Are NULL and nil equivalent?

问题

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 objectAtIndex: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.

回答1:

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).

回答2:

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.

回答3:

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.

回答4:

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]);

回答5:

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.

回答6:

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 *')

来源：https://stackoverflow.com/questions/2461617/are-null-and-nil-equivalent

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objective-c

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nsnull