Re: [boost] constexpr, cmath functions and other functions and distributions

Subject: Re: [boost] constexpr, cmath functions and other functions and distributions
From: Beman Dawes (bdawes_at_[hidden])
Date: 2016-07-29 09:07:10

On Thu, Jul 28, 2016 at 11:44 AM, Antony Polukhin <antoshkka_at_[hidden]>
wrote:

> 2016-07-28 16:24 GMT+03:00 Paul A. Bristow <pbristow_at_[hidden]>:
>
> > Is there any support for making std::abs and friends constexpr where
> > possible in future versions of the C++ Standard?
> >
>
> Here are the bad news: C++ committee wishes to keep it possible for the
> implementors of the Standard Library to reuse C headers.
> I've tried to add constexpr to the <cstring> in
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0202r0.html
> The committee sad 'No', so I've removed the changes to <cstring> and only
> then the paper passed into C++Next.
>
> There is a small chance that a paper on making some of the C math functions
> constexpr may be accepted if functions are trivial to implement
> efficiently.
>

The chance of that happening is much increased if there are National Body
comments to the C++17 ballot requesting constexpr C math functions.

So Paul should file a comment with the BSI (Roger Orr, IIRC), and anyone
else who cares should file a comment with their National Body.

> I can write such paper and represent in to the C++ committee, but I'll need
> a lot of help with:
> * writing a list of C functions that could be simply implemented from
> scratch without affecting performance (functions that usually take 1-3
> lines to implement).
> * implementing each of those function using constexpr
>

Having such a paper also would increase the chance that the C++ committee
will act.

>
> At least users will have the constexpr implementation in Boost.Math if the
> paper won't be accepted.
>
>
Again, the fact that Boost.Math has implemented such constexpr functions
increases the chance that the committee will add them to the standard.

Thanks,

--Beman