diff --git a/machine_learning/adaline_learning.cpp b/machine_learning/adaline_learning.cpp
index 73a8f4633..278376fea 100644
--- a/machine_learning/adaline_learning.cpp
+++ b/machine_learning/adaline_learning.cpp
@@ -150,7 +150,7 @@ class adaline {
                       << std::endl;
     }
 
-    friend int activation(double x) { return x > 0 ? 1 : -1; }
+    int activation(double x) { return x > 0 ? 1 : -1; }
 
  private:
     /**