Title11

The NULL value for Objective-C objects (type id) isnil.

While NULL is used for C pointers (typevoid *).

(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 NSArrays 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.