question about passing associative array to a function

[John Colvin via Digitalmars-d-learn]

On Sunday, 11 May 2014 at 14:46:35 UTC, rbutler wrote:
> I have searched and can not understand something about passing 
> AAs to a function.
> I have reduced the gist of the question to a tiny program below.
> If I put "ref"  in the function stmt it works, i.e.:
>         ref int[int] aa
> My confusion is that AAs are supposed to be passed as refs 
> anyway, so I do
> not understand why I should have to use ref to make it work.
>
> Related, it also works if I UN-comment the line    d[9] = 9;
>
> Thanks for any helpful comments you can make.
> --rbutler
>
> import std.stdio;
>
> void test(int[int] aa, int x) {
>     aa[x] = x;
>     aa[8] = 8;
> }
>
> void main() {
>     int[int] d;
>     writeln(d.length);
>     // d[9] = 9;
>     test(d, 0);
>     writeln(d);
> }

There are problems with the implementation of associative arrays. 
What you are seeing above is a consequence of the associative 
array not being correctly initialised (I think...).

I often create my associative arrays with the following function 
to avoid the problem you're having:

/// Hack to properly initialise an empty AA
auto initAA(T)()
{
	T t = [typeof(T.keys[0]).init : typeof(T.values[0]).init];
	t.remove(typeof(T.keys[0]).init);
	return t;
}

import std.stdio;

void test(int[int] aa, int x) {
     aa[x] = x;
     aa[8] = 8;
}

void main() {
     int[int] d = initAA!(int[int]);
     test(d, 0);
     writeln(d);
}