some confusions about [NSNull null]，nil [duplicate]

Question

Possible Duplicate:
What's the difference between [NSNull null] and nil?
What are the differences between nil, NULL and [NSNULL nil]?

1.

id dicValue = [aDictionary objetForKey:@"aKey"];

if(dicValue != nil)
{
     blablala...
}

or

2.

if(dicValue != [NSNull null]) 
{
     blablala...
}

should I choose the first one,or the second one?

or when it comes like this :

3.

if ([aDictionary objetForKey:@"aKey"] != nil)

or

4.

if ([aDictionary objetForKey:@"aKey"] != [NSNull null])

and what again ?

1.2.3.4. which is correct and recommended?

Answer 1

Directly from Apple Documentation.
The NSNull class defines a singleton object you use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an array or a dictionary).

NSNull *nullValue = [NSNull null];
NSArray *arrayWithNull = [NSArray arrayWithObject:nullValue];
NSLog(@"arrayWithNull: %@", arrayWithNull);
// output: "arrayWithNull: (<null>)"

It is important to appreciate that the NSNull instance is semantically different from NO or false—these both represent a logical value; the NSNull instance represents the absence of a value. The NSNull instance is semantically equivalent to nil, however it is also important to appreciate that it is not equal to nil. To test for a null object value, you must therefore make a direct object comparison.

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");
    }
}
// output: "equals NSNull instance"

Is it clear now??

Answer 2

My understanding is that NSNull is an object representing a NULL value and nil is just an empty pointer.

You can fill a NSArray with NSNull objects and it'll return null values, but you can't fill it with nils.

You can do

[myNsMutableArray addObject:[NSNull null]];

but this would crash:

[myNsMutableArray addObject:nil];

Likewise:

[[NSMutableArray alloc] initWithObjects:@"1", nil, @"2", nil];  
[[NSMutableArray alloc] initWithObjects:@"1", [NSNull null], @"2", nil]; 

The first array, when printed would output "1" The second one would output: "1, null, 2"

So you can use NSNull to represent empty spaces in an array.

You can't insert nil into a dictionary. So checking for it means the key doesn't exist. If you check for NSNull it means that the key exists but holds an empty (NSNull) value.

Answer 3

First one means there is no such key, second means that key's value is NSNull