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

From: Edward Diener (eddielee_at_[hidden])
Date: 2006-03-14 13:11:07

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.