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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Interfaces.Shift_Left
Date: Mon, 12 Dec 2011 17:36:27 -0800 (PST)
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Date: 2011-12-12T17:36:27-08:00
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On Dec 12, 3:34=A0pm, Simon Wright <si...@pushface.org> wrote:
> awdorrin <awdor...@gmail.com> writes:
> > Cur_Word :=3D (Cur_Word OR Shift_Left( Unsigned_32(Data),
> > Bits_To_Shift));
> > INT_PTR.all :=3D Integer_32(Cur_Word);
>
> > I understand that the result from the shift_left is a negative number,
> > so it is being seen by Ada as a large positive number ( > (2^31-1))
> > which is why I'm getting the Constraint error;
>
> > Maybe its my history with C that is confusing me, but shouldn't there
> > be an easy way to cast the value back to a negative Integer_32 from an
> > Unsigned_32?
>
> Ada doesn't have an easy way to do what you ask; one way you can do it
> is by instantiating Ada.Unchecked_Conversion, which is deliberately
> obvious so that reviewers know that they have to take more care than
> usual.

In fact, I think most C casts are, in essence, "unchecked
conversions".  In Ada, however, type conversions (specifically, value
conversions) are more strictly defined, and it's defined so that the
result of a type conversion has the same *value* as the operand, with
allowance for rounding.  So the idea of a type conversion whose
operand is a positive number and whose result is a negative number is
not how Ada does things.

                                 -- Adam