NULL vs nil in Objective-C

In `observeValueForKeyPath:ofObject:change:context:` - why do the docs use `NULL` instead of `nil` when not specifying a context pointer?

They're technically the same thing (0), but nil is usually used for an Objective-C object type, while NULL is used for c-style pointers (void *).

They're technically the same thing and differ only in style:

- Objective-C style says `nil` is what to use for the `id` type (and pointers to objects).
- C style says that `NULL` is what you use for `void *`.
- C++ style typically says that you should just use `0`.

I typically use the variant that matches the language where the type is *declared*.

`nil` should only be used in place of an `id`, what we Java and C++ programmers would think of as a pointer to an object. Use `NULL` for non-object pointers.

Look at the declaration of that method:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
change:(NSDictionary *)change context:(void *)context

Context is a `void *` (ie a C-style pointer), so you'd definitely use `NULL` (which is sometimes declared as `(void *)0`) rather than `nil` (which is of type `id`).

`NULL` is the **`C equivalent`** of `nil`, a pointer to nothing;

where **`nil is zero typed as id`**,

**`NULL is zero typed as void*`**.

**One important point** you can’t send a message to NULL. So it is preferred to use nil in objective-C at many places.

They almost are the same thing except,

`nil` is used in an Objective-C style.
where `NULL` is for C type pointers and is typdef'ed to `(void *)`.