在OC中可能經常會遇到 nil，Nil，NULL和NSNull，下面分析一下之間的區別：
一、nil：對象為空定義某一實例對象為空值。例如：
NSObject* obj = nil;? ? ? ? ??
if (nil == obj) {? ? ? ? ? ? ??
? ? ? ? ?NSLog(@"obj is nil");? ? ? ? ??
} else {? ? ? ? ? ? ??
? ? ? ? NSLog(@"obj is not nil");? ? ? ? ??
}?
?二、Nil：類為空定義某一類為空。例如：Class someClass = Nil;? ? ? ? ? Class anotherClass = [NSString class];? 三、NULL：基本數據對象指針為空用于c語言的各種數據類型的指針為空。例如：intint *pointerToInt = NULL;? charchar *pointerToChar = NULL;? struct TreeNode *rootNode = NULL;? 四、NSNull集合對象無法包含 nil 作為其具體值，如NSArray、NSSet和NSDictionary。相應地，nil 值用一個特定的對象 NSNull 來表示。NSNull 提供了一個單一實例用于表示對象屬性中的的nil值。@interface NSNull : NSObject+ (NSNull *)null;
@end
在NSNull單例類中，提供了唯一的方法null：Returns the singleton instance of NSNull.
例如：
NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];
mutableDictionary[@"someKey"] = [NSNull null]; // Sets value of NSNull singleton for `someKey`
NSLog(@"Keys: %@", [mutableDictionary allKeys]); // @[@"someKey"]
五、說明：
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.
六、注
下面附帶幾個有趣的例子：
（1）
NSObject *obj1 = [[NSObject alloc] init];
NSObject *obj2 = [NSNull null];
NSObject *obj3 = [NSObject new];
NSObject *obj4;
NSArray *arr1 = [NSArray arrayWithObjects:obj1, obj2, obj3, obj4, nil 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 nil];
NSLog(@"arr2 count: %ld", [arr2 count]);? //arr2 count: 0
為啥第一個數組元素有三個，而第二個數組元素為0.先看看：
NSObject* obj;
if (nil == obj) {
NSLog(@"obj is nil");
}
else {
NSLog(@"obj is not nil");
}
這個輸出：obj is nil。而NSArray是以nil結尾的。所以知道原因了吧！
（2）
//有異常！
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]);
}
}
