Some time ago, in the development process, some inexplicable places on the interface show < null > and some places show < null > which is very painful to modify. After searching for information to summarize the subtleties, so as not to spend meaningless time on this thing to modify.

First of all, I will record the solutions to these problems. The most simple and direct way is to locate the problem first, and then print out the variable in a violent way.

There are two types of printing: 1) print address at% p; 2) description of print object (string object is itself)

This article first talks about the conclusion and then unfolds it.

Summary: 0. nil, NULL are essentially the same, all point to 0x0 address, [NSNULL null] is an object, stored in the constant area, occupying a fixed address.

1.nil denotes that an object pointed by a pointer is empty, and the object type is id, which shows that it is (null) - > common in non-set classes.

2.[NSNull null] denotes the empty object itself and shows that <null> - ------------------------------------> is common in collection classes.

3.NULL is not substantially different from nil, except that the former is only in C.

Here's a detailed explanation of the three differences

============== nil ================

1. nil is the literal null value of an OC object whose type is id.

 NSString *str = nil;
 NSData *data = nil;
  NSLog(@"%@",nil);
 NSLog(@"%@",str);  
 NSLog(@"%@",data); 
  NSLog(@"%p",nil);
 NSLog(@"%p",str);  
 NSLog(@"%p",data); 
 NSLog(@"%d",(data == nil));
 
2015-10-06 13:13:45.338 test[95730:5489376] (null)
2015-10-06 13:13:45.338 test[95730:5489376] (null)
2015-10-06 13:13:45.338 test[95730:5489376] (null)
2015-10-06 13:13:45.338 test[95730:5489376] 0x0
2015-10-06 13:13:45.338 test[95730:5489376] 0x0
2015-10-06 13:13:45.338 test[95730:5489376] 0x0
2015-10-06 13:13:45.338 test[95730:5489376] 1

2. Empty objects are printed on the console as null.

3. nil definition

// objc.h
#ifndef NULL
#define NULL    __DARWIN_NULL
#endif /* ! NULL */
#ifndef nil
  #if defined(__has_feature) 
    #if __has_feature(cxx_nullptr)
      #define nil nullptr
    #else
      #define nil __DARWIN_NULL
    #endif
  #else
    #define nil __DARWIN_NULL
  #endif
#endif

// __DARWIN_NULL in _types.h 
#define __DARWIN_NULL ((void *)0)

===========NULL ================

1. NULL is an arbitrary null C-pointer

int *pointerToInt = NULL;
char *pointerToChar = NULL;
struct TreeNode *rootNode = NULL;
NSLog(@"%@",pointerToInt);
NSLog(@"%s",pointerToChar); 
NSLog(@"%@",rootNode);
NSLog(@"%d",pointerToInt==NULL);  //
NSLog(@"%d",pointerToInt==nil);   //
2015-10-06 13:38:59.927 test[95925:5515192] (null)
2015-10-06 13:38:59.927 test[95925:5515192] (null)
2015-10-06 13:38:59.927 test[95925:5515192] (null)
2015-10-06 13:38:59.927 test[95925:5515192] 1
2015-10-06 13:38:59.927 test[95925:5515192] 1

2. The console prints out (null)

3. definition

#ifndef NULL
#define NULL    __DARWIN_NULL
#endif /* ! NULL */

===========NSNULL ================

1. NSNULL is a class that represents null values

2. NSNULL has only one singleton method [NSNull null]

3. Purpose: Objects with empty set values (nil in oc cannot be stored in a collection because nil is the tag bit at the end of the collection)

 NSArray *arr = [NSArray arrayWithObjects:@"one",@"two",[NSNull null], nil];
        for (NSString *str in arr) {
            NSLog(@"%@",str);
        }
2015-10-06 16:40:25.816 test[96177:5565855] one
2015-10-06 16:40:25.817 test[96177:5565855] two
2015-10-06 16:40:25.817 test[96177:5565855] <null>

4. At this point, the console prints out <null>.

5. definition

/*	NSNull.h
	Copyright (c) 1994-2015, Apple Inc. All rights reserved.
*/

#import <Foundation/NSObject.h>

NS_ASSUME_NONNULL_BEGIN

@interface NSNull : NSObject <NSCopying, NSSecureCoding>

+ (NSNull *)null;

@end

NS_ASSUME_NONNULL_END

Reference material: http://stackoverflow.com/questions/5908936/difference-between-nil-nil-and-null-in-objective-c

            http://blog.csdn.net/shenshen123jun/article/details/38315755

             https://github.com/nicklockwood/NullSafe