RefCounted types should not have public destructors, delegate cleanup

chrome/browser/history/history.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_HISTORY_HISTORY_H_
 #define CHROME_BROWSER_HISTORY_HISTORY_H_
 #pragma once
 
 #include <set>
 #include <vector>
@@ skipping 231 matching lines @@
   // discard it.
   void SetPageContents(const GURL& url, const string16& contents);
 
   // Querying ------------------------------------------------------------------
 
   // Callback class that a client can implement to iterate over URLs. The
   // callbacks WILL BE CALLED ON THE BACKGROUND THREAD! Your implementation
   // should handle this appropriately.
   class URLEnumerator {
    public:
-    virtual ~URLEnumerator() {}
-
     // Indicates that a URL is available. There will be exactly one call for
     // every URL in history.
     virtual void OnURL(const GURL& url) = 0;
 
     // Indicates we are done iterating over URLs. Once called, there will be no
     // more callbacks made. This call is guaranteed to occur, even if there are
     // no URLs. If all URLs were iterated, success will be true.
     virtual void OnComplete(bool success) = 0;
+
+   protected:
+    virtual ~URLEnumerator() {}
   };
 
   // Enumerate all URLs in history. The given iterator will be owned by the
   // caller, so the caller should ensure it exists until OnComplete is called.
   // You should not generally use this since it will be slow to slurp all URLs
   // in from the database. It is designed for rebuilding the visited link
   // database from history.
   void IterateURLs(URLEnumerator* iterator);
 
   // Returns the information about the requested URL. If the URL is found,
@@ skipping 671 matching lines @@
   // The index used for quick history lookups.
   scoped_ptr<history::InMemoryURLIndex> in_memory_url_index_;
 
   scoped_refptr<ObserverListThreadSafe<history::VisitDatabaseObserver> >
       visit_database_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryService);
 };
 
 #endif  // CHROME_BROWSER_HISTORY_HISTORY_H_