CODEWITHSUNDEEP
objective-c
Shuaib
Shuaib

Reputation: 753

NSString is null but code does not recognize as null

I have this app where I am populating an NSDictionary and then doing computations on the dictionary.

            tmp = nil;
            tmp = [singleEvent objectForKey:@"start_date"];

            if(tmp == nil)
            {
                startDate = [dateFormatter dateFromString:@"1800/1/1 12:00:00"];
            }
            else
            {
                startDate = [dateFormatter dateFromString:tmp];
            }

tmp is an NSString object, and singleEvent is an NSDictionary. Both are declared elsewhere. I did not include that part of the code for simplicity.

The key "start_date" of the dictionary will sometimes contain a date in string format, and sometimes it will be null.

I have handled the condition where it will be null, but the code never hits that line. I found out by debugging that even when tmp is null the executions goes to else part.

In debug mode I get this when watching tmp:

tmp = (NSNull *)

But still else block is hit. So I am always getting a null pointer exception.

Upvotes: 0

Views: 150

Answers (4)

Guillaume Algis
Guillaume Algis

Reputation: 11016

The key "start_date" of the dictionary will sometimes contain a date in string format, and sometimes it will be null.

There's three cases:

  1. The key start_date is not in your dictionary : the return value of objectForKey: will be nil (first condition is true);
  2. The key start_date is in your dictionary and the value associated is [NSNull null] (of type (NSNull *) : the return value of objectForKey: will be [NSNull null] (second condition is true);
  3. The key start_date is in your dictionary and the value associated is something else : the return value of objectForKey: will be the stored object (second condition is true);

But #2 and #3 really are the same: NSNull is an object like every other.

Do not confuse a null pointer (nil) and the Null object ([NSNull null]). To test the first, you have to do:

if (!tmp)
// or
if (tmp == nil)

and for the second:

if (tmp == [NSNull null])

Upvotes: 2

Nick Forge
Nick Forge

Reputation: 21464

NSNull is different to nil. NSNull (or, more explicitly, [NSNull null]) is a singleton used to represent the concept of "nil" that can be used in the Foundation collection classes. Because it's a singleton, you don't have to use -isEqual:, you can compare it directly.

To check for NSNull, you can do this:

if (tmp == [NSNull null])

Upvotes: 9

Mostafa Berg
Mostafa Berg

Reputation: 3239

How about making it a bit smaller like that ? also I think your problem was you're using objectForKey not valueForKey, valueForKey will return Nil.

startDate = [dateFormatter dateFromString:@"1800/1/1 12:00:00"];

if([singleEvent valueForKey:@"start_date"])
        startDate = [dateFormatter dateFromString:[singleEvent valueForKey:@"start_date"]];

NSLog(@"%@", startDate);

Upvotes: -1

faffaffaff
faffaffaff

Reputation: 3549

NSNull is a different thing than nil. Perhaps this will work:

if ( [[NSNull null] isEqual:tmp] ) {

Upvotes: 1

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