String isNullOrEmpty as a category

Question

I'm trying to create a method which checks for a null/nil/empty string, and I'm trying to get it working as a category but having no luck.

I'm using this code, based on answers in this topic:

@implementation NSString (NSStringExtension)
- (BOOL)isNullOrEmpty {
    return self == nil || 
    self == (id)[NSNull null] ||
    [@"" isEqualToString:self] || 
    [[self stringByReplacingOccurrencesOfString:@" " withString:@""] length] == 0||
    [self isEqualToString:@"(null)"]
     || ([self respondsToSelector:@selector(length)] && [(NSData *) self length] == 0)
     || ([self respondsToSelector:@selector(count)] && [(NSArray *) self count] == 0)
     || [[self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0;
}
@end

Yet when I try to use this this is what I get:

NSLog([@"" isNullOrEmpty] ? @"1":@"0"); // prints 1
NSString *s1 = nil;
NSLog([s1 isNullOrEmpty] ? @"1":@"0"); // prints 0
NSLog([args.itemName isNullOrEmpty] ? @"1":@"0"); // prints 0
NSLog([(NSString*)nil isNullOrEmpty] ? @"1":@"0"); // prints 0

This is baffling me, and I can only assume that some combination of iOS5/ARC is causing the nil object to be coerced to a blank string/pointer. The debugger shows the string as 0x0, yet when I use my isNullOrEmpty method, I get false.

Answer 1

return self == nil

This can never happen. If you try to send isNullOrEmpty (or any other message) to nil, nothing happens (objc_msgSend(), the function responsible for message dispatch, checks for a nil reciever as one of the first things it does and aborts).

self == (id)[NSNull null]

This will also never happen. If you send isNullOrEmpty to an object that's an instance of NSNull, your method here, which is a method on NSString, will not be called. Instead, NSNull's version (which probably doesn't exist) will be.

Likewise, ([self respondsToSelector:@selector(count)] && [(NSArray *) self count]) is never going to happen. If the object is an NSArray, then isNullOrEmpty will never run, because, again, it's a method of NSString.

Correspondingly, [(NSData *) self length] doesn't do what you think it does. NSString instances do respond to length, but casting the object to NSData doesn't use the NSData version of the method -- it still ends up as the NSString version of length, because the object actually is an NSString (casting only happens at compile-time; it can't change anything at run-time).

[self isEqualToString:@"(null)"]

Here you appear to be checking for nil again, but you are being misled by the representation that NSLog chooses when it prints nil:

NSLog(@"%@", nil);

This displays (null) in the console, but that doesn't mean that the object itself is a string with those characters. NSLog just chooses that string to display for nil.*

Several of the things you are doing would require this to be in a category on NSObject, so that the method would in fact be called even if the object was not an NSString.

To check for a string consisting only of whitespace, all you need is the comparison to the empty string @"" after trimming whitespace:

NSString * trimmedSelf = [self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
// Then either:
[trimmedSelf isEqualToString:@""];
// Or:
([trimmedSelf length] == 0);

---

*And even better, doing NSLog(@"%@", [NSNull null]); displays <null> (angle brackets instead of parentheses), wonderfully confusing the first few times you encounter NSNull.

Answer 2

Another approach can be to define a simple macro.

#define NSStringIsNullOrEmpty(str) ((str==nil) || [(str) isEqualToString:@""])

It's simple and effective. If you do not like macros you can always convert it to a function call without affecting the rest of your code.

-- Update:

@Bryan has raised a good point. An inline function is a great way to go. Here is an updated macro that will evaluate str only once.

#define NSStringIsNullOrEmpty(str) ({ NSString *_str=(str); ((tmp==nil) || [tmp isEqualToString:@""]);})

Answer 3

In Objective-C, sending a message to nil will always return 0 (or NO, a zeroed-out struct, NULL, etc., depending on the declared return type). The isNullOrEmpty method that you wrote won't actually be invoked when you send isNullOrEmpty to nil. See the accepted answer to Sending a message to nil? for more information.

Perhaps you could change your method to be isNotNullOrEmpty. Then a return value of 0 when sending isNotNullOrEmpty to nil will make sense.

Answer 4

You aren't calling your method, but sending a message to nil.

Answer 5

This is expected behavior. You are sending a message to nil after all. So it is returning either nil (or some other 0 value). Which short circuits to false so that '0' is printed in the cases shown below:

NSLog([s1 isNullOrEmpty] ? @"1":@"0"); // prints 0
NSLog([(NSString*)nil isNullOrEmpty] ? @"1":@"0"); // prints 0

You can even confirm your message is not being called for those cases by setting a breakpoint in your new category method.

Answer 6

Like others have said, calling [nil isNullOrEmpty]; will not actually run your method. The nil object is just that : empty itself.

As a solution, I'd like to say that it's not because you're in an Object-Oriented language that you must never use functions.

BOOL IsStringNilOrEmpty(NSString*)str
{
    return str == nil ||
        str == null ||
        [@"" isEqualToString:str] ||
        [[str stringByReplacingOccurrencesOfString:@" " withString:@""] length] == 0||
        [str isEqualToString:@"(null)"]
        || ([str respondsToSelector:@selector(length)] && [(NSData *) str length] == 0)
        || ([str respondsToSelector:@selector(count)] && [(NSArray *) str count] == 0)
        || [[str stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0;   
}

Answer 7

actually I just fixed this problem by turning it around like so

-(BOOL) isNotNullOrWhiteSpace
{
        return [self length] != 0 && [[self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] != 0;
}

so instead of isNullOrWhiteSpace it's isNotNullOrWhiteSpace.

Answer 8

Here's my method of checking null/empty

-(NSString*)NULLInputinitWithString:(NSString*)InputString {

if( (InputString == nil) ||(InputString ==(NSString*)[NSNull null])||([InputString isEqual:nil])||([InputString length] == 0)||([InputString isEqualToString:@""])||([InputString isEqualToString:@"(NULL)"])||([InputString isEqualToString:@"<NULL>"])||([InputString isEqualToString:@"<null>"]||([InputString isEqualToString:@"(null)"])||([InputString isEqualToString:@"NULL"]) ||([InputString isEqualToString:@"null"])))

    return @"";
else
    return InputString ;

}

Answer 9

Have you thought about creating a class method on a category that extends NSString?

NSString+NSStringExtensions.h

#import <Foundation/Foundation.h>

@interface NSString(NSStringExtensions)
+(BOOL)isNilOrEmpty:(NSString*)string;
@end

NSString+NSStringExtensions.m

#import "NSString+NSStringExtensions.h"

@implementation NSString(NSStringExtensions)

+(BOOL)isNilOrEmpty:(NSString*)string
{
    if (nil == string)
    {
        return YES;
    }
    if (string.length == 0)
    {
        return YES;
    }
    return NO;
}

@end

Then you use it like this:

#import "NSString+NSStringExtensions.h"
...
NSLog([NSString isNilOrEmpty:@""] ? @"1":@"0");