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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: shifting bits
Date: Sun, 29 Aug 2004 08:51:55 +0200
Organization: AdaCL
Message-ID: <6476129.Z2ZE0Tfv6r@linux1.krischik.com>
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 <Xns9553DDDC8BEA9jimmaureenrogers@204.127.36.1>
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Date: 2004-08-29T08:51:55+02:00
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Jim Rogers wrote:

> fmdf@tiscali.it (fabio de francesco) wrote in
> news:ba2f9c57.0408281738.61a48643@posting.google.com:
> 
>> Hello.
>> 
>> I had to pack/unpack some variables into/from one. In order to get the
>> result I chose to shift the variables' bits using multiplications and
>> divisions by powers of two.
>> 
>> 1) Does Ada have a built-in shift operand like C++ "<<" and ">>" ?
> 
> Ada provides similar bit shifting operations in the package Interfaces.
> Those operations are defined for specific instances of modular types.
> 
>> 2) Can only variables of modular type be shifted ? If yes, why ?
> 
> Even in C and C++ bit shifting is only correct for unsigned types.
> Think about what would happen if you shift a sign bit. The
> resulting values would be really nasty depending upon the state of
> the sign bit.

And wen you still need the potentional dangerous operation you could address
map an unsigned type:

I : Interfaces.Integer_32;
U : Interfaces.Unsigned_32;

for U'Address use I'Address;

begin 
  I := 2* 30;
  Interfaces.Shift_Left (U, 1);

And I will have just the same wacky result als in C.

> Modular types are Ada's unsigned types.

To clarify: Range types alwas have sign - even when the range itself does
not allow a negative value.

With Regards

Martin

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