IOS Nil, Nil, NULL, Nsnull detailed _ios

OBJC inside a few empty value symbols will almost kill me, these basic things should be clear to the line, in order to improve the basic quality of agricultural code.

Nil

A nil is a literal null value for a OBJC object, an object that corresponds to an ID type, or a OBJC object that uses a @interface declaration.
For example:

NSString *somestring = nil;
Nsurl *someurl = nil;
ID someobject = nil;
if (Anotherobject = = nil)//do something

Defined:

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 the written null value of the OBJC class type, corresponding to the class type object.
For example:

Class SomeClass = Nil;
Class Anotherclass = [NSString class];
The definition declaration is similar to the nil, and the values are the same:

Objc.h
#ifndef Nil
# if __has_feature (cxx_nullptr)
#  define Nil nullptr
# Else
#  Define Nil __darwin_null
# endif
#endif

Null

Null is an arbitrary C-pointer null value.
For example:

int *pointertoint = NULL;
char *pointertochar = NULL;
struct TreeNode *rootnode = NULL;

Defined:

In Stddef.h 
#define NULL ((void*) 0)

Nsnull

Nsnull is a class that represents a null value and is a OBJC object. In fact, it has only one single instance method: +[nsnull null], which is generally used to represent objects with empty values in the collection.
Example Description:

Because nil is used to mark the end of a collection, nil cannot be stored in the Foundation collection.
Nsarray *array = [Nsarray arraywithobjects:@ "one", @ "two", nil];
Incorrect use of
Nsmutabledictionary *dict = [Nsmutabledictionary dictionary];
[Dict setobject:nil forkey:@ "Somekey"];
The right use
Nsmutabledictionary *dict = [Nsmutabledictionary dictionary];
[Dict setobject:[nsnull null] forkey:@ "Somekey"];
Defined:

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

Summary

Although nil, nil, and NULL values are the same, it is important to understand the written meaning between them, making the code more explicit and more readable.

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