Re: [boost] [serialization]Double rounding bug?

Subject: Re: [boost] [serialization]Double rounding bug?
From: John Maddock (john_at_[hidden])
Date: 2009-10-02 04:08:11

>>    DoubleValue doubleValue1;
>>    doubleValue1.m_value = -0.0070850229047457131;
>>    SaveDoubleValue(doubleValue1, "DoubleValue.xml");
>>
>>    DoubleValue doubleValue2;
>>    LoadDoubleValue(doubleValue2, "DoubleValue.xml");
>>    //but from file is read -0.007085022904745714
>>    assert(doubleValue1.m_value==doubleValue2.m_value);
>
> That value would require between 50 and 60 bits of precision, provided its
> representation is actually possible in a binary floating point number. 
> Typical
> double-precision floating point numbers have a fraction of 52 bits, so the
> changes you are reporting are actually to be expected.

I don't believe that's true: ideally the two values should have the same 
binary representation.  In this case the relative difference between the two 
values is 1.6e-17 or about 1eps: so my guess is they differ in the last 
significant bit - that's not a double rounding issue, or shouldn't be. 
However, I suspect this to be an MSVC issue - there are some numbers that 
MSVC's IOStreams library can't "round trip serialize" - Robert does 
Boost.Serialization rely on iostreams internally in this case?  I'm cc'ing 
Paul Bristow on this one, because I know he investigated this issue a while 
back.

HTH, John.