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?

Andras

What’s the data type of rowtwo? Why are you sending it -valueForKey:? – Fourflush

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. – Fourflush

See this Stack Overflow question for a discussion on the difference between -valueForKey: and -objectForKey:: #1062683 – Fourflush

Getting object from array populated by a database... – Andras

rowtwo is not an array, otherwise -valueForKey: would return another array. Is it a dictionary? – Fourflush

Please see my edits in how I populated it and used a NSDictionary. – Andras

It’s a dictionary, then. See my comments #2 and #3. – Fourflush

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. – Fourflush

You can check my answer https://mcmap.net/q/269991/-testing-for-nil-in-objective-c-if-x-nil-vs-if-x – Proficient

Answer 1

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.

Curricle

True, although the actual error is that [rowtwo valueForKey:@"caption"] is returning an NSNull instance instead of an NSString instance. – Fourflush

@Bavarious: See last line in answer. – Curricle

@phpnerd211: this probably doesn't solve your real problem, though -- see Bavarious's comments attached to the question. – Farkas

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. – Andras

@phpnerd211 I advise you not to use -valueForKey: because it serves a different purpose. If you’re using NSDictionary, use its -[NSDictionary objectForKey:] method. If you want to learn more about KVC and -valueForKey:, there are a couple of guides on Apple’s web site. – Fourflush

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. – Margoriemargot

I found that using '(getCaption == [NSNull null])' worked fine but got a comparison warning so I changed it to 'if (getCaption == (id)[NSNull null])' – Pentstemon

I had to check for NSNull as well, strange. Seems my NSString was set – Dichogamy

perfect answer , Clear – Lashondra

Bless you! Perfect answer! – Conradconrade

Answer 2

You should use

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

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

Frisette

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

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. – Margoriemargot

Answer 3

Preferred Way to check for the NSNULL is

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

Cinereous

Answer 4

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.

Margoriemargot

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> – Lorinalorinda

Answer 5

Just check with this code:

NSString *object;

if(object == nil)

This should work.

Alagoas