The NULL
value for Objective-C objects (type id
) is nil
.
While NULL
is used for C pointers (type void *
).
(In the end both end up holding the same value (0x0
). They differ in type however.)
In Objective-C:
nil
(all lower-case) is a null
pointer to an Objective-C object.
Nil
(capitalized) is a null pointer
to an Objective-C class.
NULL
(all caps) is a null pointer to
anything else (C pointers, that is).
[NSNull null]
is a singleton for situations where use of nil is not possible (adding/receiving nil to/from NSArray
s e.g.)
In Objective-C++:
- All of the above, plus:
null
(lowercase) or nullptr
(C++11 or later) is a null pointer to C++ objects.
So to check against nil
you should either compare against nil
(or NULL
respectively) explicitly:
if (getCaption == nil) ...
or let ObjC / C do it implicitly for you:
if (!getCaption) ...
This works as every expression in C (and with Objective-C being a superset thereof) has an implicit boolean value:
expression != 0x0 => true
expression == 0x0 => false
Now when checking for NSNull
this obviously wouldn't work as [NSNull null]
returns a pointer to a singleton instance of NSNull
, and not nil
, and therefore it is not equal to 0x0
.
So to check against NSNull
one can either use:
if ((NSNull *)getCaption == [NSNull null]) ...
or (preferred, see comments):
if ([getCaption isKindOfClass:[NSNull class]]) ...
Keep in mind that the latter (utilising a message call) will return false
if getCaption
happens to be nil
, which, while formally correct, might not be what you expect/want.
Hence if one (for whatever reason) needed to check against both nil
/NULL
and NSNull
, one would have to combine those two checks:
if (!getCaption || [getCaption isKindOfClass:[NSNull class]]) ...
For help on forming equivalent positive checks see De Morgan's laws and boolean negation.
Edit: NSHipster.com just published a great article on the subtle differences between nil, null, etc.