question about passing associative array to a function

[rbutler via Digitalmars-d-learn]

On Sunday, 11 May 2014 at 15:22:29 UTC, John Colvin wrote:
> 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);
> }

OK. :-)
That makes it difficult to talk about in a classroom, especially 
when trying to stress
adherence to the principle of least surprise.
Thanks very much for the quick reply.