Boost C++ Libraries: Ticket #5823: nextafter doesn't work correctly with non-finite values -- even in C99/TR1 mode

attachment set

Mathias Gaunard — Mon, 29 Aug 2011 13:57:46 GMT

attachment → nextafter_test.cpp

summary changed

Mathias Gaunard — Mon, 29 Aug 2011 13:59:26 GMT

summary nextafter doesn't work with denormals -- even in C99/TR1 mode → nextafter doesn't work correctly with non-finite values -- even in C99/TR1 mode

status changed; resolution set

John Maddock — Mon, 10 Oct 2011 08:52:42 GMT

status new → closed
resolution → wontfix

The current implementation deliberately treats an infinite argument as a domain error and raises an exception in C++ mode, or returns NaN in TR1 mode.

It would be easy enough to change this, but IMO it's questionable what the return value should be in this case - I'm not convinced that converting an infinite value to a finite one is the "right" answer - arguably the result could/should still be infinite.

So unless we can get a definitive answer from the C99 folks as to what the correct answer should be, I'm inclined to leave it as it is for now.

Mathias Gaunard — Mon, 10 Oct 2011 11:23:34 GMT

While the C99 standard seems allows this, this is not what existing C99 implementations do (which the attached testcase does demonstrate), nor what other standard libraries in other languages do.

With the current situation, for us Boost.Math is unfortunately not usable directly as an alternative to a standard C99 library.

Why not just add the check in boost_nextafter for feature parity?

status changed; resolution deleted

John Maddock — Mon, 10 Oct 2011 17:25:53 GMT

status closed → reopened
resolution wontfix

OK I've checked and both GLIBC and MSVC have the behaviour you describe, will fix later.

status changed; resolution set

John Maddock — Tue, 11 Oct 2011 17:28:16 GMT

status reopened → closed
resolution → fixed

(In [74918]) Change nextafter and related functions to handle infinities as arguments the same way as GLIBC and MSVC. Fixes #5823.