Re: [boost] [serialization] Serialisation/deserialisation of floating-point values

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-03-14 14:15:52

| -----Original Message-----
| From: boost-bounces_at_[hidden] 
| [mailto:boost-bounces_at_[hidden]] On Behalf Of Edward Diener
| Sent: 14 March 2006 18:11
| To: boost_at_[hidden]
| Subject: Re: [boost] [serialization] 
| Serialisation/deserialisation of floating-point values
| 
| Paul Giaccone wrote:
| > I'm having problems with deserialising floating-point 
| (double) values 
| > that are written to an XML file.  I'm reading the values 
| back in and 
| > comparing them to what I saved to ensure that my file has 
| been written 
| > correctly.  However, some of the values differ in about the 
| seventeenth 
| > significant figure (or thereabouts).
| > 
| > I thought Boost serialization used some numerical limit to 
| make sure 
| > that values are serialised exactly to full precision, so what is 
| > happening here?
| 
| This is a common cause of errors when using floating point values. 
| Writing a floating point value to a string representation, as are XML 
| values, and attempting to read that string representation 
| back, does not 
| guarantee that the floating point value will remain exactly the same 
| since there are a number of floating point values which have no exact 
| representation in the C++ floating point formats. That is 
| simply because 
| of the nature of floating point representation used in C++ and most 
| modern languages. After all, the number of floating point 
| values within 
| any range of numbers is infinite while the C++ floating point 
| representation cab not be. The only way to guarantee what you 
| want for 
| floating point values is to write and read back to a binary 
| representation of the value.

I think that this is unduly pessimistic.

In practice, provided the archive is written and read where floating point
have the same IEEE format, usually 64-bit doubles, it should work OK.

You could check in your program by reading 

if (std::numeric_limits<long double>::digits != 53)

giveup!

 that this is so, and warn if it is not.  

If the binary representation is different, it won't work anyway!  So you
would be any better off!

Paul

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Paul A Bristow
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