Actually my question here is: are null
and nil
equivalent or not?
I have an example but I am confused when they are equal when they are not.
NSNull *nullValue = [NSNull null];
NSArray *arrayWithNull = [NSArray arrayWithObject:nullValue];
NSLog(@"arrayWithNull: %@", arrayWithNull);
id aValue = [arrayWithNull objectAtI开发者_运维知识库ndex:0];
if (aValue == nil) {
NSLog(@"equals nil");
} else if (aValue == [NSNull null]) {
NSLog(@"equals NSNull instance");
if ([aValue isEqual:nil]) {
NSLog(@"isEqual:nil");
}
}
Here in the above case it shows that both null
and nil
are not equal and it displays "equals NSNull instance"
NSString *str=NULL;
id str1=nil;
if(str1 == str)
{
printf("\n IS EQUAL........");
}
else
{
printf("\n NOT EQUAL........");
}
And in the second case it shows both are equal and it displays "IS EQUAL".
Anyone's help will be much appreciated.
Thank you, Monish.
nil
and NULL
are essentially the same, nil
is something like (NSObject *)0
, while NULL
is more like (void *)0
. But both are pointers with an integer value of zero. You can send messages to nil
without raising an error.
NSNull
and NULL
(or nil
, of course) are different things, however. You just use NSNull
as a helper to add an empty
object to an NSArray
or another container class, since you can't add nil
to them. So instead, you use [NSNull null]
as a replacement, and you have to check if an array element is NSNull
, not if it's nil
(it will never be equal to nil
).
From http://www.iphonedevsdk.com/forum/iphone-sdk-development/34826-nil-vs-null.html
nil
and NULL
are 100% interchangeable.
From:
NULL
is for C-style memory pointers.nil
is for Objective-C objects.Nil
is for Objective-C classes.
Whenever you're writing Objective-C code, use nil
Whenever you're writing C code, use NULL
But ultimately they're all defined as the same thing -- (void *)0, I think -- so in practice it doesn't really matter.
The concept is the same, with the difference that it's valid to send messages (call method) to nil.
NSNull is a real (singleton) class, that can be used for arrays or dictionnaries, who don't accept NULL or nil values.
Biggest difference between them: sending a message to an NSNULL
object is probably going to cause a crash, whereas it's cool to send any message to nil
. For example, if you use a key path to get an array, like so:
NSArray *departmentNames = [departments valueForKey:@"name"];
Then you will have an NSNULL
object for any department whose name
is nil
. So, this is going to cause a crash:
for (NSString *name in departmentNames)
NSLog(@"%@", [name lowercaseString]);
whenever name
is NSNull
, because you just sent an unknown selector (lowercaseString
) to an NSNull
.
Lesson: check for the NSNull
object in an array before sending any message to its elements.
for (NSString *name in departmentNames)
if (name != [NSNull null])
NSLog(@"%@", [name lowercaseString]);
No, NSNull
and nil
are not the same. They both represent a lack of value, and you might want to treat them the same, but they are still not equal.
The NSNull
object instance represents a null value, for example when you read data from a database that has null values.
The nil
value is a null pointer, i.e. it doesn't point to any object instance.
In your second code you don't have any NSNull
instance. An NSString
pointer that contains a null pointer is not an NSNull
instance, it's still just a null pointer. You are comparing one null pointer to another, and they are of course equal.
Make sure you typecast [NSNull null] to object type that you are comparing
NSArray list;
if(list==(NSArray *)[NSNull null])
// do something
otherwise you will receive a warning message saying "Comparison of distinct pointer types('type *' and 'NSNull *')
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