Make HybridBayesTree testable and add serialization

gtsam/hybrid/HybridBayesTree.h

@@ -89,8 +89,20 @@ class GTSAM_EXPORT HybridBayesTree : public BayesTree<HybridBayesTreeClique> {
   VectorValues optimize(const DiscreteValues& assignment) const;
 
   /// @}
+
+ private:
+  /** Serialization function */
+  friend class boost::serialization::access;
+  template <class ARCHIVE>
+  void serialize(ARCHIVE& ar, const unsigned int /*version*/) {
+    ar& BOOST_SERIALIZATION_BASE_OBJECT_NVP(Base);
+  }
 };
 
+/// traits
+template <>
+struct traits<HybridBayesTree> : public Testable<HybridBayesTree> {};
+
 /**
  * @brief Class for Hybrid Bayes tree orphan subtrees.
  *

gtsam/hybrid/tests/testHybridBayesTree.cpp

@@ -16,6 +16,7 @@
  * @date    August 2022
  */
 
+#include <gtsam/base/serializationTestHelpers.h>
 #include <gtsam/discrete/DiscreteFactorGraph.h>
 #include <gtsam/hybrid/HybridBayesTree.h>
 #include <gtsam/hybrid/HybridGaussianISAM.h>
@@ -143,6 +144,20 @@ TEST(HybridBayesTree, Optimize) {
   EXPECT(assert_equal(expectedValues, delta.continuous()));
 }
 
+/* ****************************************************************************/
+// Test HybridBayesTree serialization.
+TEST(HybridBayesTree, Serialization) {
+  Switching s(4);
+  Ordering ordering = s.linearizedFactorGraph.getHybridOrdering();
+  HybridBayesTree hbt =
+      *(s.linearizedFactorGraph.eliminateMultifrontal(ordering));
+
+  using namespace gtsam::serializationTestHelpers;
+  EXPECT(equalsObj<HybridBayesTree>(hbt));
+  EXPECT(equalsXML<HybridBayesTree>(hbt));
+  EXPECT(equalsBinary<HybridBayesTree>(hbt));
+}
+
 /* ************************************************************************* */
 int main() {
   TestResult tr;