How to de/serialize a map with template class using boost::multiprecision::mpq_rational

Question

I have the following template class:

#include 

Answer 1

There's passthrough support for Boost Multiprecision serialization:

Classes number, debug_adaptor, logged_adaptor and rational_adaptor have "pass through" serialization support which requires the underlying backend to be serializable.

Backends cpp_int, cpp_bin_float, cpp_dec_float and float128 have full support for Boost.Serialization.

That is to say, it's supported iff the backend supports it: mailing list:

> Does/will multiprecesion have support for boost::serialization?

Good question, and no not yet. I guess it should do though... it's hard though as some backends (for example GMP's mpf_t) don't round trip to and from string representations, and have an internal structure that probably shouldn't be relied upon :-

You can:

• use cpp_rational (as it is supported implicitly by the above doc excerpt)
• use Boost Rational: How to serialize boost::rational

• use raw mpz_* API to serialize, see e.g. How to serialize a GMP rational number?

• you can explicitly add serialization support for the backend (note that using the gmp API's will be more efficient)

  namespace boost { namespace serialization {
  
      template 
          void save(Archive& ar, ::boost::multiprecision::backends::gmp_rational const& r, unsigned /*version*/)
          {
              std::string tmp = r.str(10000, std::ios::fixed);
              ar & tmp;
          }
  
      template 
          void load(Archive& ar, ::boost::multiprecision::backends::gmp_rational& r, unsigned /*version*/)
          {
              std::string tmp;
              ar & tmp;
              r = tmp.c_str();
          }
  
  } }
  
  BOOST_SERIALIZATION_SPLIT_FREE(::boost::multiprecision::backends::gmp_rational)
  

Here's a demo Live On Coliru with a simple roundtrip test.

Full Listing

#include 
#include 
#include 
#include 
#include 
#include 

namespace boost { namespace serialization {

    template 
        void save(Archive& ar, ::boost::multiprecision::backends::gmp_rational const& r, unsigned /*version*/)
        {
            std::string tmp = r.str(10000, std::ios::fixed);
            ar & tmp;
        }

    template 
        void load(Archive& ar, ::boost::multiprecision::backends::gmp_rational& r, unsigned /*version*/)
        {
            std::string tmp;
            ar & tmp;
            r = tmp.c_str();
        }

} }

BOOST_SERIALIZATION_SPLIT_FREE(::boost::multiprecision::backends::gmp_rational)

#include 
#include 
#include 
#include 
#include 
#include