Objective-C nil, nil, null and nsnull

In oC, nil, nil, null, and nsnull may often occur. The differences are as follows:

 

Symbol	Value	Meaning
Null	(Void *) 0	Literal null value for C pointers
Nil	(ID) 0	Literal null value for objective-C objects
Nil	(Class) 0	Literal null value for objective-C classes
Nsnull	[Nsnull null]	Singleton object used to represent null

 

1. Nil: the object is empty.

Defines an instance object as a null value. For example:

 

[Objc]View plaincopy

1. Nsobject * OBJ = nil;
2. If (nil = OBJ ){
3. Nslog (@ "obj is nil ");
4. }
5. Else {
6. Nslog (@ "obj is not nil ");
7. }

 

 

Ii. Nil: the class is empty.

Define a class as null. For example:

 

[Objc]View plaincopy

1. Class someclass = nil;
2. Class anotherclass = [nsstring class];

 

 

Iii. null: the pointer to the basic data object is null.

Pointers for various data types in C language are null. For example:

 

[Objc]View plaincopy

1. Intint * pointertoint = NULL;
2. Charchar * pointertochar = NULL;
3. Struct treenode * rootnode = NULL;

 

 

Iv. nsnull

A collection object cannot contain nil as its specific value, such as nsarray, nsset, and nsdictionary. Correspondingly, the nil value is represented by a specific object nsnull. Nsnull providesSingle InstanceIndicates the nil value in the object property.

 

[Objc]View plaincopy

1. @ Interface nsnull: nsobject <nscopying, nssecurecoding>
3. + (Nsnull *) NULL;
5. @ End

 

The unique method NULL: returns the singleton instance of nsnull is provided in the nsnull Singleton class.

For example:

 

[Objc]View plaincopy

1. Nsmutabledictionary * mutabledictionary = [nsmutabledictionary dictionary];
2. Mutabledictionary [@ "somekey"] = [nsnull null]; // sets value of nsnull Singleton for 'somekey'
3. Nslog (@ "keys: % @", [mutabledictionary allkeys]); // @ [@ "somekey"]

 

V. Description:

Technically they're all the same, but in practice they give someone reading your code some hints about what's going on; just like naming classes with a capital letter and instances with lowercase is recommended, but not required.

If someone sees you passing null, they know the specified er expects a C pointer. if they see nil, they know the specified er is expecting an object. if they see nil, they know the handler er is expecting a class. readability.

 

Vi. Note

Here are a few interesting examples:

(1)

 

[Objc]View plaincopy

1. Nsobject * obj1 = [[nsobject alloc] init];
2. Nsobject * obj2 = [nsnull null];
3. Nsobject * obj3 = [nsobject new];
4. Nsobject * obj4;
5. Nsarray * arr1 = [nsarray arraywithobjects: obj1, obj2, obj3, obj4, nil];
6. Nslog (@ "arr1 count: % lD", [arr1 count]); // arr1 count: 3
9. Nsobject * obj1;
10. Nsobject * obj2 = [[nsobject alloc] init];
11. Nsobject * obj3 = [nsnull null];
12. Nsobject * obj4 = [nsobject new];
13. Nsarray * arr2 = [nsarray arraywithobjects: obj1, obj2, obj3, obj4, nil];
14. Nslog (@ "arr2 count: % lD", [arr2 count]); // arr2 count: 0

Why is there three elements in the first array, and the second element in the array is 0? Let's take a look:

 

 

[Objc]View plaincopy

1. Nsobject * OBJ;
2. If (nil = OBJ ){
3. Nslog (@ "obj is nil ");
4. }
5. Else {
6. Nslog (@ "obj is not nil ");
7. }

This output is obj is nil. Nsarray ends with nil. So you know why!

 

(2)

 

[Objc]View plaincopy

1. // An exception occurred!
2. Nsobject * obj1 = [nsnull null];
3. Nsarray * arr1 = [nsarray arraywithobjects: @ "one", @ "two", obj1, @ "three", nil];
4. For (nsstring * STR in arr1 ){
5. Nslog (@ "array object: % @", [STR lowercasestring]);
6. }
8. // Modify
9. Nsobject * obj1 = [nsnull null];
10. Nsarray * arr1 = [nsarray arraywithobjects: @ "one", @ "two", obj1, @ "three", nil];
11. For (nsstring * STR in arr1 ){
12. If (! [STR isequal: [nsnull null]) {
13. Nslog (@ "array object: % @", [STR lowercasestring]);
14. }
15. }

Objective-C nil, nil, null and nsnull