objective c - What describes nil best? What's that really?

Question

Currently I understand it as a kind of "empty object". But what's it really?

Answer 1

Objective-C objects

First of all, when you call this:

id someObject = [NSArray array];

someObject isn't the array object directly but only a pointer to it. That means, if someObject is equal to 0x1234 there's an object at that address in the memory.

That's the reason why

id someOtherObject = someObject;

doesn't copy the object. Both pointers point now to the same object.

Pointer to 0x0

So, how is nil defined? Let's take a look at the source code:

objc.h

#define nil __DARWIN_NULL    /* id of Nil instance */

_types.h

#ifdef __cplusplus
…
#else /* ! __cplusplus */
#define __DARWIN_NULL ((void *)0)
#endif /* __cplusplus */

Looks like nil is a pointer to the address 0x0.

So what?

Let's see what the Objective-C Programming Reference has to say:

Sending Messages to nil

In Objective-C, it is valid to send a message to nil—it simply has no effect at runtime. There are several patterns in Cocoa that take advantage of this fact. The value returned from a message to nil may also be valid: …

The returned values are either nil, 0 or a struct with all variables initialized to 0. Which one it is depends on the expected return type. There is an explicit check in the objective-c runtime for messages to nil, that means it's really fast.

Nil, nil, NULL

Those are the 3 types. Here are all the definitions:

#define Nil __DARWIN_NULL   /* id of Nil class */
#define nil __DARWIN_NULL   /* id of Nil instance */
#define NULL __DARWIN_NULL
#define __DARWIN_NULL ((void *)0)

As can be seen, they are all exactly the same. Nil and nil are defined by Objective-C, NULL comes from C.

What's the difference then? It's only about style. It makes the code more readable.

• Nil is used as a non-existent class: Class someClass = Nil.
• nil is used as a non-existent instance: id someInstance = nil.
• NULL is a pointer to a non-existent memory part: char *theString = NULL.

Short

nil isn't an empty object but a non-existent one. A method -getSomeObject doesn't return an empty object if it doesn't exist but returns nil which tells the user that there is no object.

Maybe this makes sense: (Both would compile and run.)

if (anObject == nil) {   // One cannot compare nothing to nothing,
                         // that wouldn't make sense.

if (anObject) {          // Correct, one checks for the existence of anObject