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From: Ken Garlington <garlingtonke@lmtas.lmco.com>
Subject: Re: Rules for Representation of Subtypes
Date: 1996/10/11
Message-ID: <325E5259.6D3E@lmtas.lmco.com>#1/1
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references: <9610101740.AA09737@most>
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organization: Lockheed Martin Tactical Aircraft Systems
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Date: 1996-10-11T00:00:00+00:00
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W. Wesley Groleau (Wes) wrote:
> [snip]
> I haven't used Verdix in 18 months, but if I remember right, it allowed
> source to be larger than target WITH a warning.  Similar memory for
> VAX Ada, though I haven't used that in five years.  Unfortunately, many
> coders suppress warnings because "there are so @$#^$%^*&$ many of them,
> they just hide the real problems."

DEC Ada v3.2 still permits dissimilar sizes. Here's the code I tried:

with Unchecked_Conversion;
package UC_Pkg is

  type S_Type is range 0 .. 16#7FFF_FFFF#;
  for S_Type'Size use 32;

  type T_Type is range 0 .. 10;
  for T_Type'Size use 4;

  function S_to_T is new Unchecked_Conversion (S_Type, T_Type);

  S : S_Type := 16#FFFF#;
  T : T_Type;

end;

with UC_Pkg; use UC_Pkg;
with Text_IO;
procedure UC is
begin
  T := S_to_T(S);  -- examining T in the DEC debugger after this statement
                   -- shows T to be equal to 16#F#.

  Text_IO.Put_Line(T_Type'Image(T));  -- prints "-1" (16#F# as two's complement)
end;

In Ada 83, there's an AI that permits the unchecked conversion to be rejected,
but it's not required.

> 
> ---------------------------------------------------------------------------
> W. Wesley Groleau (Wes)                                Office: 219-429-4923
> Hughes Defense Communications (MS 10-40)                 Home: 219-471-7206
> Fort Wayne,  IN   46808                  (Unix): wwgrol@pseserv3.fw.hac.com
> ---------------------------------------------------------------------------

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