The difference between Nil, NULL, Nil, and Nsnull in iOS

1.nil

>defines the ID of a null instance.

Defines an instance that is empty, pointing to an empty pointer to an object in OC.

> Sample code:

NSString *somestring = nil;

Nsurl *someurl = nil;

ID someobject = nil;

if (Anotherobject = = nil)//do something

> When you release an object, it is best to assign them to nil, which ensures security, which can cause a crash if you do not assign a value of nil.

Nsarray * array = [Nsarray arraywithobjects:@ "test", @ "Test1", nil];

[Array release];

if (array)

{

Only the array release, not assigned to nil, somewhere in the program if you continue to operate the array, the program directly crashes

NSString * string = [array objectatindex:0];

NSLog (@ "%@", string);

}

2.NULL

>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, nil objects are designed to be associated with NULL null pointers. The difference is that nil is an object, and null is just a value. And we call the method on the nil without generating crash or throwing an exception.

3.Nil

>defines the ID of a null class.

To define an empty 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 single instance object to represent the null value of a collection object

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