id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
7886,Use 9 decimal digits for text serialization of float,Adam Lerer <adam.lerer@…>,Robert Ramey,"The boost serialization of float doesn't appear to use enough digits to guarantee that deserialization of a serialized float will produce the same value. The relevant code is in boost/archive/basic_text_oprimitive.hpp:

{{{
void save(const float t)
{
    os << std::setprecision(std::numeric_limits<float>::digits10 + 2);
    ... 
}
}}}


std::numeric_limits<float>::digits10+2=8, but float actually requires numeric_limits<float>::max_digits10=9 decimal digits to guarantee unique representation.

In order to work around a bug in VS's max_digits10, it might be better to use
numeric_limits<T>::digits * 3010 / 10000. See http://boost.2283326.n4.nabble.com/serialization-Serialisation-deserialisation-of-floating-point-values-td2604169i20.html for details.

On the boost mailing list, it was correctly pointed out that serializing floating-point to text cannot always be guaranteed to deserialize to the same value (e.g. across architectures), but for the common case, 9 digits is what is required to guarantee correct roundtrip serialization. Also, other libraries like boost::lexical_cast use 9 digits.

----
The code below (with boost/1.52) demonstrates the problem:

{{{
#include <sstream>
#include ""boost/archive/text_iarchive.hpp""
#include ""boost/archive/text_oarchive.hpp""
#include <iostream>

int main(int argc, char **argv){
float oldf = -1.20786635e-05;
std::ostringstream os;
boost::archive::text_oarchive oa(os);
oa & BOOST_SERIALIZATION_NVP(oldf);

float newf;
std::istringstream is(os.str());
boost::archive::text_iarchive ia(is);
ia & BOOST_SERIALIZATION_NVP(newf);
printf(""before serialization: %.12g\nafter serialization:  %.12g\n"", oldf, newf);
}

///Output:
///before serialization: -1.20786635307e-05
///after serialization:  -1.20786644402e-05
}}}
",Bugs,closed,To Be Determined,serialization,Boost 1.52.0,Problem,fixed,,