diff --git a/tools/llvm-diff/DifferenceEngine.h b/tools/llvm-diff/DifferenceEngine.h
index 5b4f80b..7ea79e4 100644
--- a/tools/llvm-diff/DifferenceEngine.h
+++ b/tools/llvm-diff/DifferenceEngine.h
@@ -50,7 +50,9 @@
 
     /// An oracle for answering whether two values are equivalent as
     /// operands.
-    struct Oracle {
+    class Oracle {
+      virtual void anchor();
+    public:
       virtual bool operator()(Value *L, Value *R) = 0;
 
     protected: