Re: Interesting effects in array renaming

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

Jeffrey Carter wrote:

> Dmitry A. Kazakov wrote:
>> ---------- test.adb
>> with Ada.Text_IO;  use Ada.Text_IO;
>> 
>> procedure Test is
>>    type Some_Array is array (Integer range <>) of Integer;
>> 
>>    procedure Foo (X : Some_Array) is
>>       subtype Constrained is Some_Array (1..X'Length);
>>       XX : Constrained renames X;
>>    begin
>>       Put_Line
>>       (  "XX'Range:"
>>       &  Integer'Image (XX'First)
>>       &  ".."
>>       &  Integer'Image (XX'Last)
>>       &  ", Constrained'Range:"
>>       &  Integer'Image (Constrained'First)
>>       &  ".."
>>       &  Integer'Image (Constrained'Last)
>>       );
>>    end Foo;
>> 
>>    Object : Some_Array (-3..2) := (-3, -2, -1, 0, 1, 2);
>> begin
>>    Foo (Object);
>> end Test;
>> ---------- test.adb
>> 
>> Surprisingly the above produces different ranges for XX and its declared
>> subtype. Isn't something wrong here?
> 
> Note that you can also do
> 
> subtype Constrained is Some_Array (1..X'Length-1);
> 
> or
> 
> subtype Constrained is Some_Array (1..3);
> 
> with similar results. You can also do
>
> XX : Some_Array renames X;
> 
> which perhaps demonstrates more clearly where the object's subtype comes
> from.

I see no link with subtypes. ARM clearly states: 8.5.1(6)

"... any constraint implied by the subtype_mark of the 
object_renaming_declaration is ingored)."

I.e. whatever subtype you, a programmer, might specify I, the compiler, will 
shamelessly ignore it!

How safe! Do you really think it is OK? Consider this:

X1 : Constrained := Constrained (X);
X2 : Constrained renames X;

What is OK here, that X1 and X2 have different constraints or that X2 is 
potentially invalid?

I see only two alternatives:

Either to add a dynamic semantics ensuring that the result of renaming is a 
valid object of the declared subtype. [X2 raises Constraint_Error if 
Constrained'Range /= X'Range]

Or [painful] to require X1 and X2 be same in all cases, and thus:

   XX : Some_Array renames X (X'First + 1 .. X'Last);

would have length different from X, so a new array dope has to be generated 
etc.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de