Re: [boost] [XInt] review

Subject: Re: [boost] [XInt] review
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-03-09 09:41:58

Chad Nelson wrote:
> Steven Watanabe <watanabesj_at_[hidden]> wrote:
>
> > Why can the comparison operators throw? I would have
> > expected that they could be implemented to never throw.
>
> So far as I know, the only reason they would ever throw is if
> they're passed a Not-a-Number value, or if something
> unpredictable has gone seriously wrong.

I would hope you could be more definitive than that and clearly document the behavior.

> > When you test for integer_t<BOOST_XINT_APARAMS>::Nothrow &&
> > n.is_nan(), you could just test n.is_nan() and make sure
> > that n.is_nan() is always false if Nothrow is true.
>
> Couldn't that prevent some less-intelligent compilers from
> optimizing out the test?

Only if is_nan() isn't inline or the optimizer is really stupid (in which case you shouldn't care because there will be many other failures to optimize).

> > magnitude_t:
> > * It probably needs to be POD for you to make assumptions
> > about the layout.
>
> The "struct hack" is a standard C idiom, and has been for as
> long as C has been around. It only became explicitly
> documented behavior in C99, but it works with all known C and
> C++ compilers; I checked very thoroughly, and even asked on
> this list (where one might reasonably expect to find people
> with experience with really odd architectures and compiler
> setups), and wasn't able to find a single report of a compiler
> where it would fail.

A C struct is POD.

> > gcd.hpp:
> > The behavior of invmod when the inverse doesn't exist is to
> > return zero.  Wouldn't an exception/NaN be more consistant?
>
> I prefer to reserve exceptions for exceptional behavior,
> primarily errors. A number that doesn't have an inverse isn't
> really exceptional, there are many such values.

Returning NaN poisons subsequent calculations.  Zero doesn't.

_____
Rob Stewart
Software Engineer, Core Software
Susquehanna International Group, LLP

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