Check object if it's equal to nil or to [NSNull null]

Question

if object set to nil and I want to check for its value.

if object != nil {
  // do something
}

or

if object != [NSNull null] {
  // do something
}

I guess the second condition will be triggered but I am not sure why.

Can someone explain please?

Answer 1

If you want to make sure object isn't nil and it isn't NSNull null then do:

if (object && object != [NSNull null]) {
    // do something with object
} // else it's either nil or NSNull null

Answer 2

As others have pointed out, object != nil && object != NSNull.null would give you the expected behaviour, however it might be tedious and error prone to write this pair of conditions every time.

Alternatively you can use inverted logic, by adding an nonNullValue method to virtually almost all objects in the system:

// in some header file
@interface NSObject(NullExtension)
- (instancetype)nonNullValue;
@end

// in some implementation file    
@implementation NSObject(NullExtension)

// regular objects are not null (right?)
- (instancetype)nonNullValue {
    return self;
}
@end

@implementation NSNull(NullExtension)

// let's make NSNull report as being null
- (instancetype)nonNullValue {
    return nil;
}
@end

// adding this just for the sake of completeness, it's highly unlikely
// that JSON's will decode NSProxy instances
@implementation NSProxy(NullExtension)

- (instancetype)nonNullValue {
    return self;
}
@end

you can then simply use it on your pointers:

// since we're in Objective-C, a nil pointer will always fail the check
// and thanks to the NSNull extension, NSNull instances will behave the same
if ([object nonNullValue] != nil) {
    // NSNull won't get here
    // nils won't get here
    // other objects will get here
}

This approach is a little bit invasive as it touches all NSObject subclasses, however it eliminates the need of writing multiple conditions.

Answer 3

The difference between [NSNull null] and nil is that nil is an empty object that has completely disappeared from memory, and we use [NSNull null] when we want to express the idea that "we need a container that has nothing in it," which is "an object whose value is null." If you look up the development documentation you'll see that the class NSNull inherits NSObject and has only one "+ (NSNull *) null;" Class methods. This means that the NSNull object has a valid memory address, so any reference to it in the program will not cause the program to crash.