Add Model and Configurable interface. (#4945)

* Apply Configurable to objective functions.
* Apply Model to Learner and Regtree, gbm.
* Add Load/SaveConfig to objs.
* Refactor obj tests to use smart pointer.
* Dummy methods for Save/Load Model.

tests/cpp/objective/test_ranking_obj.cc

@@ -2,33 +2,59 @@
 #include <xgboost/objective.h>
 #include <xgboost/generic_parameters.h>
 #include "../helpers.h"
+#include <xgboost/json.h>
+
+namespace xgboost {
 
 TEST(Objective, PairwiseRankingGPair) {
   xgboost::GenericParameter tparam;
   std::vector<std::pair<std::string, std::string>> args;
   tparam.InitAllowUnknown(args);
 
-  xgboost::ObjFunction * obj =
-      xgboost::ObjFunction::Create("rank:pairwise", &tparam);
+  std::unique_ptr<xgboost::ObjFunction> obj {
+    xgboost::ObjFunction::Create("rank:pairwise", &tparam)
+  };
   obj->Configure(args);
+  CheckConfigReload(obj, "rank:pairwise");
+
   // Test with setting sample weight to second query group
   CheckRankingObjFunction(obj,
-                   {0, 0.1f, 0, 0.1f},
-                   {0,   1, 0, 1},
-                   {2.0f, 0.0f},
-                   {0, 2, 4},
-                   {1.9f, -1.9f, 0.0f, 0.0f},
-                   {1.995f, 1.995f, 0.0f, 0.0f});
+                          {0, 0.1f, 0, 0.1f},
+                          {0,   1, 0, 1},
+                          {2.0f, 0.0f},
+                          {0, 2, 4},
+                          {1.9f, -1.9f, 0.0f, 0.0f},
+                          {1.995f, 1.995f, 0.0f, 0.0f});
 
   CheckRankingObjFunction(obj,
-                   {0, 0.1f, 0, 0.1f},
-                   {0,   1, 0, 1},
-                   {1.0f, 1.0f},
-                   {0, 2, 4},
-                   {0.95f, -0.95f,  0.95f, -0.95f},
-                   {0.9975f, 0.9975f, 0.9975f, 0.9975f});
+                          {0, 0.1f, 0, 0.1f},
+                          {0,   1, 0, 1},
+                          {1.0f, 1.0f},
+                          {0, 2, 4},
+                          {0.95f, -0.95f,  0.95f, -0.95f},
+                          {0.9975f, 0.9975f, 0.9975f, 0.9975f});
 
   ASSERT_NO_THROW(obj->DefaultEvalMetric());
-
-  delete obj;
 }
+
+TEST(Objective, NDCG_Json_IO) {
+  xgboost::GenericParameter tparam;
+  tparam.InitAllowUnknown(Args{});
+
+  std::unique_ptr<xgboost::ObjFunction> obj {
+    xgboost::ObjFunction::Create("rank:ndcg", &tparam)
+  };
+
+  obj->Configure(Args{});
+  Json j_obj {Object()};
+  obj->SaveConfig(&j_obj);
+
+  ASSERT_EQ(get<String>(j_obj["name"]), "rank:ndcg");;
+
+  auto const& j_param = j_obj["lambda_rank_param"];
+
+  ASSERT_EQ(get<String>(j_param["num_pairsample"]), "1");
+  ASSERT_EQ(get<String>(j_param["fix_list_weight"]), "0");
+}
+
+}  // namespace xgboost