[swig] Refactor StringArray* with unique_ptr

swig/StringArray_API_extensions.i

@@ -18,6 +18,7 @@
 %apply char **STRING_ARRAY {char **StringArrayHandle_get_strings};
 
 #include <iostream>
+#include <memory>
 
 %inline %{
 
@@ -225,21 +226,20 @@ class StringArray
                                      int eval_counts,
                                      StringArrayHandle *strings)
     {
+        strings = nullptr;
+        std::unique_ptr<StringArray> strings_ptr(nullptr);
+
         try {
             // TODO: @reviewer, 128 was the chosen size, any particular reason for this constraint?
-            *strings = new StringArray(eval_counts, 128);
-        } catch (std::bad_alloc &e) {
-            *strings = nullptr;
-            LGBM_SetLastError("Failure to allocate memory."); // TODO: @reviewer is not setting the message.
+            strings_ptr.reset(new StringArray(eval_counts, 128));
+        } catch (std::bad_alloc &e) {            
+            LGBM_SetLastError("Failure to allocate memory."); // TODO: @reviewer - why is this not setting the message?
             return -1;
         }
 
-        int api_return = LGBM_BoosterGetEvalNames(handle, &eval_counts,
-                                                  static_cast<StringArray*>(*strings)->data());
-        if (api_return == -1) {
-            // Call failed, no point in returning memory to free later:
-            StringArrayHandle_free(*strings);
-            *strings = nullptr;
+        int api_return = LGBM_BoosterGetEvalNames(handle, &eval_counts, strings_ptr->data());
+        if (api_return == 0) {
+            *strings = strings_ptr.release();
         }
 
         return api_return;
@@ -259,32 +259,30 @@ class StringArray
                                         int max_feature_name_size,
                                         StringArrayHandle * strings)
     {
+        std::unique_ptr<StringArray> strings_ptr(nullptr);
         int api_return;
 
         // 1) To preallocate memory extract number of features first:
         int num_features;
         api_return = LGBM_BoosterGetNumFeature(handle, &num_features);
-        if (api_return == -1)
+        if (api_return == -1) {
             return -1;
+        }
 
         // 2) Allocate an array of strings:
         try {
             // TODO: @reviewer should we also figure out the size of the biggest string and remove parameter max_feature_name_size?
-            *strings = new StringArray(num_features, max_feature_name_size);
+            strings_ptr.reset(new StringArray(num_features, max_feature_name_size));
         } catch (std::bad_alloc &e) {
-            *strings = nullptr;
-            LGBM_SetLastError("Failure to allocate memory."); // TODO: @reviewer is not setting the message.
+            LGBM_SetLastError("Failure to allocate memory."); // TODO: @reviewer - why is this not setting the message?
             return -1;
         }
 
         // 3) Extract feature names:
         int _dummy_out_num_features; // already know how many there are (to allocate memory).
-        api_return = LGBM_BoosterGetFeatureNames(handle, &_dummy_out_num_features,
-                                                 static_cast<StringArray*>(*strings)->data());
-        if (api_return == -1)
-        {
-            StringArrayHandle_free(*strings);
-            *strings = nullptr;
+        api_return = LGBM_BoosterGetFeatureNames(handle, &_dummy_out_num_features, strings_ptr->data());
+        if (api_return == 0) {
+            *strings = strings_ptr.release();
         }
 
         return api_return;