In observeValueForKeyPath:ofObject:change:context: - why do the docs use NULL instead of nil when not specifying a context pointer?
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in my opinion, object is nil, class is Nil, and NULL using for object or classThanh Vũ Trần– Thanh Vũ Trần2012-11-09 01:45:23 +00:00Commented Nov 9, 2012 at 1:45
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5 Answers
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).
answered Feb 17, 2009 at 16:23
14 Comments
erikprice
But since the type of
context in observeValueForKeyPath:ofObject:change:context: is void *, doesn't that mean that the data passed as the context could be an object pointer? I would think that to be a common case. That's why I'm confused as to why the docs always use NULL instead of nil.
Paul Tomblin
The type of context: in that method is "void *". "nil" is not a "void *", but NULL is.
Peter Hosey
You can. void * is any pointer. Nonetheless, you are absolutely right that NULL is the correct constant there.
Peter Hosey
They said void *. NULL is for void * and nil is for id. Therefore, you pass NULL. If you pass nil, you are lying to your reader, who will think this method takes an id.
Paul Tomblin
Or to think of it another way, NULL is a broader type, and nil is a subset of NULL. In general, use the broadest type you can get away with (ie in Java, write your method to expect a Collection instead of a Vector, unless you need something specific from Vector)
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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 *).
answered Feb 17, 2009 at 16:21
4 Comments
jbat100
@WTP if you read through MacTypes.h, it declares #define nil NULL
Brennan
That is very interesting. It seems it does not matter than other than for style points. It's like YES/TRUE and NO/FALSE.
Micaiah Wallace
@Brennan, That's not entirely true, just because nil is defined as NULL doesn't mean there's some other hidden implementation behind the scenes. For example, IBAction is defined as void but it has a different meaning when using interface builder when displaying methods to attach to actions on buttons and such.
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.
Comments
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 *).
1 Comment
Pang
Isn't this pretty much a repeat of this existing answer?