why?

Here I go through the steps:

1. Memory for NSDateFormatter is getting allocated and instantiated in the heap. dateFormatter is pointing to that location, got it:

   NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
   

2. Now dateFormatter is being told how to interpret NSString‘s as a date, ok:

   [dateFormatter setDateFormat:@"yyyyMMdd"];
   

3. Here’s the part I’m hazy on, inspectionDate pointer is set to nil. It’s not pointing to anything:

   NSDate *inspectionDate;
   

4. How is dateFormatter returning a pointer to a NSDate when there was no alloc orinit called to inspectionDate? Is the implementation of dateFromString doing the alloc and init in its implementation?

    inspectionDate = [dateFormatter dateFromString:inspDate];
   

Helping me visualize this would be a tremendous help. Thank you!

Answer 1

The dateFromString method will instantiate (likely through an alloc/init call at some point, or an NSDate convenience method which itself will do the allocation) an NSDate object and you assign it to your inspectionDate variable. As with any “convenience” method that does not start with alloc, new, or copy, there is no transfer of object ownership, i.e. the instance returned is autoreleased.