How to detect if NSString is null?

Question

I have a piece of code that detects if a NSString is NULL, nil, etc. However, it crashes. Here is my code:

NSArray *resultstwo = [database executeQuery:@"SELECT * FROM processes WHERE ready='yes' LIMIT 0,1"];
for (NSDictionary *rowtwo in resultstwo) {

NSString *getCaption = [rowtwo valueForKey:@"caption"];

if (getCaption == NULL) {
theCaption = @"Photo uploaded...";
} else if (getCaption == nil) {
theCaption = @"Photo uploaded...";
} else if ([getCaption isEqualToString:@""]) {
theCaption = @"Photo uploaded...";
} else if ([getCaption isEqualToString:@" "]) {
theCaption = @"Photo uploaded...";
}

}

And here's the error:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull isEqualToString:]: unrecognized selector sent to instance 0x3eba63d4'

Am I doing something wrong? Do I need to do it a different way?

What’s the data type of rowtwo? Why are you sending it -valueForKey:? — user557219
– user557219, Commented Apr 16, 2011 at 2:47
I’ll leave this comment for further reference: -valueForKey: is a KVC method. In KVC, [NSNull null] represents the absence of a value for a given key. If you’re using an NSDictionary in a context that doesn’t need KVC, use -objectForKey: instead — it will return nil instead of [NSNull null] when a given key is not present in the dictionary. — user557219
– user557219, Commented Apr 16, 2011 at 3:03
See this Stack Overflow question for a discussion on the difference between -valueForKey: and -objectForKey:: stackoverflow.com/questions/1062183/… — user557219
– user557219, Commented Apr 16, 2011 at 3:04
One more note (ha! :-)) — it looks like you’re using some non-Apple class to fetch results from a database query. If a column is NULL (as in SQL NULL), this class has two options when populating a dictionary: either don’t include the corresponding key, or include the corresponding key mapping to an [NSNull null] value. So it is up to this class how to represent SQL NULL values, and your test needs to consider the specifics of this class. — user557219
– user557219, Commented Apr 16, 2011 at 3:47

Regexident · Accepted Answer · 2013-11-15 11:05:13Z

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 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.

True, although the actual error is that [rowtwo valueForKey:@"caption"] is returning an NSNull instance instead of an NSString instance.
@phpnerd211: this probably doesn't solve your real problem, though -- see Bavarious's comments attached to the question.
He was mistaken. I use this method all over my app and it functions properly -- see my edits in my code on how I populated the array, etc.
getCaption == [NSNull null] is a bad idea to use. It's better to do [getCaption class] == [NSNull class]. You're free to create new instances of NSNull and some JSON parsers do make new ones instead of using the singleton.

ludesign · Accepted Answer · 2011-04-16 02:46:59Z

24

You should use

if ([myNSString isEqual:[NSNull null]])

This will check if object myNSString is equal to NSNull object.

answered Apr 16, 2011 at 2:46

ludesign

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2 Comments

Caleb Over a year ago

Since NSNull is a singleton, you could actually compare pointers directly: if (getCaption == [NSNull null])...

Dustin Over a year ago

But comparing against the singleton is a bad idea! Don't do it =X. [NSNull null] may be a singleton but NSNull is a class like any other.

Baby Groot · Accepted Answer · 2014-11-24 07:01:15Z

6

Preferred Way to check for the NSNULL is

if(!getCaption || [getCaption isKindOfClass:[NSNull class]])

edited Nov 24, 2014 at 7:01

Baby Groot

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answered Nov 24, 2014 at 6:37

Aadi007

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Comments

Dustin · Accepted Answer · 2012-02-29 01:08:13Z

2

if([getCaption class] == [NSNull class])
    ...

You can also do

if([getCaption isKindOfClass:[NSNull class]])
    ...

if you want to be future proof against new subclasses of NSNull.

answered Feb 29, 2012 at 1:08

Dustin

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1 Comment

Evan R Over a year ago

Of all the answers on this page, only your top answer is working for me in Xcode 6.1, using an NSString property called body that's null. Debugger output: _body NSString * class name = NSNull 0x0000000107279ce0 and Printing description of ad->_body: <null>

Abdul Jamil · Accepted Answer · 2017-08-09 14:10:26Z

2

Just check with this code:

NSString *object;

if(object == nil)

This should work.

edited Aug 9, 2017 at 14:10

Abdul Jamil

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answered Feb 23, 2016 at 10:51

Surjeet Rajput

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