Difference between nil, NIL and, null in Objective-C

I want to know the difference between nil, NIL and null. I've googled around and found this:

nil -> null pointer to Objective-C object

NIL -> null pointer to Objective-C class

null -> null pointer to primitive type or absence of data

But I'm not able to understand the terms "Objective-C object" and "class" clearly.

Please explain this to me. Also, is there any word like NSNull or NSNil in Objective-C? If so, then please explain for what it is for.

nil is the literal null value for Objective-C objects, corresponding to the abstract type id or any Objective-C type declared via @interface. For instance:

NSString *someString = nil;
NSURL *someURL = nil;
id someObject = nil;

if (anotherObject == nil) // do something

Nil is the literal null value for Objective-C classes, corresponding to the type Class. Since most code doesn’t need variables to reference classes, its use is not common. One example is:

Class someClass = Nil;
Class anotherClass = [NSString class];

NULL is the literal null value for arbitrary C pointers. For instance,

int *pointerToInt = NULL;
char *pointerToChar = NULL;
struct TreeNode *rootNode = NULL;

NSNull is a class for objects that represent null. In fact, there’s only one object, namely the one returned by +[NSNull null]. It is different from nil because nil is a literal null value, i.e., it isn’t an object. The single instance of NSNull, on the other hand, is a proper object.

NSNull is often used in Foundation collections since they cannot store nil values. In the case of dictionaries, -objectForKey: returns nil to indicate that a given key has no corresponding object in the dictionary, i.e., the key hasn’t been added to the dictionary. If you want to make it explicit that you have a certain key but it doesn’t have a value yet, you can use [NSNull null].

For instance, the following throws an exception because dictionaries cannot store nil values:

NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setObject:nil forKey:@"someKey"];

On the other hand, the following code is valid since [NSNull null] is a non-nil object:

NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setObject:[NSNull null] forKey:@"someKey"];

It’s worth mentioning that Foundation collections have initialisers that use nil as a marker for the end of a list of objects without having to specify the number of elements in the list. This can only happen because nil cannot be stored in a Foundation collection. For instance,

NSArray *array = [NSArray arrayWithObjects:@"one", @"two", nil];

As for NIL or NSNil, there are no such things in Objective-C or Apple Foundation.

I am not sure but i think nil should only be used in place of an id, what Java and C++ programmers would think of as a pointer to an object. Use NULL for non-object pointers.

nil is usually used for an Objective-C object type, while NULL is used for c-style pointers

Nil,Null and nil are used with below

1> Nil for Objective c Class

2> nil for Objective c object

3> Null for C pointer

Example:

1>Class A=Nil;

2>NSString strName=nil;

3>char *pointerChar = NULL;

Suppose you have a class MyClass then by convention nil is used if you want to initialize its instance to null value (same as null in java) i.e.

MyClass *obj = nil;

and if you want to initialize a primitive pointer to null value (same as in c) you use

int *ptr = NULL; 

and if you want to initialize to Class reference to null value (same as null in java) then use

Class classRefOfMyClass = Nil;

It's just a convention otherwise Nil or nil have same meaning and perhaps NULL , nil or Nil all are same.

Here is the definition for these in objc.h file

#ifndef Nil
# if __has_feature(cxx_nullptr)
#   define Nil nullptr
# else
#   define Nil __DARWIN_NULL
# endif
#endif

#ifndef nil
# if __has_feature(cxx_nullptr)
#   define nil nullptr
# else
#   define nil __DARWIN_NULL
# endif
#endif

And in stddef.h

#define NULL ((void*)0)

And the definition of __DARWIN_NULL in _types.h

#define __DARWIN_NULL ((void *)0)

So there is no difference logically. The main idea here is to initialize a pointer whether C or Objective-C to 0. If you have knowledge of C then you can assign

int *ptr = 0;

without type casting 0 to a pointer. As you don't need to typecast 0 to assign it to a pointer.

In short they all are 0 and nothing else.

This will help you to understand the difference between nil,NIL and null.

> All three of these values represent null, or zero pointer, values. The > difference is that while NULL represents zero for any pointer, nil is > specific to objects (e.g., id) and Nil is specific to class pointers. > It should be considered a best practice of sorts to use the right null > object in the right circumstance for documentation purposes, even > though there is nothing stopping someone from mixing and matching as > they go along.

The below link may help you in some way:

http://nshipster.com/nil/

Here is some important part from the link:

nil, NIL and null. is depended on your requirement.

NSNull

collections like NSArray and NSDictionary not being able to contain nil values.

NSMutableDictionary *MymutableDictionary = [NSMutableDictionary dictionary];
MymutableDictionary[@"someKey"] = [NSNull null]; // Sets value of NSNull singleton for "someKey"
NSLog(@"Keys: %@", [mutableDictionary allKeys]);

nil

all pointers that object has to other objects begin as nil, so it's unnecessary to, for instance, set self.(association) = nil in init methods.

In other languages, like C++, this would crash your program, but in Objective-C, invoking a method on nil returns a zero value.

if (name != nil)
{
........
}

Symbol Value Meaning

nil (id)0 literal null value for Objective-C objects

Nil (Class)0 literal null value for Objective-C classes