Check if NSDictionary is Null?
if (TheDict == (NSDictionary*) [NSNull null]){
//TheDict is null
}
else{
//TheDict is not null
}
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 NULL
, or with other pointers.
So:
( myDict == NULL )
and
( myDict == [ NSNull null ] )
are both valid.
The first one will check if the pointer is NULL
. NULL
is usually defined as a void *
with a value of 0.
Note that, for Objective-C objects, we usually use nil
. nil
is also defined as a void *
with a value of 0, so it equals NULL
. It's just here to denote a NULL
pointer to an object, rather than a standard pointer.
The second one compares the address of myDict
with the singleton instance of the NSNull
class. So you are here comparing two pointers values.
So to quickly resume:
NULL == nil == Nil == 0
And as [ NSNull null ]
is a valid instance:
NULL != [ NSNull null ]
Now about this:
( [ myDict count ] == 0 )
It may crash if you have a wild pointer:
NSDictionary * myDict;
[ myDict count ];
Unless using ARC, it will surely crash, because the myDict
variable has not been initialised, and may actually point to anything.
It may also crash if you have a dangling pointer:
NSDictionary * myDict;
myDict = [ [ NSDictionary alloc ] init ];
[ myDict release ];
[ myDict count ];
Then you'll try to send a message to a deallocated object.
Sending a message to nil
/NULL
is always valid in Objective-C.
So it depends if you want to check if a dictionary is nil
, or if it doesn't have values (as a valid dictionary instance may be empty).
In the first case, compare with nil
. Otherwise, checks if count is 0, and ensure you're instance is still valid. Maybe you just forgot a retain somewhere.