Create ForTesting method rather than declaring a dozen "friend" classes.

base/metrics/statistics_recorder.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.
 
 // StatisticsRecorder holds all Histograms and BucketRanges that are used by
 // Histograms in the system. It provides a general place for
 // Histograms/BucketRanges to register, and supports a global API for accessing
 // (i.e., dumping, or graphing) the data.
 
 #ifndef BASE_METRICS_STATISTICS_RECORDER_H_
@@ skipping 154 matching lines @@
   static OnSampleCallback FindCallback(const std::string& histogram_name);
 
   // Returns the number of known histograms.
   static size_t GetHistogramCount();
 
   // Removes a histogram from the internal set of known ones. This can be
   // necessary during testing persistent histograms where the underlying
   // memory is being released.
   static void ForgetHistogramForTesting(base::StringPiece name);
 
-  // Reset any global instance of the statistics-recorder that was created
+  // Creates a local StatisticsRecorder object for testing purposes. All new
+  // histograms will be registered in it until it is destructed or pushed
+  // aside for the lifetime of yet another SR object. The destruction of the
+  // returned object will re-activate the previous one. Always release SR
+  // objects in the opposite order to which they're created.
+  static std::unique_ptr<StatisticsRecorder> CreateTemporaryForTesting()
+      WARN_UNUSED_RESULT;
+
+  // Resets any global instance of the statistics-recorder that was created
   // by a call to Initialize().
   static void UninitializeForTesting();
 
  private:
   // We keep a map of callbacks to histograms, so that as histograms are
   // created, we can set the callback properly.
   typedef std::map<std::string, OnSampleCallback> CallbackMap;
 
   // We keep all |bucket_ranges_| in a map, from checksum to a list of
   // |bucket_ranges_|.  Checksum is calculated from the |ranges_| in
   // |bucket_ranges_|.
   typedef std::map<uint32_t, std::list<const BucketRanges*>*> RangesMap;
 
   friend struct DefaultLazyInstanceTraits<StatisticsRecorder>;
-  friend class ::SubprocessMetricsProviderTest;
-  friend class HistogramBaseTest;
-  friend class HistogramSnapshotManagerTest;
-  friend class HistogramTest;
-  friend class JsonPrefStoreTest;
-  friend class PersistentHistogramAllocatorTest;
-  friend class SharedHistogramTest;
-  friend class SparseHistogramTest;
-  friend class StatisticsRecorderTest;
-  FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,
-                           DeserializeHistogramAndAddSamples);
 
   // Imports histograms from global persistent memory. The global lock must
   // not be held during this call.
   static void ImportGlobalPersistentHistograms();
 
   // The constructor just initializes static members. Usually client code should
   // use Initialize to do this. But in test code, you can friend this class and
   // call the constructor to get a clean StatisticsRecorder.
   StatisticsRecorder();
 
@@ skipping 13 matching lines @@
 
   // Lock protects access to above maps.
   static base::Lock* lock_;
 
   DISALLOW_COPY_AND_ASSIGN(StatisticsRecorder);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_STATISTICS_RECORDER_H_