diff --git a/src/scanner.hpp b/src/scanner.hpp
index 9e4a6f6..df933d8 100644
--- a/src/scanner.hpp
+++ b/src/scanner.hpp
@@ -19,6 +19,7 @@ public:
     }
 
 private:
-    virtual int yylex();
+    // implemented by flex
+    virtual int yylex() override;
     yy::parser::semantic_type *yylval;
 };
\ No newline at end of file