A.cpp

#include "A.hpp"
#include <cereal/types/polymorphic.hpp>
#include <cereal/archives/json.hpp>

struct Cat : Animal
{
  int x;
  template <class Archive>
  void serialize( Archive & ar )
  {
    ar( x );
  }

  std::string name() { return "cat"; }
};

std::unique_ptr<Animal> makeAnimal(std::string const & animalType)
{
  if( animalType == "cat" )
  {
    return std::unique_ptr<Animal>( new Cat() );
  }
}

CEREAL_REGISTER_TYPE(Cat);

A.hpp

#pragma once
#include <string>
#include <memory>

struct Animal
{
  virtual std::string name() = 0;
};

std::unique_ptr<Animal> makeAnimal(std::string const & animalType);

gistfile1.json

{
    "value0": {
        "polymorphic_id": 2147483649,
        "polymorphic_name": "Cat",
        "ptr_wrapper": {
            "valid": 1,
            "data": {
                "value0": 0
            }
        }
    }
}

main.cpp

#include "A.hpp"
#include <cereal/archives/json.hpp>
#include <cereal/types/polymorphic.hpp>
#include <iostream>

int main()
{
  auto a = makeAnimal("cat");
  {
    cereal::JSONOutputArchive oar(std::cout);
    oar( a );
  }
}

Makefile

all:
	g++ -std=c++11 -I${CEREAL_INCLUDE} -c A.cpp
	ar rvs A.a A.o	
	g++ -std=c++11 -I${CEREAL_INCLUDE} main.cpp A.a -o main