How to detect if NSString is null?

问题:

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?

回答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.

回答2:

You should use

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

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

回答3:

Preferred Way to check for the NSNULL is

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

回答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.

回答5:

Just check with this code:

NSString *object;

if(object == nil)

This should work.