NULL vs nil in Objective-C

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).

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.