IOS about nil and nil and the difference between null and <null>

The problem is this.

Nsdictionary *sample = [nsjsonserialization jsonobjectwithdata:received options:nsjsonreadingmutableleaves Error: &*messageinfo = [Sample objectforkey:@ "message"];

Sample is a dictionary, Messsageinfo is obtained from the dictionary according to the key value, and then through the log you can know that the value of Messageinfo is <null>.

At this point, if you need to determine whether its value is empty. Then you should do this:

For example, you need to pop up a message box.

if ([null]]) {        *alter = [[Uialertview alloc] Initwithtitle:@"Title" message:@ " incorrect number format, please re-enter " delegate: nil cancelbuttontitle:@ "OK"  Otherbuttontitles:nil];        [Alter show];    }

the difference between nil, NULL, nil, nsnull in iOS

1.nil
>defines The ID of a null instance.
Defines a null pointer to an object in OC that has an empty instance. is a object value.　　
> Sample code:
NSString *somestring = nil;
Nsurl *someurl = nil;
ID someobject = nil;
if (Anotherobject = nil)//do something

> when an object is Rele ASE is also best to assign them to nil , which ensures security and, if not assigned nil, can cause the program to crash.
    nsarray * array = [Nsarray arraywithobjects:@ "test", @ "Test1", nil];
    [array release];
    *release should be assigned a value of nil

Otherwise the object will become a zombie object.
if (array)
{
Only an array of release, and no assignment to nil, somewhere in the program if you continue the array operation, the program directly crashes
NSString * string = [array objectatindex:0];
NSLog (@ "%@", string);
}

2 . NULL: is a general-purpose pointer (generic pointer)

>these macros define null values for classes and instances.
Null can be used on various pointers in the C language,
#define __DARWIN_NULL #define__DARWIN_NULLConstants

> Sample code:
int *pointertoint = NULL;
char *pointertochar = NULL;
struct TreeNode *rootnode = NULL;

> in Objective-c, the nil object is designed to be associated with a NULL null pointer. The difference is that nil is an object, and null is just a value. And we're not going to generate crash or throw exceptions for the nil call method.

　　 3.Nil
>defines the ID of a null class.
　　 defines an empty class, a null pointer to an OBJECTIVE-C class.
Available in Mac OS X v10.0 through Mac OS X v10.4.
Declared in NSObjCRuntime.h.
Declared Inobjc.h
> Sample code:
Class SomeClass = Nil;
Class Anotherclass = [NSString class];

　　 4.NSNull
>the NSNull class defines a singleton object used to represent null values in collection objects (which don ' t allow Nil Values).
The Nsnull class defines a singleton object used to represent the null value of a collection object

> Collection objects cannot contain nil as their specific value, such as Nsarray, Nsset, and Nsdictionary. Accordingly, the nil value is represented by a specific object nsnull. Nsnull provides a single instance for representing the nil value in the object's properties. The default implementation method, Dictionarywithvaluesforkeys: And setvaluesforkeyswithdictionary: automatically converts nsnull and nil to each other, so your object does not require nsnull test operations

IOS about nil and nil and the difference between null and <null>