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Date: Fri, 12 Aug 2011 09:41:34 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Why use C++?
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Date: 2011-08-12T09:41:35+02:00
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On 12.08.11 08:51, Paavo Helde wrote:

> I guess that they were instead thinking about intermediate results. As
> the wrap-around point (zero) is very close to the common usage range of
> unsigned integers, it may well happen that in a complex arithmetic
> expression a temporary intermediate result may fall below zero whereas
> the final result will be OK again. This would in effect mean that the
> addition-subtraction operations would lose the associativity property,
> which would cause some headaches. E.g. the programmer should always use
> parens for indicating the relative order of addition and subtraction,
> e.g. a+(b-c) and take care that no intermediate result would overflow. As
> the common C/C++ implementations do not bother to check overflow even for
> the signed types, this would just mean that there would be a lot more of
> formally undefined behavior which seems to work as expected on a lot of
> hardware. So, instead of introducing a lot of UB they chose to make
> things like a+b-c legal by a simple trick of declaring the unsigned types
> wrap-around.
>
> Yes, Ada could have been wiser, I'm sorry to hear this is not the case.

Sort of; Ada does have a number of definitions around overflow and
when overflow matters, when in intermediate results.

      1. with System;
      2. procedure ovfl is
      3.   type I is range 1 .. System.MAX_INT;
      4.   r, a, b, c: I;
      5. begin
      6.   a := 1;
      7.   b := I'Last;
      8.   c := 2;
      9.   r := a + b - c;  -- overflow
                  |
         >>> warning: value not in range of type "I" defined at line 3
         >>> warning: "Constraint_Error" will be raised at run time

     10.   r := a + (b - c);  -- o.K.
     11.   r := (a + b) - c;  -- overflow
                   |
         >>> warning: value not in range of type "I" defined at line 3
         >>> warning: "Constraint_Error" will be raised at run time

     12. end ovfl;


$ ./ovfl

raised CONSTRAINT_ERROR : ovfl.adb:9 overflow check failed
$