Re: ANN: AdaTutor on the Web - done

[AdaMagica <christ-usch.grein@t-online.de>]

This text in the tutorial about UC is nonsense:

<quote>
One use of Ada.Unchecked_Conversion might be to allow us to and two Integers.  (Ada 95 allows us to and two objects of a modular type, but not two Integers.) Some Ada compilers come with a package that enables us to and two Integers, but many compilers have no such package.  Suppose that types Integer 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 = 1. "and" will pick either the MSB or the LSB of Left and Right, depending on compiler resp. 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 awkward.)

UC between Integer and Natural might work or include a multiplication/division by 2 because Natural'Size=Integer'Size-1. I have been bitten by UC between 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 matters for UC is the subtype's size, which is given by 'Size - this is why GNAT has an attribute 'Object_Size.