Re: [boost] A question regarding serialization lib

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2007-12-30 17:58:49

>-----Original Message-----
>From: boost-bounces_at_[hidden] 
>[mailto:boost-bounces_at_[hidden]] On Behalf Of Robert Ramey
>Sent: 30 December 2007 18:59
>To: boost_at_[hidden]
>Subject: Re: [boost] A question regarding serialization lib
>
>Paul A Bristow wrote:
>
>> The chances of this a very small 1 in 3 values of a very narrow
>> range-
>
>I disagree with this.  I don't think the chances are very small.

This is not conjecture - it was an observed behaviour as my original report shows.

It only emerged by accident - and was only confirmed by a random sampling of round-tripping values.

My understanding was that other compilers did NOT show this problem.
 
>> only MS is known to have this problem

>I don't think it's isolated to microsoft. I think it's an inherent 
>limitation of trying to represent some binary fractions exactly as 
>decimal fractions.

I am confident from my investigation that it is failure to provide the *nearest* representable floating-point value from a decimal
digit string - but, bizarrely, only in a very small region, (IIRC from 0.0001 to 0.0005).

With this (as is provided by the C++ *compiler* standard when 'reading' decimal digit strings into floating-points like float,
double & long double), you can round-trip to decimal digit strings and back - provided of course that you use enough decimal digits.
But there is no similar requirement on reading with std:: iostreams, perhaps surprisingly (I think the authors assumed it but didn't
think to specify it).

(Of course, you can only expect 'round-tripping' to work if the floating point format is the same).

The same applies to lexical_cast.

So an answer to the first question: probably - but if the floating-point types differ, then all float-point value 'round-tripped'
will be a few bits wrong, and you might get a tiny proportion wrong for the reasons in my original (rejected) report to Microsoft.

Paul

---
Paul A Bristow
Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB
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