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Subject: Re: [boost] a safe integer library
From: John Maddock (jz.maddock_at_[hidden])
Date: 2015-12-10 13:47:48

On 10/12/2015 07:49, Robert Ramey wrote:
> Arithmetic operations in C++ are NOT guaranteed to yield a correct
> mathematical result. This feature is inherited from the early days of
> C. The behavior of int, unsigned int and others were designed to map
> closely to the underlying hardware. Computer hardware implements these
> types as a fixed number of bits. When the result of arithmetic
> operations exceeds this number of bits, the result will not be
> arithmetically correct.
> I have crafted a library to address this issue once and for all. You
> can find out more about this by checking out the page for Safe
> Numerics at the boost library incubator.
> I hereby request that this library be added to the boost review queue.

In case you haven't seen it, Multiprecision's cpp_int type has a very
similar "checked" mode that adds similar safety.

> I've also made a proposal for the C++ Standards committee to include a
> simplified version of this library as part of he C++ standard.
> You can see the proposal at


"Performance will depend on the implementation and subject to the
constraints above. This design will permit the usage of template
meta-programming to eliminate runtime performance penalties in some
cases. In the following example, there is no runtime penalty required to
guarantee that incorrect results will never be generated."

The example that follows surely can overflow:

void f(safe<int8_t> i){
safe<int8_t> j;
j = i * i; // C++ promotion rules make overflow impossible!
std::cout << j;

OK so the temporary returned by i*i *may* not overflow (depending on the
size of type int), but the assignment to j most certainly can - I would
expect that to throw, and if not it drives a coach and horses right
through the proposal IMO.

What happens with floating point types? Can values overflow to infinity
or is this trapped? How about divide by zero or indeed zero/zero
(NaN)? What about underflow?

And finally, I think there needs to be good constexpr support - in fact
that could be the killer feature that sets the library apart.

Finally your documents title is "Java Printing" which you may wish to
correct ;)

Best, John.


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