On Thursday, 7 August 2014 at 16:53:24 UTC, H. S. Teoh via
Digitalmars-d wrote:
On Thu, Aug 07, 2014 at 11:46:48AM +0000, via Digitalmars-d
wrote:
AAs are (like regular dynamic arrays) initialized to `null`.
On first
modification (i.e. assignment of an element), memory is
allocated and
the AA variable updated to point at it.
However, is there a guarantee that AAs are never reallocated
once they
are initialized, i.e. is it then safe to assume that changes
made
through a copy of an AA variable (e.g. pass by value) are
visible
through the original variable?
The current implementation behaves like this, but the language
reference does not mention it, AFAICS. I'd like to amend the
documentation if this behaviour is reliable.
[...]
I'm not the one to make the decision, but I'd vote for
codifying this
behaviour in the language reference. From what I understand, at
least,
it seems that this is the intention anyway, and the current
implementation certainly suggests so. Otherwise, it leads to
strange
awkward semantics where passing AA's around may sometimes
change the
original "view" of it, and sometimes not.
It's really just the .init value of null which causes odd
behaviour with
empty AA's. Fun fact:
void changeAA(int[string] aa) {
aa["a"] = 123;
}
// Null AA:
int[string] aa1; // null
assert(aa1.length == 0);
changeAA(aa1); // no effect
for most of the new users the WAT part is actually here :-)
In all other occations AA behaves just like a reference type: You
want to changeAA with assignment and you get an empty(null) AA,
you should be able to change it.
refering to an null AA and not wanting to modify it is not a
common case.
So I'd like to suggest a rule here similar to what assignment
does to null AA:
If someone refers to an uninitialized null AA ( in
implementation, that maybe, a copy of a null AA struct happens),
allocate it first.
assert(aa1.length == 0);
// Empty but non-null AA:
int[string] aa2; // null
aa2["a"] = 0;
aa2.remove("a"); // empty but non-null
assert(aa2.length == 0);
changeAA(aa2); // has effect!
assert(aa2.length == 1); // WAT :-)
T
