--- a/trainer.cpp
+++ b/trainer.cpp
@@ -103,7 +103,7 @@
     if (!fn) {
       return false;
     }
-    x_.push_back(std::make_pair<std::string, FeatureNode *>(y, fn));
+    x_.push_back(std::make_pair(y, fn));
     return true;
   }