nil、Nil、NULL、NSNull、[NSNull null] in Objective-C

很多初学者对nil，Nil，NULL，NSNull，[NSNull null]这五个概念很混淆!先借用网上的解释：

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

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

NULL: A null pointer to anything else. (primitive type or absence of data)
 

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

[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;
nil和NULL从字面意思来理解比较简单，nil是一个对象，而NULL是一个值，我的理解为nil是将对象设置为空，而NULL是将基本类型设置为空的。而且我们对于nil调用方法，不会产生crash或者抛出异常。
看一下用法
NSURL *url = nil；
Class  class = Nil；
int *pointerInt = NULL；
nil是一个对象指针为空，Nil是一个类指针为空，NULL是基本数据类型为空。

 这五个概念很容易混淆，在一些场合下他们具有同样的用法。首先解释一下“空指针”意思，空指针是指该指针未指向一块有意义的内存区。如int *p; int *p = NULL;

        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");
        }

        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]);    //count: 3 因为obj=nil，在加入obj4时就结束添加对象
        
        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]);   //count: 0，因为obj1=nil，所以后面的对象没有添加进去

[NSNull null]通常作为占位符作用，如下：

        NSObject *obj1 = [NSNull null];
        NSArray *arr1 = [NSArray arrayWithObjects:@"One", @"TWO", obj1,@"three",nil];
        for (NSString *str in arr1) {
            NSLog(@"array object: %@", str);
        } //result：One、Two、<NULL>、three

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