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.( #define nil ((id)0)  )
Nil: A null pointer to an Objective-C class.
 NULL: A null pointer to anything else. ?( #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).
[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;

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