Re: [boost] [xint] Boost.XInt formal review

Subject: Re: [boost] [xint] Boost.XInt formal review
From: Thomas Klimpel (Thomas.Klimpel_at_[hidden])
Date: 2011-03-15 14:48:38

John Bytheway wrote:
> I tend to use
> 
>   r = m%n;
>   if (r < 0) r += n;
> 
> which the compiler is more likely to implement using a conditional
> move,
> rather than a branch (but it does have the issue that it's not an
> expression).

I have now implemented modulo as

template <class T>
T modulo(T m, const T& n)
{
  m %= n;
  if (m < 0) m += n;
  return m;
}

and replaced 10 occurrences (all I could find) of (m%n+n)%n with it. Two of these occurrences had the form "(m%std::ptrdiff_t(n)+n)%n", which surprised me a bit. When I replaced them with "modulo(m, n)", the compiler refused to compile it. So I wrote "modulo(m, std::ptrdiff_t(n))" instead, and this compiled fine. Thinking a bit about this, I realized that the template does the right thing by refusing to compile this, whereas the "(m%n+n)%n" construct can lead to surprising results in case n is of type std::size_t.

So I think I'm quite happy with my new template.

Regards,
Thomas