Objective-C about null values nil and null and nsnull

1. Nil

A null pointer pointing to an objective-C object

RepresentsObjectNull: nsstring * STR = nil; (# define nil (ID) 0 ))

2. Nil

A null pointer pointing to an objective-C Class Object.

RepresentsClassObjectNull: Class class = nil;

3. null

The objective-C value type is null: int number = NULL; (equivalent to int number = 0;) (# define null (void *) 0 ))

4. nsnull

A class definesSingleton objectUsed to indicate the null value in the collection object (nil value not allowed ).

#import <Foundation/NSObject.h>@interface NSNull : NSObject <NSCopying, NSCoding>+ (NSNull *)null;@end

[Nsnull null]; a singleton object is null.

5. Differences between nil and nsnull object message calls

If an object is equal to nil, no message is called and no exception is thrown;

If the object is of the nsnull type, an exception is thrown when a message is sent.

6. Note:

In objective-C, nsarray, nsdictionary, and nsset may all contain nsnull objects,

Therefore, if they contain nsnull objects, the program will crash.

Reference: http://blog.csdn.net/yhawaii/article/details/7442529