[PATCH] Define 3-argument overloads of std::hypot for C++17 (P0030R1)

[Marc Glisse]

On Wed, 28 Sep 2016, Jonathan Wakely wrote:

> On 27/09/16 23:28 +0000, Joseph Myers wrote:
>> On Tue, 27 Sep 2016, Jonathan Wakely wrote:
>> 
>>> This adds the new 3D std::hypot() functions. This implementation seems
>>> to be faster than the naïve sqrt(x*x + y*y + z*z) implementation, or
>>> hypot(hypot(x, y), z), and should be a bit more accurate at very large
>>> or very small values due to reducing the arguments by the largest one.
>>> Improvements welcome though, as this is not my forte.
>> 
>> Should I take it from this implementation that C++ is not concerned by
>> certain considerations that would arise for C: spurious underflow
>> exceptions from the scaling when some arguments much larger than others;
>> spurious "invalid" exceptions from the comparisons when any argument is
>> (quiet) NaN; handling of mixed (quiet) NaN and Inf arguments (where ISO C
>> Annex F has Inf returned, not NaN)?
>
> The entire spec from the C++ draft is:  Returns: √ x^2 + y^2 + z^2
>
> It doesn't mention "without undue overflow or underflow" as the C
> standard does for hypot(x, y). Handling those conditions would of
> course be nice, but I'm not capable of doing so, and I'd rather have a
> poor implementation than none.

p0030r0 had "In addition to the two-argument versions of certain math
functions in <cmath> and its float and long double overloads, C++ adds
three-argument versions of these functions that follow similar semantics
as their two-argument counterparts."

One could interpret that as an intent to follow the C semantics. On the
other hand, the paper mentions speed and convenience as the main selling
points. Maybe LWG could clarify the intent?

-- 
Marc Glisse