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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