Re: Valid Attribute and Unchecked Conversion

[mazzanti@iei.pi.cnr.it (Franco Mazzanti)]

Robert I. Eachus wrote:

>    It is erroneous.  That is the point of this discussion change to:
> 
>      declare
>          type T is range 1 .. 10;
>          for T'SIZE use Integer'SIZE;
>          function To_T is new Unchecked_Conversion (Integer, T'Base);
>          O : constant T := To_T (0);
>       begin
>          if O'Valid then
>             ...
>       end;
> 
>     Now To_T returns a bit pattern that is legal for the type (T'Base),
> and the constraint check occurs on the assignment.  You never get to
> the call to O'Valid.
> 

However, we must be very careful not to generalize this approach.
E.g.  in the following very similar case:

      declare
          type T1 is new Integer range 1 .. 10;
          type T2 is new Integer range 1 .. 10;
          for T2'SIZE use T1'SIZE;
          function To_T is new Unchecked_Conversion (T1, T2'Base);
          S1:T1;  -- maybe initialized with zero
          S2:T2;
       begin
          S2:= 1;
          ...
          S2:= To_T2(S1);
          ...
          if O'Valid then
             ...
       end;

if S1 is not initialized, and has an invalid value, the call of To_T2 is
still directly erroneous (because the unchecked conversion still tries to
return an invalid scalar value!). 

It seems, in fact, that it is a generic principle in the Reference
Manual that invalid objects can be created ONLY by a missing initialization.
Once initialized, an object can no-more become "invalid". It can become,
at best, abnormal, and just storing its value in an object is erroneous.

My impression is that the safest generic approach still requires the use of 
a "Checked_Scalar_Conversion" as shown inside the previous thread
(Re: Rules for Representation of Subtypes).

Franco Mazzanti