De-serializing and stack sizes

De-serializing and stack sizes

[Alex R. Mosteo]

I'm now facing a problem that I don't know how to solve. I'm going to give
some thought to it over the week-end and any ideas are welcome.

I have some objects that I want to serialize to files. For their creation I
use access types so they get created in heap and there's no problem. I
simply output them with

Item_Type'Output (Stream, Item.all);

However, you see the problem, loading them back is a no-no:

Item : constant Item_Access := new Item_Type'(Item_Type'Input (Stream));

The 'Input attribute is trying to return the object in the stack and so it
doesn't work. My items are around 20 MB in size (now, they could grow
bigger) and I'd like not to have to raise my stack to these sizes (also
making it doable from the environment task would be a plus).

Thanks in advance,

A. Mosteo.

Re: De-serializing and stack sizes

[Dmitry A. Kazakov]

On Fri, 01 Sep 2006 19:57:55 +0200, Alex R. Mosteo wrote:

> I'm now facing a problem that I don't know how to solve. I'm going to give
> some thought to it over the week-end and any ideas are welcome.
> 
> I have some objects that I want to serialize to files. For their creation I
> use access types so they get created in heap and there's no problem. I
> simply output them with
> 
> Item_Type'Output (Stream, Item.all);
> 
> However, you see the problem, loading them back is a no-no:
> 
> Item : constant Item_Access := new Item_Type'(Item_Type'Input (Stream));
> 
> The 'Input attribute is trying to return the object in the stack and so it
> doesn't work. My items are around 20 MB in size (now, they could grow
> bigger) and I'd like not to have to raise my stack to these sizes (also
> making it doable from the environment task would be a plus).

I think what you have is stream attributes for *limited* types. You didn't
declare Item_Type limited, but in fact it is, because you want to prevent
copying it, for example, from the stack to the heap. As we know it didn't
work in Ada 95. It must be different in Ada 05.

For Ada 95, in an analogous case, I wrote a limited types factory of my
own. It was necessary anyway, because stream attributes are not portable.
There are clear disadvantages of such approach (again for Ada 95), because
of single inheritance. In Ada 05, it should be much better, because the
number of ancestors is unlimited as long as they are interfaces.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: De-serializing and stack sizes

[Simon Wright]

"Alex R. Mosteo" <devnull@mailinator.com> writes:

> Item : constant Item_Access := new Item_Type'(Item_Type'Input (Stream));
>
> The 'Input attribute is trying to return the object in the stack and
> so it doesn't work. My items are around 20 MB in size (now, they
> could grow bigger) and I'd like not to have to raise my stack to
> these sizes (also making it doable from the environment task would
> be a plus).

Could you maybe override Item_Access'Input? (or even 'Read? I came
across a GNAT bug a while back, possibly 3.16, where default 'Input
didn't use overridden 'Read if the type was private ..)

Re: De-serializing and stack sizes

[Alex R. Mosteo]

Dmitry A. Kazakov wrote:

> On Fri, 01 Sep 2006 19:57:55 +0200, Alex R. Mosteo wrote:
> 
>> I'm now facing a problem that I don't know how to solve. I'm going to
>> give some thought to it over the week-end and any ideas are welcome.
>> 
>> I have some objects that I want to serialize to files. For their creation
>> I use access types so they get created in heap and there's no problem. I
>> simply output them with
>> 
>> Item_Type'Output (Stream, Item.all);
>> 
>> However, you see the problem, loading them back is a no-no:
>> 
>> Item : constant Item_Access := new Item_Type'(Item_Type'Input (Stream));
>> 
>> The 'Input attribute is trying to return the object in the stack and so
>> it doesn't work. My items are around 20 MB in size (now, they could grow
>> bigger) and I'd like not to have to raise my stack to these sizes (also
>> making it doable from the environment task would be a plus).
> 
> I think what you have is stream attributes for *limited* types. You didn't
> declare Item_Type limited, but in fact it is, because you want to prevent
> copying it, for example, from the stack to the heap. As we know it didn't
> work in Ada 95. It must be different in Ada 05.

I think you mean "what you need"? I see your point if that's the case. My
type isn't intrinsically limited, it wasn't until I hit the stack problem,
now I see it may be a possible approach. 

I see I can use customized stream attributes, but is an artificial need
imposed by the stack limitation. It's not a problem doing it once, but
could become a hassle if necessary more often. That's the reason I'm trying
to get some general solution.

> For Ada 95, in an analogous case, I wrote a limited types factory of my
> own. It was necessary anyway, because stream attributes are not portable.
> There are clear disadvantages of such approach (again for Ada 95), because
> of single inheritance. In Ada 05, it should be much better, because the
> number of ancestors is unlimited as long as they are interfaces.
 
Portability is not an issue in this case, that's a reason I'm going for the
fastest solution possible.

Thanks for your comments,

A. Mosteo.

Re: De-serializing and stack sizes

[Alex R. Mosteo]

Simon Wright wrote:

> "Alex R. Mosteo" <devnull@mailinator.com> writes:
> 
>> Item : constant Item_Access := new Item_Type'(Item_Type'Input (Stream));
>>
>> The 'Input attribute is trying to return the object in the stack and
>> so it doesn't work. My items are around 20 MB in size (now, they
>> could grow bigger) and I'd like not to have to raise my stack to
>> these sizes (also making it doable from the environment task would
>> be a plus).
> 
> Could you maybe override Item_Access'Input? (or even 'Read? I came
> across a GNAT bug a while back, possibly 3.16, where default 'Input
> didn't use overridden 'Read if the type was private ..)

Yep, I can't think of anything better. Thanks for your comments,

A. Mosteo.