Stupid question about AA. The following code works but I don't undrstand why?!

[Uranuz via Digitalmars-d-learn]

On Saturday, 9 April 2016 at 18:27:11 UTC, ag0aep6g wrote:
> On Saturday, 9 April 2016 at 18:06:52 UTC, Uranuz wrote:
>> Thanks. It's clear now. AA holds not `array struct` itself 
>> inside, but pointer to it.
>
> How the array is stored in the AA doesn't matter, as far as I 
> can see. The point is that you obtain a pointer to the array 
> struct in the AA, not a copy.
>
> If you had tried it like the following, mapElem would be a copy 
> of the array struct in the AA, and the append would not affect 
> mapka["item"]:
> ----
> string[] mapElem = "item" in mapka ? mapka["item"] : 
> (mapka["item"] = []);
> mapElem ~= ["dog", "cat", "horse", "penguin", "fish", "frog"];
> ----
>
>> So reallocation affects ptr to allocated memory but not 
>> pointer to `array struct`. I think that's it.
>
> Correct.

Another observation is illustrated with the foloving code:
http://dpaste.dzfl.pl/8d68fd5922b7

Because AA and arrays are not created before they were assigned 
some value it leads to inconsistency in behavior. And will 
produce unexpected and hidden bugs that is not good. It's not 
very good side of D's array and AA. But classes could be also 
affected by this *feature* (or bug, as you wish). So we must 
always construct reference semantics types before passing them to 
functions that will modify it. For classes it's obvious but for 
AA and dynamic arrays is not.

Another solution is to pass reference types by *ref*. So you will 
not have such bugs in implementation