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