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From: Gautier <Gautier.deMontmollin@Maths.UniNe.CH>
Subject: Re: Ada vs Delphi?
Date: 1999/08/12
Message-ID: <37B27FE8.6E09569E@Maths.UniNe.CH>#1/1
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To: Andre Ratel <aratel@total.net>
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Newsgroups: comp.lang.ada
Date: 1999-08-12T00:00:00+00:00
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> {$RANGECHECK ON}
> 
>   var
>     n1, n2: shortint; {range: -128 .. 127}
>     N: integer; {range: 2147483648 .. 2147483647}
> 
>   begin
>     n1:= 120;
>     n2:= 120;
>     N:= n1 + n2;
>     Writeln(N);
>   end;
> >>>>>>>>>>

> The result was, appropriately 240.

> Now, if I know that n1 and n2 are both in the range -128..127
> while their sum is outside this range, I don't understand what's
> wrong with assigning (n1 + n2) to an integer.

Neither Delphi nor you can guess if the result of an expression
like (n1 + n2) alone will be in some range. You just were lucky because
Borland has choosen to convert your shortints *before* the addition!
Let's see what happens in Turbo Pascal 6 (16 bit):

{$R+}

  var
    n1, n2: shortint; {range: -128 .. 127}
    i1,i2:  integer;  {range: -32768..32767}
    N: longint; {range: -2147483648 .. 2147483647}

  begin
    n1:= 120;
    n2:= 120;
    N:= n1 + n2;
    Writeln(N);

    i1:= 32000;
    i2:= 32000;
    N:= i1 + i2;
    Writeln(N);
  end.

The 1st result is 240: Borland has converted the signed 8-bit n1,n2
to 16-bit, then added them, converted to a 32-bit one (N).

The second is catastrophic: -1536 with range checks ON !
What happened ? The addition is a normal 16-bit one. BUT it's
an UNSIGNED one (0..65536), so not catched by range check: 64000
Then the thing (now again understood as a SIGNED thing: -1536)
is converted to the signed longint !!

The problem with these automatic castings is double, since
Borland chooses the signed/unsigned and the 8/16/32/64 bits
representation for you!

In Ada you can handle ranges and bits independently:
e.g. a 32-bit subtype of range -127..128, etc. etc.
In the same subtype family (e.g. integer, natural, positive)
you can write n1+n2 safely, without explicit conversion, as
there is no bad surprise with hidden type conversions; ranges
will be checked correctly - provided you didn't suppress them
at compile time...

-- 
Gautier

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