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+// Copyright 2008, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef BASE_REVOCABLE_STORE_H__
+#define BASE_REVOCABLE_STORE_H__
+
+#include "base/basictypes.h"
+#include "base/logging.h"
+#include "base/ref_counted.h"
+
+// |RevocableStore| is a container of items that can be removed from the store.
+class RevocableStore {
+ public:
+  // A |StoreRef| is used to link the |RevocableStore| to its items.  There is
+  // one StoreRef per store, and each item holds a reference to it.  If the
+  // store wishes to revoke its items, it sets |store_| to null.  Items are
+  // permitted to release their reference to the |StoreRef| when they no longer
+  // require the store.
+ class StoreRef : public base::RefCounted<StoreRef> {
+   public:
+    StoreRef(RevocableStore* store) : store_(store) { }
+
+    void set_store(RevocableStore* store) { store_ = store; }
+    RevocableStore* store() const { return store_; }
+
+   private:
+    RevocableStore* store_;
+
+    DISALLOW_EVIL_CONSTRUCTORS(StoreRef);
+  };
+
+  // An item in the store.  On construction, the object adds itself to the
+  // store.
+  class Revocable {
+   public:
+    Revocable(RevocableStore* store);
+    ~Revocable();
+
+    // This item has been revoked if it no longer has a pointer to the store.
+    bool revoked() const { return !store_reference_->store(); }
+
+  private:
+    // We hold a reference to the store through this ref pointer.  We release
+    // this reference on destruction.
+    scoped_refptr<StoreRef> store_reference_;
+
+    DISALLOW_EVIL_CONSTRUCTORS(Revocable);
+  };
+
+  RevocableStore();
+  ~RevocableStore();
+
+  // Revokes all the items in the store.
+  void RevokeAll();
+
+  // Returns true if there are no items in the store.
+  bool empty() const { return count_ == 0; }
+
+ private:
+  friend class Revocable;
+
+  // Adds an item to the store.  To add an item to the store, construct it
+  // with a pointer to the store.
+  void Add(Revocable* item);
+
+  // This is the reference the unrevoked items in the store hold.
+  scoped_refptr<StoreRef> owning_reference_;
+
+  // The number of unrevoked items in the store.
+  int count_;
+
+  DISALLOW_EVIL_CONSTRUCTORS(RevocableStore);
+};
+
+#endif  // BASE_REVOCABLE_STORE_H__