Re: Restarting Tread: Why isn't this program working with Unchecked_Converstion

[sjw <simon.j.wright@mac.com>]

On Jan 16, 12:18 pm, "Stuart" <stu...@0.0> wrote:
> "ChristopherL" <clusard...@aol.com> wrote in message
>
> news:83d19cd8-71ca-43c1-a3eb-05832290e565@r36g2000prf.googlegroups.com...
>
> > I'm restarting the tread [How to put 200 into an integer sub-type of
> > 16 bits (code included)] with a new subject because the last was too
> > long!
>
> <snip problem with Unchecked_Conversion>
>
> > I want to copy bits from an Ada float to a user defined short integer
> > maintaining the same bit representation.

(I'm sorry to reply to Stuart's post rather than OP's, but Google
fails to show OP's posts at all! (perhaps there's a reason for that?))

Sorry to shout, Chris, but you CAN NOT FIT THE BITS OF A STANDARD
FLOAT INTO ANYTHING SMALLER!!! IF YOU WANT THE SAME BIT REPRESENTATION
YOU HAVE TO USE THE SAME NUMBER OF BITS!!!

The following is another way of achieving unchecked conversion - the
results on a little-endian machine (i686) are

 2.00000E+02 ->  0  0  72  67
 3.14159E+00 ->  219  15  73  64

with Ada.Numerics;
with Ada.Text_IO; use Ada.Text_IO;
with System.Storage_Elements;

procedure Foo is
   F : Float;
   A : System.Storage_Elements.Storage_Array (1 .. 4);
   for A'Address use F'Address;
   pragma Import (Ada, A);  -- prevent unwanted auto-initialisations
   function Image (B : System.Storage_Elements.Storage_Element) return
String
     renames System.Storage_Elements.Storage_Element'Image;
begin
   F := 200.0;
   Put_Line (Float'Image (F)
               & " -> "
               & Image (A (1))
               & " "
               & Image (A (2))
               & " "
               & Image (A (3))
               & " "
               & Image (A (4)));
   F := Ada.Numerics.Pi;
   Put_Line (Float'Image (F)
               & " -> "
               & Image (A (1))
               & " "
               & Image (A (2))
               & " "
               & Image (A (3))
               & " "
               & Image (A (4)));
end Foo;