| -----Original Message----- | From: boost-bounces@lists.boost.org | [mailto:boost-bounces@lists.boost.org] On Behalf Of John Maddock | Sent: 16 March 2006 13:43 | To: boost@lists.boost.org | 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 -- Paul A Bristow Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB Phone and SMS text +44 1539 561830, Mobile and SMS text +44 7714 330204 mailto: pbristow@hetp.u-net.com http://www.hetp.u-net.com/index.html http://www.hetp.u-net.com/Paul%20A%20Bristow%20info.html