[Blocking] Fix #3840: Clean up logic for parsing tree_method parameter

Makefile

@@ -250,6 +250,8 @@ Rpack: clean_all
 	cp -r src xgboost/src/src
 	cp -r include xgboost/src/include
 	cp -r amalgamation xgboost/src/amalgamation
+	mkdir -p xgboost/src/tests/cpp
+	cp tests/cpp/test_learner.h xgboost/src/tests/cpp
 	mkdir -p xgboost/src/rabit
 	cp -r rabit/include xgboost/src/rabit/include
 	cp -r rabit/src xgboost/src/rabit/src

src/common/enum_class_param.h

@@ -0,0 +1,81 @@
+/*!
+ * Copyright 2018 by Contributors
+ * \file enum_class_param.h
+ * \brief macro for using C++11 enum class as DMLC parameter
+ * \author Hyunsu Philip Cho
+ */
+
+#ifndef XGBOOST_COMMON_ENUM_CLASS_PARAM_H_
+#define XGBOOST_COMMON_ENUM_CLASS_PARAM_H_
+
+#include <dmlc/parameter.h>
+#include <string>
+#include <type_traits>
+
+/*!
+ * \brief Specialization of FieldEntry for enum class (backed by int)
+ *
+ * Use this macro to use C++11 enum class as DMLC parameters
+ *
+ * Usage:
+ *
+ * \code{.cpp}
+ *
+ *   // enum class must inherit from int type
+ *   enum class Foo : int {
+ *     kBar = 0, kFrog = 1, kCat = 2, kDog = 3
+ *   };
+ *
+ *   // This line is needed to prevent compilation error
+ *   DECLARE_FIELD_ENUM_CLASS(Foo);
+ *
+ *   // Now define DMLC parameter as usual;
+ *   //   enum classes can now be members.
+ *   struct MyParam : dmlc::Parameter<MyParam> {
+ *     Foo foo;
+ *     DMLC_DECLARE_PARAMETER(MyParam) {
+ *       DMLC_DECLARE_FIELD(foo)
+ *         .set_default(Foo::kBar)
+ *         .add_enum("bar", Foo::kBar)
+ *         .add_enum("frog", Foo::kFrog)
+ *         .add_enum("cat", Foo::kCat)
+ *         .add_enum("dog", Foo::kDog);
+ *     }
+ *   };
+ *
+ *   DMLC_REGISTER_PARAMETER(MyParam);
+ * \endcode
+ */
+#define DECLARE_FIELD_ENUM_CLASS(EnumClass) \
+namespace dmlc {  \
+namespace parameter {  \
+template <>  \
+class FieldEntry<EnumClass> : public FieldEntry<int> {  \
+ public:  \
+  FieldEntry<EnumClass>() {  \
+    static_assert(  \
+      std::is_same<int, typename std::underlying_type<EnumClass>::type>::value,  \
+      "enum class must be backed by int");  \
+    is_enum_ = true;  \
+  }  \
+  using Super = FieldEntry<int>;  \
+  void Set(void *head, const std::string &value) const override {  \
+    Super::Set(head, value);  \
+  }  \
+  inline FieldEntry<EnumClass>& add_enum(const std::string &key, EnumClass value) {  \
+    Super::add_enum(key, static_cast<int>(value));  \
+    return *this;  \
+  }  \
+  inline FieldEntry<EnumClass>& set_default(const EnumClass& default_value) {  \
+    default_value_ = static_cast<int>(default_value);  \
+    has_default_ = true;  \
+    return *this;  \
+  }  \
+  inline void Init(const std::string &key, void *head, EnumClass& ref) {  /* NOLINT */  \
+    Super::Init(key, head, *reinterpret_cast<int*>(&ref));  \
+  }  \
+};  \
+}  /* namespace parameter */  \
+}  /* namespace dmlc */
+
+#endif  // XGBOOST_COMMON_ENUM_CLASS_PARAM_H_