1 files changed, 2 insertions, 1 deletions
diff --git a/callback.h b/callback.h
index ca088ac..487fd40 100644
--- a/callback.h
+++ b/callback.h
@@ -26,7 +26,8 @@ namespace i18n {
 namespace phonenumbers {
 
 template <typename R, typename A1, typename A2, typename A3, typename A4>
-struct ResultCallback4 {
+class ResultCallback4 {
+ public:
   virtual ~ResultCallback4() {}
   virtual R Run(A1 a1, A2 a2, A3 a3, A4 a4) = 0;
 };