问题
This is sample code:
NSDictionary *myDictionary = [NSDictionary dictionary];
NSNumber *myNumber = [myDictionary valueForKey: @\"MyNumber\"];
NSLog(@\"myNumber = %@\", myNumber); // output myNumber = (null)
if (myNumber == nil)
NSLog(@\"test 1 myNumber == nil\");
if (myNumber == NULL)
NSLog(@\"test 2 myNumber == NULL\");
if ([myNumber isEqual:[NSNull null]])
NSLog(@\"test 3 myNumber == [NSNull null]\");
When should I use nil, NULL and [NSNull null]?
回答1:
You can use nil about anywhere you can use null. The main difference is that you can send messages to nil, so you can use it in some places where null cant work.
In general, just use nil.
回答2:
They differ in their types. They're all zero, but NULL is a void *, nil is an id, and Nil is a Class pointer.
回答3:
nil is an empty value bound/corresponding with an object (the id type in Objective-C). nil got no reference/address, just an empty value.
NSString *str = nil;
So nil should be used, if we are dealing with an object.
if(str==nil)
NSLog("str is empty");
Now NULL is used for non-object pointer (like a C pointer) in Objective-C. Like nil , NULL got no value nor address.
char *myChar = NULL;
struct MyStruct *dStruct = NULL;
So if there is a situation, when I need to check my struct (structure type variable) is empty or not then, I will use:
if (dStruct == NULL)
NSLog("The struct is empty");
Let’s have another example, the
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
Of key-value observing, the context should be a C pointer or an object reference. Here for the context we can not use nil; we have to use NULL.
Finally the NSNull class defines a singleton object used to represent null values in collection objects(NSArray, NSDictionary). The [NSNull null] will returns the singleton instance of NSNull. Basically [NSNull null] is a proper object.
There is no way to insert a nil object into a collection type object. Let's have an example:
NSMutableArray *check = [[NSMutableArray alloc] init];
[check addObject:[NSNull null]];
[check addObject:nil];
On the second line, we will not get any error, because it is perfectly fair to insert a NSNull object into a collection type object. On the third line, we will get "object cannot be nil" error. Because nil is not an object.
回答4:
NULL and nil are equal to each other, but nil is an object value while NULL is a generic pointer value ((void*)0, to be specific). [NSNull null] is an object that's meant to stand in for nil in situations where nil isn't allowed. For example, you can't have a nil value in an NSArray. So if you need to represent a "nil", you can use [NSNull null].
回答5:
I've found the following:
objc.h
#define Nil __DARWIN_NULL /* id of Nil class */
#define nil __DARWIN_NULL /* id of Nil instance */
_types.h
#define __DARWIN_NULL ((void *)0)
stddef.h
#undef NULL
#ifdef __cplusplus
#undef __null // VC++ hack.
#define NULL __null
#else
#define NULL ((void*)0)
#endif
MacTypes.h
#ifndef NULL
#define NULL __DARWIN_NULL
#endif /* ! NULL */
#ifndef nil
#define nil NULL
#endif /* ! nil */
The way it looks, there's no difference but a conceptual one.
回答6:
Beware that if([NSNull null]) returns true.
回答7:
They both are just typecast zero's. Functionally, there's no difference between them. ie.,
#define NULL ((void*)0)
#define nil ((id)0)
There is a difference, but only to yourself and other humans that read the code, the compiler doesn't care.
One more thing nil is an object value while NULL is a generic pointer value.
回答8:
To expand on a comment from @cobbal:
MacTypes.h contains:
#ifndef nil
#define nil NULL
#endif
回答9:
In modern OS X and iOS SDKs:
nilandNilandNULLare identical in Objective-C and in Objective-C++ before C++11.nilandNilandstd::nullptrare identical in Objective-C++ with C++11.
Stylistically, many people prefer to use nil for Objective-C objects and NULL or nullptr for other pointer types. I myself now use nil everywhere.
[NSNull null] is a singleton object use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an NSArray or NSDictionary). Number and Value Programming Topics: Using NSNull
回答10:
As already mentioned, they are the same, but I use either the one or the other depending on the language in which the corresponding framework was written.
For everything related to Objective-C, I use nil. For example:
- (BOOL)doSomethingWithObjectsInArray:(NSArray *)anArray {
if (anArray == nil) return NO;
// process elements
...
}
However, when checking validity of data models from a C-framework (like AddressBook framework and CoreFoundation), I use NULL. For example:
- (BOOL)showABUnknownPersonVCForABRecordRef:(ABRecordRef)aRecord {
if (aRecord == NULL) return NO;
// set-up the ABUnknownPersonViewController and display it on screen
..
}
This way, I have subtle clues in my code if I'm dealing with Obj-C or C based code.
回答11:
There is a difference in some contexts.
Literally, Null is a character: ASCII 0.
Nil is equivalent to blank, no value.
Depending on the programming context, this can be a big difference.
回答12:
nil is an object pointer to nothing. Although semantically distinct from NULL, they are technically equivalent to one another.
On the framework level, Foundation defines NSNull, which defines a class method, +null, which returns the singleton NSNull object. NSNull is different from nil or NULL, in that it is an actual object, rather than a zero value.
Additionally, in Foundation/NSObjCRuntime.h, Nil is defined as a class pointer to nothing.
Refer this for further info - nil / Nil / NULL / NSNull
回答13:
Use NULL for example when you invoke an Objective-C method with an output parameter of type (NSError **).
I see lots of example code on the web where people provide nil instead of NULL in this case. This is because it's a pointer to a pointer and thus not directly an Objective-C object type. As said above, nil should be used for Objective-C object types.
回答14:
nil means absence of value while NULL represent No Object,
NSArray *array = @[@"Hello World !", @101,[NSNULL null] ];
Here [NSNULL null] is an object which means no object, at the same time you cannot add nil to indicate absence of object.
you can use both nil and [NSNUll null] for checking too.
回答15:
This will help you to understand the difference between nil, NIL and null.
The below link may help you in some way:
http://nshipster.com/nil/
nil -> literal null value for Objective-C objects.
Nil -> literal null value for Objective-C classes.
NULL -> literal null value for C pointers.
NSNULL -> singleton object used to represent null.
回答16:
Basically: nil: null pointer on an object and null: is for other type pointer
来源：https://stackoverflow.com/questions/1564410/when-should-i-use-nil-and-null-in-objective-c