Re: [boost] iround() and itrunc() in Conceptual Requirements for Real Number Types

Subject: Re: [boost] iround() and itrunc() in Conceptual Requirements for Real Number Types
From: Matt Pulver (mpulver_at_[hidden])
Date: 2019-01-04 20:50:31

On Fri, Jan 4, 2019 at 2:49 PM John Maddock via Boost <boost_at_[hidden]>
wrote:

>
> On 04/01/2019 18:24, Matt Pulver via Boost wrote:
> > Hello,
> >
> > Regarding the page Conceptual Requirements for Real Number Types
> >
> https://www.boost.org/doc/libs/1_69_0/libs/math/doc/html/math_toolkit/real_concepts.html
> >
> > Is it still accurate to list these 2 rows under Standard Library Support
> > Requirements?
> >
> >     - iround(cr1) | int
> >     - itrunc(cr1) | int
> >
> > The motivation in asking this is two-fold:
> >
> >     1. I don't see either of these functions listed in the Standard
> Library.
> >     Perhaps this is just old documentation, or I am not looking in the
> right
> >     place. Instead, I do find lround() and truncl().
>
> I should make it clearer that these are not *std* lib functions -
> they're Boost.Math functions, but will be called via ADL so can be in
> the same namespace as the number type.
>

I should have checked there.

The motivation for these, is that there are many special functions which
> require conversion of the real-number-type to an integer as part of
> their implementation (when figuring out recurrence relations for
> example).  However, conversion of arbitrary number types to integers
> can't be guaranteed, and when it does work - as with built in floating
> point types - then it's unsafe (overflow etc).  So these functions
> provide a customisation point for this conversion.
>
> You may well find that Boost.Math's default versions of these work for
> your number type - have you tried it to see?
>

I just did, and found I had to make a couple of minor modifications to get
boost::math::iround() to work:

1. Added an explicit int() operator to the autodiff class.

2. Changed line 38 of round.hpp
https://github.com/boostorg/math/blob/d26f04a9c9e3004904dd466249ddf84eae76ec83/include/boost/math/special_functions/round.hpp#L38
from

return 0;

to

return static_cast<result_type>(0);

No changes were needed to trunc.hpp since a similar
static_cast<result_type>() is already done there.

This is because in the particular case of autodiff, the single-parameter
constructors are declared explicit. I can attempt to justify this design
choice if this seems questionable.

I'm happy to submit a pull request for this change if you like.

Cheers,
Matt