data containers

[Koroskin Denis]

On Fri, 04 Jul 2008 03:22:03 +0400, Jarrett Billingsley  
<kb3ctd2 at yahoo.com> wrote:

> "Moritz" <mpipahl at gspgmbh.com> wrote in message
> news:g4jm5u$1p9r$1 at digitalmars.com...
>> torhu schrieb:
>>> Does it work if you make this change?
>>>
>>> -     if(layer <= layer_count)
>>> +     if(layer < layer_map.length)
>>
>> Well, my program DOES work, but I doesnt do what I want it to, because  
>> Im
>> now unable to insert ScreenElements into the array.
>>
>> Do I need to initialize the array slots before I assign an object to  
>> them?
>> The way I see it, the array has length 0 at start, and I cant put  
>> anything
>> in slot 0 or Ill get an array out of bounds error :-(
>
> No, it's because you're using a temp variable when you shouldn't be.
>
> ScreenElement[] single_layer = layer_map[layer];
> single_layer ~= new_element;
>
> That will get a reference the layer map at index `layer`, but when you
> concatenate, only the local single_layer will refer to the newer, longer
> array.  Just do it in place:
>
> layer_map[layer] ~= new_element;
>
> (Also if you don't feel like typing all those long types for variables,  
> it's
> completely unnecessary.
>
> auto single_layer = layer_map[layer];
>
> Dah.)
>
>> I guess my next problem after storing them will be to keep the garbage
>> collector from deleting them after I leave the function where the
>> ScreenElements  are created and stored into the array, or does the array
>> reference protect the ScreenElements from GC?.
>
> Arrays are on the heap.  As long as something references said array, the  
> GC
> won't collect objects the array holds.  That's kind of the point.  The GC
> isn't out to make your life hard; quite the opposite in fact :)
>
> The GC is actually fairly conservative, and if it's not sure whether
> something is referenced or not (i.e. say you have some random-looking  
> data,
> like sound data, and the GC says "hm, this looks like a pointer into an
> object"), it won't collect it.
>
>

Or, better, use:
auto single_layer = layer in layer_map;

to avoid lookup on insertion:
layer_map[layer] ~= new_element; -> single_layer ~= new_element;

right?