How to serialize boost::rational - serialization

Question

I cannot serialize boost::rational<int>. I searched for a boost/serialize/rational.h header but it does not exist.
/usr/include/boost/serialization/access.hpp:118:9: error: ‘class boost::rational<int>’ has no member named ‘serialize’
Is there a way to achieve it?

Answer 1

Just serialize the numerator and denominator.
Here's the legwork, in semi-generic form (supports archives with named nodes, like XML serialization, too): Live On Coliru
#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>
#include <boost/serialization/serialization.hpp>
#include <boost/rational.hpp>
#include <iostream>
namespace boost { namespace serialization {
template <typename Archive, typename T>
void save(Archive& ar, ::boost::rational<T> const& r, unsigned /*version*/)
{
int n = r.numerator(), d = r.denominator();
ar & boost::serialization::make_nvp("numerator", n);
ar & boost::serialization::make_nvp("denominator", d);
}
template <typename Archive, typename T>
void load(Archive& ar, ::boost::rational<T>& r, unsigned /*version*/)
{
int n, d;
ar & boost::serialization::make_nvp("numerator", n);
ar & boost::serialization::make_nvp("denominator", d);
r = ::boost::rational<T>(n, d);
}
} }
BOOST_SERIALIZATION_SPLIT_FREE(boost::rational<int>);
using namespace boost;
#include <iostream>
#include <sstream>
int main()
{
rational<int> number(2, 3), other;
std::stringstream ss;
{
archive::xml_oarchive oa(ss);
oa << serialization::make_nvp("rational", number);
}
std::cout << "Serialized: '" << ss.str() << "'\n";
{
archive::xml_iarchive ia(ss);
ia >> serialization::make_nvp("rational", other);
}
std::cout << "Deserialized: " << other;
}
Prints
Serialized: '<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<!DOCTYPE boost_serialization>
<boost_serialization signature="serialization::archive" version="10">
<rational class_id="0" tracking_level="0" version="0">
<numerator>2</numerator>
<denominator>3</denominator>
</rational>
</boost_serialization>
'
Deserialized: 2/3

Answer 2

Use the provided input/output functions: https://www.boost.org/doc/libs/1_64_0/libs/rational/rational.html#Input%20and%20Output
Serialize to a std::string:
template <typename I>
std::string serialize(boost::rational<I>& rational) const {
std::stringstream rational_ss;
rational_ss << rational;
return rational_ss.str();
}
This serializes the rational to a string that looks like "3/5" e.g.
To deserialize, use boost::rational's >>.