diff --git a/lang/c++/impl/Stream.cc b/lang/c++/impl/Stream.cc
index 63a8b4e8fc5..60524f01d90 100644
--- a/lang/c++/impl/Stream.cc
+++ b/lang/c++/impl/Stream.cc
@@ -117,7 +117,7 @@ class MemoryInputStream2 : public InputStream {
     }
 };
 
-class MemoryOutputStream : public OutputStream {
+class MemoryOutputStream final : public OutputStream {
 public:
     const size_t chunkSize_;
     std::vector<uint8_t *> data_;