Re: [boost] several messages

Subject: Re: [boost] several messages
From: Marc Glisse (marc.glisse_at_[hidden])
Date: 2012-08-08 15:01:47

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?

The issue here is that setting it to zero is expensive (less than a copy, 
but still quite a bit), so John instead decided to put it in some "broken" 
state that he can detect at destruction/assignment time. This means that 
doing x-y or x<y may crash (although if he sets _mp_size to 0, most read 
operations are likely not to notice that this isn't a true zero). This is 
probably a good compromise: better performance for regular code, crashes 
only for very odd code. The question is where the limit is between regular 
and odd code...

-- 
Marc Glisse