Objective-c 中 nil, Nil, NULL和NSNull的区别

nil: A null pointer to an Objective-C object.

( #define nil ((id)0)  )

Nil: A null pointer to an Objective-C class.

NULL: A null pointer to anything else,  is for C-style memory pointers.

( #define NULL ((void *)0)  )

NSNull: A class defines a singleton object used to represent null values in collection objects (which don't allow nil values).

[NSNull null]: The singleton instance of NSNull.

 

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 receiver expects a C pointer. If they see nil, they know the receiver is expecting an object. If they see Nil, they know the receiver is expecting a class. Readability.

 

if obj is nil , [obj message] will return NO, without NSException
if obj is NSNull , [obj message will throw a NSException

Demo1:

[NSApp beginSheet:sheet
modalForWindow:mainWindow

modalDelegate:nil       //pointing to an object
didEndSelector:NULL     //pointing to a non object/class
contextInfo:NULL];      //pointing to a non object/class

Demo2:

NSObject *obj1;
if (obj1 != nil) {
NSLog(@"object is not nil");
}else
{
NSLog(@"object is nil");
}

testClass *c1;
if (c1 != Nil) {
NSLog(@"class is not Nil");
}else
{
NSLog(@"class is Nil");
}

int *money;
if (money != NULL) {
NSLog(@"money is not NULL");
}else
{
NSLog(@"money is NULL");
}

Demo3:

NSObject *obj1 = [[NSObject alloc] init];
NSObject *obj2 = [NSNull null];
NSObject *obj3 = [NSObject new];
NSObject *obj4;
NSArray *arr1 = [NSArray arrayWithObjects:obj1, obj2, obj3, obj4, nil];
NSLog(@"arr1 count: %ld", [arr1 count]);    //arr1 count: 3

NSObject *obj1;
NSObject *obj2 = [[NSObject alloc] init];
NSObject *obj3 = [NSNull null];
NSObject *obj4 = [NSObject new];
NSArray *arr2 = [NSArray arrayWithObjects:obj1, obj2, obj3, obj4, nil];
NSLog(@"arr2 count: %ld", [arr2 count]);   //arr2 count: 0

Demo4:

//有异常！
NSObject *obj1 = [NSNull null];
NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil];
for (NSString *str in arr1) {
NSLog(@"array object: %@", [str lowercaseString]);
}

//修改
NSObject *obj1 = [NSNull null];
NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1, @"three" ,nil];
for (NSString *str in arr1) {
if (![str isEqual:[NSNull null]]){
NSLog(@"array object: %@", [str lowercaseString]);
}
}