Representation clause of a discriminated record

Representation clause of a discriminated record

[David SAUVAGE]

Hi.all,

For a data frame which length can vary, we have described this data frame as a record, and wish to describe it's representation clause.

What would be a valid representation clause for the following discriminated record ? (taken from the Ada wikibook [1])

   type Discriminated_Record (Size : Natural) is 
    record
       A : String (1 .. Size);
    end record;

[1] https://en.wikibooks.org/wiki/Ada_Programming/Types/record#Discriminated_record

Re: Representation clause of a discriminated record

[Jeffrey R. Carter]

On 08/02/2016 10:04 PM, David SAUVAGE wrote:
> 
> What would be a valid representation clause for the following discriminated record ? (taken from the Ada wikibook [1])
> 
>    type Discriminated_Record (Size : Natural) is 
>     record
>        A : String (1 .. Size);
>     end record;

for Discriminated_Record use record
   Size at 0 range 0 .. 31;
end record;

if Integer'Size = 32. You can't give a representation for A because you don't
know it's size.

-- 
Jeff Carter
"Mr. President, we must not allow a mine-shaft gap!"
Dr. Strangelove
33

Re: Representation clause of a discriminated record

[Per Sandberg]

Well
I would add a Aspect/Pragma "pack" as well, since some architectures 
may insert padding to start objects on boundaries /= 32 bit.

    type Discriminated_Record (Size : Natural) is
       record
          A : String (1 .. Size);
       end record with Pack => True;
    for Discriminated_Record use record
       Size at 0 range 0 .. 31;
    end record;
/Per

Den 2016-08-03 kl. 08:13, skrev Jeffrey R. Carter:
> On 08/02/2016 10:04 PM, David SAUVAGE wrote:
>>
>> What would be a valid representation clause for the following discriminated record ? (taken from the Ada wikibook [1])
>>
>>    type Discriminated_Record (Size : Natural) is
>>     record
>>        A : String (1 .. Size);
>>     end record;
>
> for Discriminated_Record use record
>    Size at 0 range 0 .. 31;
> end record;
>
> if Integer'Size = 32. You can't give a representation for A because you don't
> know it's size.
>

Re: Representation clause of a discriminated record

[david.sauvage]

>On Wednesday, August 3, 2016 at 7:23:02 PM UTC+4, Per Sandberg wrote:
>> Den 2016-08-03 kl. 08:13, skrev Jeffrey R. Carter:

Thanks all for your feedbacks, my wish is about adding A in the representation clause

Re: Representation clause of a discriminated record

[David SAUVAGE]

>On Wednesday, August 3, 2016 at 7:23:02 PM UTC+4, Per Sandberg wrote:
>> Den 2016-08-03 kl. 08:13, skrev Jeffrey R. Carter:


Thanks all for your feedbacks, my wish is about adding A in the representation clause 

Re: Representation clause of a discriminated record

[Randy Brukardt]

"David SAUVAGE" <david.sauvage@adalabs.com> wrote in message 
news:c3625d1a-62a5-4507-9c37-1baa81c74f61@googlegroups.com...
> >On Wednesday, August 3, 2016 at 7:23:02 PM UTC+4, Per Sandberg wrote:
>>> Den 2016-08-03 kl. 08:13, skrev Jeffrey R. Carter:
>
>
> Thanks all for your feedbacks, my wish is about adding A in the 
> representation clause

You can't do that, the size of items in representation clauses have to be 
static (that is, known at compile-time). It doesn't make sense to specify 
the location of something of unknown properties.

Note that by giving all of the other components, you effectively place the A 
component in a particular location (the one left over). But I'd suggest 
using some sort of static data structure.

                              Randy.