Serialize and deserialize Eigen Matrix using jsoncpp

eigen_json_serialization.cpp

#include <iostream>
#include <sstream>
#include <string>
#include <Eigen/Core>
#include <vector>
#include <memory>

#include "json/json.h"

template<typename EigenType>
Json::Value toJSON(const std::string& name, const Eigen::PlainObjectBase<EigenType>& mat)
{
    Json::Value traj;
    traj["rows"] = mat.rows();
    traj["cols"] = mat.cols();
    const std::size_t size = mat.size() * sizeof(typename EigenType::Scalar);
    std::vector<double> vec(mat.data(), mat.data() + mat.rows() * mat.cols());
    Json::Value v;
    for (auto&& element: vec) {
        v.append(element);
     }
    traj["data"] = v;
    Json::Value root;
    root[name] = traj;
    return root;
}

Eigen::MatrixXd toEigen(Json::Value jv)
{
    Eigen::MatrixXd ret;
    int rows = jv["rows"].asInt();
    int cols = jv["cols"].asInt();
    Json::Value data = jv["data"];    
    Eigen::MatrixXd a;
    a.resize(rows, cols);
    
    for(int i=0; i<rows; ++i)
    {
        for(int j=0; j<cols; ++j)
        {
            a(i,j) = data[i + j*rows].asDouble();
        }
    }
    return a;
}

Json::Value read4mJson(std::string strr)
{
    std::stringstream ss;
    ss << strr;
    Json::Value root;
    Json::CharReaderBuilder builder;
    builder["collectComments"] = true;
    JSONCPP_STRING errs;
    if(!parseFromStream(builder, ss, &root, &errs))
    {
        std::cout << errs << std::endl;
        return EXIT_FAILURE;
    }

    return root;
}


int main() {
  
  Eigen::MatrixXd a(3,4);
  a << 1,2,3,4,5,6,7,8,-1,-2,-3,-4;

  std::cout << "Matrix to serialize : \n"<< a << std::endl;

  Json::Value jv = toJSON("mat", a);
  Json::StreamWriterBuilder builder;
  const std::unique_ptr<Json::StreamWriter> writer(builder.newStreamWriter());
  std::stringstream ss;
  writer->write(jv, &ss);

  Json::Value rv = read4mJson(ss.str());
  Eigen::MatrixXd k = toEigen(rv["mat"]);
  std::cout << "De-serialized matrix : \n"<< k << std::endl;
  return EXIT_SUCCESS;
}

https://godbolt.org/z/zc3faePPY