Re: NSNull and @""
Re: NSNull and @""
- Subject: Re: NSNull and @""
- From: Andrew Farmer <email@hidden>
- Date: Sat, 11 Oct 2008 02:51:34 -0700
On 10 Oct 08, at 19:28, Ian Joyner wrote:
So it's really a workaround for this situation. I think therefore
there are several constructs to represent the same concept of "lack
of presence" – nil, Nil, Null, and NSNull.
Fewer than you think, actually. nil, Nil, and NULL are all internally
synonymous: the only difference is that nil and Nil have traditional
meanings in ObjC. (nil is (id) NULL, and Nil is (Class) NULL). NSNull
is the only odd case out - it's used in situations where a concrete
object is needed, such as in collections.
Sorry, but NULL values need very careful reasoning and handling.
(See C.J. Date "Database Systems" 5.6 Nulls (A Digression).) Thanks
for your insights.
Null values in C (and C++ and ObjC) have much saner semantics than
they do in SQL. In particular, comparisons with NULL aren't treated
specially.
*: It's similar to integer 0 in some respects, but not fully
interchangable._______________________________________________
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