NULL vs nil in Objective-C

问题:

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

回答1:

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).

回答2:

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 *).

回答3:

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.

回答4:

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.

回答5:

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 *).