nil -> Null-pointer to objective- c object
Nil -> Null-pointer to objective- c class
null-> null pointer to primitive type or absence of data.
例如:
NSURL *url = nil;
Class class = Nil;
int *pointerInt = NULL;
nil是令一个对象指针为空,Nil是令一个类指针为空,NULL是令基本数据类型指针为空。