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

From: Paul Giaccone (paulg_at_[hidden])
Date: 2006-03-14 12:38:48

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?

Example:
Value in original object, written to file: 0.0019075645054089487
Value actually stored in file (by examination of XML file): 
0.0019075645054089487 [identical to value written to file]
Value after deserialisation: 0.0019075645054089489

It looks like there is a difference in the least-significant bit, as 
examining the memory for these two values gives:

Original value: b4 83 9b ca e7 40 5f 3f
Deserialised value: b5 83 9b ca e7 40 5f 3f

(where the least-significant byte is on the left)

Note the difference in the first bytes.

I'm using Boost 1.33.1 with Visual Studio 7.1.3088 in debug mode.

Paul