Re: [boost] several messages

Subject: Re: [boost] several messages
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-08-18 20:52:21

on Wed Aug 08 2012, Marc Glisse <marc.glisse-AT-inria.fr> wrote:

> On Wed, 8 Aug 2012, Dave Abrahams wrote:
>
>>> Most uses of a moved-from object I can think of other than destruction
>>> and assignment are rather contrived. For a container, it can make
>>> sense to look at its size and assign to the elements. For a bignum,
>>> maybe if you are interested in an arbitrary number < y (to pass it to
>>> a nextafter-like function), before setting x to y-1, you could check
>>> if x<y and not want that to crash the program. If you are going to use
>>> a bignum as a bitfield, the same argument as for the container could
>>> be made.
>>
>> This sounds kind of like programming with iostreams whose badbit has
>> been set.  Lots of times it's possible to write the code so that it can
>> barrel ahead as though nothing is wrong and only check for a problem
>> much later.  But it sounds kind of implausible that you'd be able to
>> pick a value for empty bignums that would allow that in most or even
>> many cases.
>
> Hi Dave,
>
> I am not sure what you mean here. The safe solution is that whenever
> you move-construct from a bignum, you set it to zero (move-assignment
> is a swap). I am not sure, but it sounds like you are saying this
> wouldn't be safe?

Not exactly.  With floats, we have non-signalling NaN, which IIUC is
used in exactly that way: you might end up with a few NaNs in your big
linear algebra solution but they might well turn out to be unimportant.
What you do know is that anything that touches them will also yield a
NaN, sort of like anything that uses a stream with badbit set yields a
stream with badbit set.  Zero doesn't work that way: after all, it's an
additive identity.

-- 
Dave Abrahams
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