原文地址：http://blog.csdn.net/yhawaii/article/details/7442529
nil: A null pointer to an Objective-C object. （一个Objective-C对象的空指针）
( #define nil ((id)0) )
Nil: A null pointer to an Objective-C class. （一个Objective-C类的空指针）
NULL: A null pointer to anything else, is for C-style memory pointers. （一个C类型的空指针）
( #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). （在不允许使用nil的集合对象中，可用该类的单例对象[NSNull null]表示空值）
[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]); } }