IOS (null), & lt; null & gt;, nil, iosnil

IOS (null), <null>, nil problems, iosnil
Abstract: Have you ever experienced something like <null> or (null. I have already said this again. Today I will summarize the solutions to this problem.

Some time ago, during the development process, some of the interface was inexplicably displayed in some places (null) and some areas showed <null>. The modification was very painful. After searching for information, I summarized the details, so that you do not need to spend any time on this item.

 

First, record the problem-solving ideas I encountered. The simplest and most direct method is: first locate the problem and print the variable in brute force mode!

There are two types of printing: ① % p print address; ② % @ print the description of the object (the string object is its own)

This article begins with the conclusion and expands.

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

1. nil indicates that the object pointed to by a pointer is null. The object type is id and is displayed as (null) ---> common in non-collection classes

2. [NSNull null] indicates the empty object, which is displayed as <null> ------------------------> common in the Collection class.

3. There is no substantive difference between NULL and nil, except that the former is only in the C Language

The following describes the differences among the three methods in detail.

================== Nil ============================

 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] 0x02015-10-06 13:13:45.338 test[95730:5489376] 0x02015-10-06 13:13:45.338 test[95730:5489376] 0x02015-10-06 13:13:45.338 test[95730:5489376] 1

2. The empty object is (null) printed on the console)

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 ==========================

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] 12015-10-06 13:38:59.927 test[95925:5515192] 1

2. the console prints it as (null)

3. Definition

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

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

 

 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] one2015-10-06 16:40:25.817 test[96177:5565855] two2015-10-06 16:40:25.817 test[96177:5565855] <null>

4. What is printed on the console at this time is <null>

5. Definition

/*NSNull.hCopyright (c) 1994-2015, Apple Inc. All rights reserved.*/#import <Foundation/NSObject.h>NS_ASSUME_NONNULL_BEGIN@interface NSNull : NSObject <NSCopying, NSSecureCoding>+ (NSNull *)null;@endNS_ASSUME_NONNULL_END

References: 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