Re: Discriminants constraining unconstrained array types

[Damien Carbonne <damien.carbonne@free.fr>]

James Rogers a �crit :

> Matthew Woodcraft wrote:
> >
> > The following rule appears in the GNAT coding style guide:
> >
> > | Do not declare discriminated record types where the discriminant is
> > | used for constraining an unconstrained array type. (Discriminated
> > | records for a variant part are allowed.)
> >
> > Does anyone know a reason to avoid this feature of the language in
> > general, or is it more likely that there's just some issue with the
> > runtime or bootstrap behaviour which makes it inadvisable to use in the
> > compiler itself?
>
> I was not involved in the team that wrote this guideline.
>
> I can explain some of the problems with declaring discriminated
> record types used for constraining unconstrained array types.
>
> The first problems is memory usage. Take the following example:
>
> type Unconstrained_Type is array(Positive range <>) of Integer;
>
> type Inadvised_Record_Type (Max : Positive) is record
>    Buffer : Unconstrained_Type(1..Max);
> end record;
>

I think what you explain further is right only if the declaration was:

type Inadvised_Record_Type (Max : Positive :=  1) is record
   Buffer : Unconstrained_Type(1..Max);
end record;

Note: "1" could be replaced by any other valid value.
The issue is with default values and declarations without constraint.