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From: "Randy Brukardt" <randy@rrsoftware.com>
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Subject: Re: Ada.Containers.Vectors - querying multiple elements
Date: Sun, 1 May 2005 22:21:42 -0500
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Date: 2005-05-01T22:21:42-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:txah28f4zlnp$.1pqea609lovyn.dlg@40tude.net...
...
> Now consider the following test:
>
>    type T is mod 1;
>    type T_Array is array (T range <>);
>    X : T_Array := <somehow-obtained-empty-array>;
> begin
>    for I in X'Range loop -- This is OK
>       null;
>    end loop;
>    for I in X'First..X'Last loop -- This cannot be OK
>       null;
>    end loop;
>
> Which is in a perfect contradiction with 3.6.2(7). It could probably be
> mended too if we had universal index types. But this is yet another story.

In Ada as currently defined, you would get Constraint_Error from the attempt
to define the bounds. That happens with string types:

    type T is mod 1;
    type T_Array is array (T range <>) of Character;
    X : T_Array := ""; -- Raises Constraint_Error

There is actually an ACATS test to check this. (I remember it because one
vendor disputed the test; it was upheld.)

You'd get the same result if there was a way to describe a null array
aggregate:

   X : T_Array := (null array); -- Not Ada, and if it was, it would raise
Constraint_Error here.

It turns out that Interfaces.C.To_C has a similar problem, and it too raises
Constraint_Error for a null string (if not null terminated).

Of course, you could conceivably use other rules, but they'd be incompatible
with the current ones, which wouldn't be a problem in cases like these, but
they might be in other cases.

                     Randy.