StackOverflow2013

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plurals

PO
primarykey
Id
13960627
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2012-12-19T20:28:54.640
FavoriteCount
0
LastActivityDate
2012-12-19T20:41:18.920
LastEditDate
2012-12-19T20:41:18.920
LastEditorUserId
182676
OwnerUserId
182676
ParentId
13960209
PostTypeId
2
Score
31
ViewCount
0
LastEditorDisplayName
text
Body
It looks like you have a dangling or wild pointer. You can consider Objective-C objects as pointers to structs. You can then of course compare them with <code>NULL</code>, or with other pointers. So: <pre><code>( myDict == NULL ) </code></pre> and <pre><code>( myDict == [ NSNull null ] ) </code></pre> are both valid. The first one will check if the pointer is <code>NULL</code>. <code>NULL</code> is usually defined as a <code>void *</code> with a value of 0. Note that, for Objective-C objects, we usually use <code>nil</code>. <code>nil</code> is also defined as a <code>void *</code> with a value of 0, so it equals <code>NULL</code>. It's just here to denote a <code>NULL</code> pointer to an object, rather than a standard pointer. The second one compares the address of <code>myDict</code> with the singleton instance of the <code>NSNull</code> class. So you are here comparing two pointers values. So to quickly resume: <pre><code>NULL == nil == Nil == 0 </code></pre> And as <code>[ NSNull null ]</code> is a valid instance: <pre><code>NULL != [ NSNull null ] </code></pre> Now about this: <pre><code>( [ myDict count ] == 0 ) </code></pre> It may crash if you have a wild pointer: <pre><code>NSDictionary * myDict; [ myDict count ]; </code></pre> Unless using ARC, it will surely crash, because the <code>myDict</code> variable has not been initialised, and may actually point to anything. It may also crash if you have a dangling pointer: <pre><code>NSDictionary * myDict; myDict = [ [ NSDictionary alloc ] init ]; [ myDict release ]; [ myDict count ]; </code></pre> Then you'll try to send a message to a deallocated object. Sending a message to <code>nil</code>/<code>NULL</code> is always valid in Objective-C. So it depends if you want to check if a dictionary is <code>nil</code>, or if it doesn't have values (as a valid dictionary instance may be empty). In the first case, compare with <code>nil</code>. Otherwise, checks if count is 0, and ensure you're instance is still valid. Maybe you just forgot a retain somewhere.
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1. POCheck if NSDictionary is Null?
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