Re: [boost] [serialization] round-trip serialization of float

Subject: Re: [boost] [serialization] round-trip serialization of float
From: Paul A. Bristow (pbristow_at_[hidden])
Date: 2013-01-10 12:02:33

> -----Original Message-----
> From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of Robert Ramey
> Sent: Thursday, January 10, 2013 4:28 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] [serialization] round-trip serialization of float
> 
> Adam Lerer wrote:
> > Note: There's some discussion of this topic in a slightly different
> > context here:
> > http://boost.2283326.n4.nabble.com/serialization-Serialisation-deseria
> > lisation-of-floating-point-values-td2604169i20.html
> 
> Remember that the serialization library uses standard i/o for input/output.
> So this discussion above
> is actually the same issue.
> 
> In general, libraries won't guarentee bit for bit equality.  This is due to a number of reasons:
> 
> a) not every binary floating number has en exact representation when rendered as decimal.
> 
> b) different libraries can't be expected to handle this in all the same way.
> 
> c) text type archives are meant to be portable between architectures.  So, for example, on can
create
> floating point numbers in an environment which uses an 80 ieee754 representation, write them to a
file,
> and read them on another architecture - say a 64 bit ieee754 representation and expect things to
work.
> That is, the streams i/o attampts to preserve values (rather than bit representation
> ) to the extent that it makes sense to do so.
> 
> d) It gets much worse with floating point values such as NaN - since different libraries differ on
how to
> address this.
> 
> e) Remember, this is not a serialization library issue-- but rather one related to floating point
numbers and
> standard i/o.  i
> 
> So, If you feel you need to preserver the exact set of bits - don't use floating point - use
something else.

As the originator of the thread above, I just want to add that I agree with this summary.

You can only find out if serialization-deserialization will work by both examination of the bit
layouts and careful testing.

Your best bet is to use the max_digits10 precision (or, sadly for now, better use the formula given
- because on the previous VS version, std::numeric_limits<float>::max_digits10 was wrong :-)

and to use scientific format (to avoid a previous VS buglet).

Nan and infinity now has a fair chance of working, after work by Johan Rode, but testing is still
essential.

Good luck!

Paul

---
Paul A. Bristow,
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[phone]  07714330204
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