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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Normalizing array indices
Date: Fri, 28 Oct 2011 13:36:19 -0700 (PDT)
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Date: 2011-10-28T13:36:19-07:00
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On Oct 28, 11:58=A0am, Stefan.Lu...@uni-weimar.de wrote:
> Hi all, does anyone know a way to change the array indices of a subprogra=
m
> parameter to start with a default index? This question occurred to me whe=
n
> I happened to discover a subtle bug in a sort procedure I had implemented=
.
>
> =A0 generic
> =A0 =A0 type Element_Type is private;
> =A0 =A0 type Sort_Array_Type is array (Positive range <>) of Element_Type=
;
> =A0 =A0 with function "<" (Left, Right: Element_Type) return Boolean is <=
>;
> =A0 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 =A0
> =A0 =A0Positive'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 =3D Positive'Last. To rescue my
> implementation I considered something like
>
> =A0 procedure Sort(A: in out Sort_Array_Type) is
> =A0 =A0 Alias_A: Sort_Array_Type(1 .. A'Length) renames A;
> =A0 begin
> =A0 =A0 ... -- apply your favorite sorting algorithm to Alias_A;
> =A0 end Sort;
>
> but the compiler didn't like that renaming:
> =A0 "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