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 *)contextContext 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
nilis what to use for theidtype (and pointers to objects). - C style says that
NULLis what you use forvoid *. - C++ style typically says that you should just use
0.
I typically use the variant that matches the language where the type is declared.