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Date: 1 Sep 93 15:47:15 GMT
From: butch!rapnet!lvonrude@uunet.uu.net  (Lowell S. VonRuden x5294)
Subject: Unchecked_Conversion question
Message-ID: <1993Sep1.154715.10498@Rapnet.Sanders.Lockheed.Com>
List-Id: <comp.lang.ada>

I am doing something that seems to work using a Verdix compiler, but I
have not been able to determine if this is something that will be safely
transportable.  Hopefully, someone here can tell me.

I have a 32 bit value coming in from an external interface as an
integer, which I am mapping to an enumeration type.  The enumeration
type has representation clauses for both size (Integer'Size) and
implementation values.  Assigning the result of an unchecked conversion
from the integer to an object of the enumeration type doesn't raise any
exception if the integer is out of range for the enumeration type
representation.  I found that if I do an explicit conversion of the
enumeration object to its type, then the range gets checked.

procedure Sample (Int : Integer) is

  type Enum is (AAA, BBB, CCC, DDD);
  for  Enum use (AAA=> 1,
                 BBB=> 2,
                 CCC=> 13,
                 DDD=> 14);
  for  Enum'Size use Integer'Size;

  function Convert is new Unchecked_Conversion (Source => Integer,
                                                Target => Enum);
begin

  E := Convert (Int);  -- no exception raised here if Int is out of range

  E := Enum (Convert (Int));  -- this does raise constraint error if
                              -- Int is out of range

end Sample;


So, is this a dependable thing to do?

-- 
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-- Usual disclaimers apply...               Lowell Von Ruden      --
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