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Subject: Re: ANN: AdaTutor on the Web - done
From: AdaMagica <christ-usch.grein@t-online.de>
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Date: 2012-09-29T03:22:41-07:00
List-Id: <comp.lang.ada>

This text in the tutorial about UC is nonsense:

<quote>
One use of Ada.Unchecked_Conversion might be to allow us to and two Integer=
s.  (Ada 95 allows us to and two objects of a modular type, but not two Int=
egers.) Some Ada compilers come with a package that enables us to and two I=
ntegers, but many compilers have no such package.  Suppose that types Integ=
er and Boolean occupy the same amount of storage.  If our program says with=
 Ada.Unchecked_Conversion; we could write

function Int_To_Bool is new Ada.Unchecked_Conversion(Integer, Boolean);
function Bool_To_Int is new Ada.Unchecked_Conversion(Boolean, Integer);
function "and"(Left, Right : in Integer) return Integer is
begin
   return Bool_To_Int(Int_To_Bool(Left) and Int_To_Bool(Right));
end "and";
</quote>

Integer and Boolean have never the same size. Boolean'Size =3D 1. "and" wil=
l pick either the MSB or the LSB of Left and Right, depending on compiler r=
esp. hardware.

(Ada83 was a bit unclear about the meaning of 'Size, there are differences =
in the behaviour of compilers. Ada95 fixed that - in a way that some find a=
wkward.)

UC between Integer and Natural might work or include a multiplication/divis=
ion by 2 because Natural'Size=3DInteger'Size-1. I have been bitten by UC be=
tween objects of types with different sizes when porting some legacy Ada83 =
code to a different hardware and compiler. Be careful: Stand-along objects =
of subtype Natural and of Integer have of course the same size - what matte=
rs for UC is the subtype's size, which is given by 'Size - this is why GNAT=
 has an attribute 'Object_Size.