Explanation of nil, Nil, NULL, and NSNull in iOS, nilnsnull

Explanation of nil, Nil, NULL, and NSNull in iOS, nilnsnull
Nil

• Nil is the literal null value of the ObjC object, corresponding to the id type object, or the ObjC object declared by @ interface.
• For example:

  NSString *someString = nil;NSURL *someURL = nil;id someObject = nil; if (anotherObject == nil) // do something

• Definition:

  // objc.h#ifndef nil# if __has_feature(cxx_nullptr)#   define nil nullptr# else#   define nil __DARWIN_NULL# endif#endif // __DARWIN_NULL in _types.h #define __DARWIN_NULL ((void *)0)

Nil

• Nil is a written null value of the ObjC Class type, which corresponds to a Class object.
• For example:

  Class someClass = Nil;Class anotherClass = [NSString class];

  　　

• The definition Definition is similar to that of nil, with the same value:

  // objc.h#ifndef Nil# if __has_feature(cxx_nullptr)#   define Nil nullptr# else#   define Nil __DARWIN_NULL# endif#endif

NULL

• NULL is a NULL value of any C pointer.
• For example:

  int *pointerToInt = NULL;char *pointerToChar = NULL;struct TreeNode *rootNode = NULL;

• Definition:

  // in stddef.h#define NULL ((void*)0)

NSNull

• NSNull is a class that represents null values and is an ObjC object. In fact, it only has one Singleton method: + [NSNull null], which is generally used to indicate objects with null values in the set.
• Example:

  // Nil cannot be stored in the Foundation set because nil is used to end the set. NSArray * array = [NSArray arrayWithObjects: @ "one", @ "two", nil]; // NSMutableDictionary * dict = [NSMutableDictionary]; [dict setObject: nil forKey: @ "someKey"]; // use NSMutableDictionary correctly * dict = [NSMutableDictionary dictionary]; [dict setObject: [NSNull null] forKey: @ "someKey"];

• Definition:

  /*  NSNull.h    Copyright (c) 1994-2012, Apple Inc. All rights reserved.*/ #import <Foundation/NSObject.h> @interface NSNull : NSObject <NSCopying, NSSecureCoding> + (NSNull *)null; @end

  　

  Summary:

  Nil: NULL pointer to an object

  Nil: NULL pointer to a class

  NULL: NULL pointer to other types (such as basic type and C type)

  NSNull: Null Value in the set.