On 19/02/2012 03:31, Daniel Murphy wrote:
"Ben Davis"<[email protected]> wrote in message
news:[email protected]...
I've seen some line-blurring between 'null' and 'empty' for dynamic arrays
(non-associative). Specifically, I read that array.init returns null for
both static and dynamic, but I think I also read that a dynamic array's
default value is the empty array. I also observed that null~[1] == [1],
and I wondered if actually 'null' becomes an empty array when cast to
dynamic array and they're effectively the same thing.
null is not the same thing as an empty array, and I'm not aware of any
situations where null will implicitly turn into one.
null == { length == 0, ptr == null }
empty == { length == 0, ptr != null }
To make an empty array you (generally) need to allocate memory for it, and
having this happen implicitly would be a problem.
So 'null' implicitly turns into { length == 0, ptr == null } when
implicitly cast to the array type (and then it behaves like an empty
array in many situations). That needs documenting :) Coming from any of
C, C++ or Java, you would think of null (or NULL) as a pointer to 0,
which will crash if you try to dereference it in any way - so the fact
that (null array).length is valid and gives 0 is not obvious!
Thanks for explaining it to me in any case :)
If I'm right, then the same could be true for assoc arrays - that 'null'
cast to an assoc array type becomes an empty assoc array. Which would
explain the magic you're seeing.
This is not what's happening.
With the lvalue AA lookup, the call turns into this:
*_d_aaGet(&AA, keyinformation ...) = value;
Because it passes a pointer to the actual AA variable, if the AA doesn't
exist it is created. All of the rvalue AA methods behave the same for null
and empty AAs.
... Cool!
Except for this magic initialization, AAs behave the same as classes - ie a
reference type.
That's not quite true, because 'length' is passed around by value
alongside the reference, leading to semantics you could never reproduce
with classes, unless I'm mistaken.
