fix warnings (#3399)

* fix warnings

* Apply suggestions from code review

* Update feature_group.h

* Update feature_group.h

* Update src/treelearner/serial_tree_learner.cpp

* Update multiclass_metric.hpp

include/LightGBM/feature_group.h

@@ -29,9 +29,9 @@ class FeatureGroup {
   * \param num_data Total number of data
   * \param is_enable_sparse True if enable sparse feature
   */
-  FeatureGroup(int num_feature, bool is_multi_val,
+  FeatureGroup(int num_feature, int8_t is_multi_val,
     std::vector<std::unique_ptr<BinMapper>>* bin_mappers,
-    data_size_t num_data) : num_feature_(num_feature), is_multi_val_(is_multi_val), is_sparse_(false) {
+    data_size_t num_data) : num_feature_(num_feature), is_multi_val_(is_multi_val > 0), is_sparse_(false) {
     CHECK_EQ(static_cast<int>(bin_mappers->size()), num_feature);
     // use bin at zero to store most_freq_bin
     num_total_bin_ = 1;

src/metric/multiclass_metric.hpp

@@ -336,7 +336,7 @@ class AucMuMetric : public Metric {
       }
     }
     ans = (2.0 * ans / num_class_) / (num_class_ - 1);
-    return std::vector<double>(1.0, ans);
+    return std::vector<double>(1, ans);
   }
 
  private:

src/treelearner/serial_tree_learner.cpp

@@ -716,7 +716,7 @@ void SerialTreeLearner::RenewTreeOutput(Tree* tree, const ObjectiveFunction* obj
 
 void SerialTreeLearner::ComputeBestSplitForFeature(
     FeatureHistogram* histogram_array_, int feature_index, int real_fidx,
-    bool is_feature_used, int num_data, const LeafSplits* leaf_splits,
+    int8_t is_feature_used, int num_data, const LeafSplits* leaf_splits,
     SplitInfo* best_split, double parent_output) {
   bool is_feature_numerical = train_data_->FeatureBinMapper(feature_index)
                                   ->bin_type() == BinType::NumericalBin;

src/treelearner/serial_tree_learner.h

@@ -120,7 +120,7 @@ class SerialTreeLearner: public TreeLearner {
  protected:
   void ComputeBestSplitForFeature(FeatureHistogram* histogram_array_,
                                   int feature_index, int real_fidx,
-                                  bool is_feature_used, int num_data,
+                                  int8_t is_feature_used, int num_data,
                                   const LeafSplits* leaf_splits,
                                   SplitInfo* best_split, double parent_output);