1.nil: 指向一個(gè)對(duì)象的空指針, 對(duì)Objective-C中id對(duì)象付空值.
2.Nil：指向一個(gè)類的空指針, a null pointer to a Objective-C class.表示對(duì)類進(jìn)行付空值.
3.NULL：指向其它類型的空指針（如：C類性，基本類型）A null pointer to anything else, is for C-style memory pointers.
4.NSNull：Objective-C中的一個(gè)類，包含 + （NSNull *)null；單例方法，多用于集合中值為空的對(duì)象。
For example:
NSString *string = [NSString stringWithFormat:@"Test"]; string = nil;

NSURL *url = nil;

NSArray * array = [NSArray arrayWithObjects:@"test",@"test1" ,nil];
[array release];

if (array)
{

//僅僅對(duì)數(shù)組release，并沒有賦空值的話，在程序某個(gè)地方如果繼續(xù)對(duì)數(shù)組操縱，程序直接崩潰
NSString * string = [array objectAtIndex:0];
NSLog(@"%@",string);
}

Class class = Nil;
int *pointer = NULL;
NSArray *array = [NSArray arrayWithObjects:[[NSObject alloc] init], [NSNull null], [[NSObject alloc] init], [NSNull null], nil];

NSArray 和 NSDictionary 都是以nil結(jié)尾;

NSMutableDictionary *dictionary = [[NSMutableDictionary alloc] init];
[dictionary setObject:nil forKey:@"Key-nil"]; 會(huì)crash
[dictionary setObject:[NSNull null] forKey:@"Key-nil"];不會(huì)引起crash
[dictionary setObject:(value == nil ? [NSNull null] : value) forKey:@"key"];