Serializing ISAM2

[ScottMcMichael]

Description

I am trying to back up an ISAM2 solver object to disk but it does not work properly once it is loaded back from file. This is the gtest code I used to test it (I can also provide the includes and exports used):

TEST(Gtsam, serializeTest) {

  gtsam::ISAM2Params parameters;
  gtsam::ISAM2 solver(parameters);
  gtsam::NonlinearFactorGraph graph;
  gtsam::Values initialValues;
  initialValues.clear();

  gtsam::Vector6 temp6;
  for (int i = 0; i < 6; ++i) {
    temp6[i] = 0.0001;
  }
  gtsam::noiseModel::Diagonal::shared_ptr noiseModel = gtsam::noiseModel::Diagonal::Sigmas(temp6);

  gtsam::Pose3 pose0(gtsam::Rot3(), gtsam::Point3(0, 0, 0));
  gtsam::Symbol symbol0('x', 0);
  graph.add(gtsam::PriorFactor<gtsam::Pose3>(symbol0, pose0, noiseModel));
  initialValues.insert(symbol0, pose0);

  solver.update(graph, initialValues, gtsam::FastVector<size_t>());

  std::string binaryPath = "saved_solver.dat";
  {
    std::ofstream outputStream(binaryPath);
    boost::archive::binary_oarchive outputArchive(outputStream);
    outputArchive << solver;
  }

  gtsam::ISAM2 solverFromDisk;
  {
    std::ifstream ifs(binaryPath);
    boost::archive::binary_iarchive inputArchive(ifs);  
    inputArchive >> solverFromDisk;
  }

  EXPECT_NO_THROW(solver.calculateEstimate<gtsam::Pose3>(symbol0)); // Works
  EXPECT_NO_THROW(solverFromDisk.calculateEstimate<gtsam::Pose3>(symbol0)); // Throws
}

The exception text is: Requested variable 'x0' is not in this VectorValues.

Environment

I am using gtsam version 4.0.3 on Ubuntu 20.04.

Additional information

Looking through the source code, the ISAM2 class inherits the "serialize" function from the BayesTree parent class. ISAM2 adds several member values such as delta_ and theta_ which are not included in the serialize function and this seems to be where the problem comes from. I'm not sure if it would be appropriate to write a new serialize function which includes these or if ISAM2 is not intended to be serialized.

[varunagrawal]

This seems straightforward to fix by adding the necessary Boost Name-Value-Pairs (nvp). Am I missing something?

@ScottMcMichael I don't have a lot of bandwidth right now to fix this for the next few weeks, but if you submit a PR I can provide reviews and recommendations.