diff --git a/fake_prefs.h b/fake_prefs.h
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+++ b/fake_prefs.h
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+// Copyright (c) 2012 The Chromium OS 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 CHROMEOS_PLATFORM_UPDATE_ENGINE_FAKE_PREFS_H_
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_FAKE_PREFS_H_
+
+#include <map>
+#include <string>
+
+#include <base/basictypes.h>
+
+#include "update_engine/prefs_interface.h"
+
+namespace chromeos_update_engine {
+
+// Implements a fake preference store by storing the value associated with
+// a key in a std::map, suitable for testing. It doesn't allow to set a value on
+// a key with a different type than the previously set type. This enforces the
+// type of a given key to be fixed. Also the class checks that the Get*()
+// methods aren't called on a key set with a different type.
+
+class FakePrefs : public PrefsInterface {
+ public:
+  FakePrefs() {}
+
+  // PrefsInterface methods.
+  bool GetString(const std::string& key, std::string* value) override;
+  bool SetString(const std::string& key, const std::string& value) override;
+  bool GetInt64(const std::string& key, int64_t* value) override;
+  bool SetInt64(const std::string& key, const int64_t value) override;
+  bool GetBoolean(const std::string& key, bool* value) override;
+  bool SetBoolean(const std::string& key, const bool value) override;
+
+  bool Exists(const std::string& key) override;
+  bool Delete(const std::string& key) override;
+
+ private:
+  enum class PrefType {
+    kString,
+    kInt64,
+    kBool,
+  };
+  struct PrefValue {
+    std::string as_str;
+    int64_t as_int64;
+    bool as_bool;
+  };
+
+  struct PrefTypeValue {
+    PrefType type;
+    PrefValue value;
+  };
+
+  // Class to store compile-time type dependant constants.
+  template<typename T>
+  class PrefConsts {
+   public:
+    // The PrefType associated with T.
+    static FakePrefs::PrefType const type;
+
+    // The data member pointer to PrefValue associated with T.
+    static T FakePrefs::PrefValue::* const member;
+  };
+
+  // Returns a string representation of the PrefType useful for logging.
+  static std::string GetTypeName(PrefType type);
+
+  // Checks that the |key| is either not present or has the given |type|.
+  void CheckKeyType(const std::string& key, PrefType type) const;
+
+  // Helper function to set a value of the passed |key|. It sets the type based
+  // on the template parameter T.
+  template<typename T>
+  void SetValue(const std::string& key, const T& value);
+
+  // Helper function to get a value from the map checking for invalid calls.
+  // The function fails the test if you attempt to read a value  defined as a
+  // different type. Returns whether the get succeeded.
+  template<typename T>
+  bool GetValue(const std::string& key, T* value) const;
+
+  // Container for all the key/value pairs.
+  std::map<std::string, PrefTypeValue> values_;
+
+  DISALLOW_COPY_AND_ASSIGN(FakePrefs);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_FAKE_PREFS_H_