resolve comments

include/grpcpp/impl/codegen/message_allocator.h

@@ -39,9 +39,14 @@ class RpcAllocatorState {
 template <typename RequestT, typename ResponseT>
 class MessageHolder : public RpcAllocatorState {
  public:
-  virtual void Release() { delete this; }
+  // Release this object. For example, if the custom allocator's
+  // AllocateMessasge creates an instance of a subclass with new, the Release()
+  // should do a "delete this;".
+  virtual void Release() = 0;
   RequestT* request() { return request_; }
   ResponseT* response() { return response_; }
+
+ protected:
   void set_request(RequestT* request) { request_ = request; }
   void set_response(ResponseT* response) { response_ = response; }
 

test/cpp/end2end/message_allocator_end2end_test.cc

@@ -346,6 +346,7 @@ class ArenaAllocatorTest : public MessageAllocatorEnd2endTestBase {
         set_response(
             google::protobuf::Arena::CreateMessage<EchoResponse>(&arena_));
       }
+      void Release() override { delete this; }
       void FreeRequest() override { GPR_ASSERT(0); }
 
      private: