Re: [boost] Infinite precision integer draft

From: Maarten Kronenburg (M.Kronenburg_at_[hidden])
Date: 2006-06-01 08:20:21

Martin,
This problem is solved,
see below:

The arithmetic member operators
returning integer by value are changed
to non-virtual.
The virtual integer & operator=(
  const integer & );
now has the following note:
"Derived classes must override this
member operator and use it to convert
(temporary) objects of type integer
back to the derived class."
Also it is explained that the
arithmetic non-member operators and
the arithmetic member operators
returning integer by value must call
integer member functions and operators,
not the derived ones.
Now in expressions with unsigned_integer
variables, temporary results can be
negative. Only when a negative end-result
is assigned, an exception will be thrown.
So when a is 3 and b = 4,
x = -a + b;
will not throw an exception.
Regards, Maarten.

"Martin Bonner" <martin.bonner_at_[hidden]> wrote in message
news:D997BF79D1E92C4793B7FCC04B4F90A555D1E0_at_pigeon.pi.local...
> ----Original Message----
> From: boost-bounces_at_[hidden]
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Maarten Kronenburg
> Sent: 31 May 2006 21:25 To: boost_at_[hidden]
> Subject: Re: [boost] Infinite precision integer draft
>
> > This problem with unsigned_integer
> > throwing an exception when negative
> > is already apparent for the two
> > expressions
> > a-b
> > and
> > -a+b  [presumably -b+a is intended]
> [snip]
>
> > a true unsigned infinite precision integer
> > which may or may not throw exceptions in
> > equivalent expressions
>
> But those expressions are not equivalent for non-negative integers.
>
> a-b is defined as the value x such that b+x is equal to a.
>
> For non-negative integers that means a-b is only defined if a >= b.
>
> I would argue that -a is not really meaningful for non-negative
> integers, but the most obvious definition is that -a is the value x such
> that a+x = 0.  That is only defined for a = 0 (and gives x=0).
>
> Thus your first expression is defined for any b <= a, whereas your
> second is only defined for b=0.
>
> non-negative integers are not the same as Z (but are still useful).
>
> -- 
> Martin Bonner
> Martin.Bonner_at_[hidden]
> Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
> ENGLAND Tel: [phone]
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