在OC中可能经常会遇到 nil，Nil，NULL和NSNull，下面分析一下之间的区别：
| Symbol | Value | Meaning |
|---|---|---|
| NULL | (void *)0 | literal null value for C pointers |
| nil | (id)0 | literal null value for Objective-C objects |
| Nil | (Class)0 | literal null value for Objective-C classes |
| NSNull | [NSNull null] | singleton object used to represent null |
一、nil：对象为空
定义某一实例对象为空值。例如：
NSObject* obj = nil;
if (nil == obj) {
NSLog(@"obj is nil");
}
else {
NSLog(@"obj is not nil");
}
二、Nil：类为空
定义某一类为空。例如：
Class someClass = Nil;
Class anotherClass = [NSString class];
三、NULL：基本数据对象指针为空
用于c语言的各种数据类型的指针为空。例如：
int *pointerToInt = NULL; char *pointerToChar = NULL; struct TreeNode *rootNode = NULL;
四、NSNull
集合对象无法包含 nil 作为其具体值，如NSArray、NSSet和NSDictionary。相应地，nil 值用一个特定的对象 NSNull 来表示。NSNull 提供了一个单一实例用于表示对象属性中的的nil值。
@interface NSNull : NSObject <NSCopying, NSSecureCoding> + (NSNull *)null; @end
在NSNull单例类中，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、说明：
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
下面附带几个有趣的例子：
（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];
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
为啥第一个数组元素有三个，而第二个数组元素为0.先看看：
NSObject* obj;
if (nil == obj) {
NSLog(@"obj is nil");
}
else {
NSLog(@"obj is not nil");
}这个输出：obj is nil。而NSArray是以nil结尾的。所以知道原因了吧！（2）
//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]);
}
}