Re: Normalizing array indices

[Adam Beneschan <adam@irvine.com>]

On Oct 28, 11:58 am, Stefan.Lu...@uni-weimar.de wrote:
> Hi all, does anyone know a way to change the array indices of a subprogram
> parameter to start with a default index? This question occurred to me when
> I happened to discover a subtle bug in a sort procedure I had implemented.
>
>   generic
>     type Element_Type is private;
>     type Sort_Array_Type is array (Positive range <>) of Element_Type;
>     with function "<" (Left, Right: Element_Type) return Boolean is <>;
>   procedure Sort(A: in out Sort_Array_Type);
>
> I had a reasonable amount of black box tests and Sort passed all of them.
>
> Some time later, I added a test with A'range being  
>    Positive'Last -2 .. Positive'Last
> and boooom -- got a Constraint_Error. As it turned out, there was a
> Positive index variable which could take the value A'Last+1 -- which is
> perfectly OK except when A'Last = Positive'Last. To rescue my
> implementation I considered something like
>
>   procedure Sort(A: in out Sort_Array_Type) is
>     Alias_A: Sort_Array_Type(1 .. A'Length) renames A;
>   begin
>     ... -- apply your favorite sorting algorithm to Alias_A;
>   end Sort;
>
> but the compiler didn't like that renaming:
>   "constraint not allowed in object renaming declaration".

procedure Sort (A : in out Sort_Array_Type) is
  subtype A_Type is Sort_Array_Type (1 .. A'Length);
  Alias_A : A_Type renames A_Type(A);
begin
  ...
end Sort;

The type conversion is necessary in order to get the indexes to
"slide".  If you do this:

  Alias_A : A_Type renames A;

the compiler will accept it, but you're likely to get
Constraint_Errors at runtime.

                        -- Adam