Re: [boost] [serialization]Serialisation/deserialisationoffloating-point values

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-03-16 09:19:05

| -----Original Message-----
| From: boost-bounces_at_[hidden] 
| [mailto:boost-bounces_at_[hidden]] On Behalf Of John Maddock
| Sent: 16 March 2006 13:43
| To: boost_at_[hidden]
| Subject: Re: 
| [boost][serialization]Serialisation/deserialisationoffloating-
| point values
| 
| > Anything that guarantees a round trip MUST be a good.
| >
| > (Getting output and input right would be even better!  There are
| > papers which present methods for doing it which claim to be proven
| > correct - but these are not the methods used for popular
| > implmentations.)
| >
| > Is a hex fully portable?  It surely just promises to be as close as
| > the FP representation will allow?  Or should we store the FP
| > representation in the serialization and only deserialize if it
| > matches exactly?  This sounds a prudent move to me.
| 
| Um, what do you mean by fully portable?  It is in the sense that:
| 
| * If you do a write-then-read cycle on the same machine you 
| get back exactly 
| the same result.
| * If you do a write-then-read cycle on different machines you 
| only get the 
| same result back if the machine reading the value has at 
| least as many bits 
| in it's mantissa as the machine used for writing.  But that 
| goes without 
| saying really.
| 
| I guess I really should put my money where my mouth is and 
| present some 
| sample code, I'll see what I can do later....
| 
| John. 

That would be excellent.

My suggestion is to store the FP format somehow and somewhere in the
serialization. 

http://babbage.cs.qc.edu/courses/cs341/IEEE-754references.html#tables

lists half a dozen IEEE formats, so a single byte would suffice, but it
might be better to cater for User Defined Types by storing the number of
significand and exponent bit counts separately? Some 128-bit types like
doubledouble Darwin and 265-bit reals are in use, as well as arbitrary
precision like NTL ZZ.

Some users might also want to use 'exact reals', for example

http://keithbriggs.info/xrc.html  (note that C and C++ implmentations exist)

so it might be useful to cater for this as well.

Paul

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Paul A Bristow
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