but otherwise once a NaN always a NaN :-) Agreed - but worth a comment?
Agreed.
That's the problem: there is no portably way to create NaN's with other payloads, not even with ldexp.
That is true.
And it leaves me most unsatisfied, feeling frustrated that we are not getting what the IEEE754 designers intended.
We don't seem to have made ANY progress since Kahan's note in 1997
http://www.cs.berkeley.edu/~wkahan/ieee754status/IEEE754.PDF
http://www.cs.berkeley.edu/~wkahan/ieee754status/why-ieee.pdf
Very depressing reading.
I feel that there __ought__ to be some debug use in getting the NaN values, as Guillaume Melquiond noted, but do need to be able to get/put the bits.
Considering the amount of MACRO bodging that Boost is doing to create portability, I wonder if we should not try to produce some way of getting/putting at the 'internal' IEEE representation at least. There are surely not THAT many combinations? We should know how many exponet and significand bits from numeric_limits (so also know if VAX/Alpha without denorm), and if big or little-endian. What others (apart from decimal, a different ball game).
Stephen Moshier has already done a LOT of this bit twiddling for Cephes for IBMPC (X86), DEC (VAX) and M68000 for 32, 64, 80 and 128 bit sizes.
Yep: the formats are mostly well known and the bit twiddling isn't that hard once you know what the format is, but there's at least one problem case: long double's on Darwin. I can't find any information on these after a quick google search, but I believe they're effectively a sum of two doubles. If so this is exceptionally bad form because it violates the most fundamental principal of IEEE arithmatic that a number is n*2^k. (Background: epsilon is very small for this type, it's possible to add a number "delta" to 1.0L and still get a distict value when "delta" is much smaller than the number of bit's in a long double would otherwise indicate. This makes a *lot* of IEEE based reasoning invalid. If anyone has more information on this type I'd appreciate a reference). John.