The difference between nil, nil, null and Nsnull

1.nil pointer to an object is empty the definition in objc.h is as follows:

#ifndef nil# if __has_feature (cxx_nullptr) #   define nil nullptr# else#   define nil __darwin_null# endif#endif

Object in objective-c for ID type

NSString *name = nil; Nsurl    *url  = nil;id object      = nil;

2.Nil pointers to a class are null defined as follows:

#ifndef nil# if __has_feature (cxx_nullptr) #   define nil nullptr# else#   define Nil __darwin_null# endif#endif

Objects for class type in Objective-c

Class aclass = Nil; Clsss Bclass = [Nsurl class];

3.NULL pointer to type C is empty as defined in Stddef.h:

#if defined (__need_null) #undef null#ifdef __cplusplus#  if!defined (__mingw32__) &&!defined (_msc_ver) #    define NULL __null#  else#    define NULL 0#  endif#else#  define NULL ((void*) 0) #endif

This is used for the following examples:

int   *pint     

4.NSNull is a class in objective-c, but there is a null,nsnull in the name+ (NSNull*) null; A singleton method that is used for objects with null values in the collection (nsarray,nsdictionary)

Nsarray *array = [Nsarray arraywithobjects:                      [[NSObject alloc] init],                      [NSNull null],                      @ "AAA",                      Nil,                      [[NSObject alloc] init],                      [[NSObject alloc] init], nil]; NSLog (@ "%ld", Array.count); Output 3,nsarray ends with nil

Nsdictionary *dictionary = [[Nsdictionary alloc] Initwithobjectsandkeys:                                @ "Object0", @ "Key0",                                @ "Object1", @ " Key1 ",                                Nil,        @" Key-nil "                                @" Object2 ", @" Key2 ",                                Nil]; NSLog (@ "%@", dictionary); Output of 2 Key-value,nsdictionary is also terminated with nil

Nsmutabledictionary *mutabledictionary = [[Nsmutabledictionary alloc] init]; [Mutabledictionary setobject:nil forkey:@ "Key-nil"]; will cause crash[mutabledictionary setobject:[nsnull null] forkey:@ "Key-nil"]; does not cause crash

So in use, the following methods are relatively safe

[Mutabledictionary setobject: (nil = = value?) [NSNull NULL]: value)                      forkey:@ "Key"];

The difference between nil, nil, null and Nsnull