IOS Study Notes 8-nil, Nil, null, NSNULL differences and usage

1. nil: Generally, it is assigned to an empty object;

2. NULL: It is generally assigned to NULL values other than nil. Such as SEL;

Give me a chestnut (so heavy ~) :

[NSApp beginSheet: sheet
ModalForWindow: mainWindow

ModalDelegate: nil // pointing to an object

DidEndSelector: NULL // pointing to a non object/class

ContextInfo: NULL]; // pointing to a non object/class

3. NSNULL: NSNull has only one method: + (NSNull *) null;

[NSNull null] is used to add a non-nil (indicating the end of the list) null value to NSArray and NSDictionary. [NSNull null] is an object, which is used when nil cannot be used.

4. When a message is sent to nil, NO is returned and NO exception is thrown. The program continues to run;

An exception is thrown when a message is sent to an NSNull object.

Because nil in NSArray and NSDictionary has a special meaning (indicating the end of the list), the nil value cannot be placed in the set. To store a value indicating "nothing", you can use the NSNull class. There is only one NSNull method:

+ (NSNull *) null;

 

Nil is an object pointer with NULL values, Nil is a class pointer with NULL values, and NULL is a basic data type with NULL values. These can be understood as the difference between nil, Nil, and NULL.