Optimize memory use of registered histograms.

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 58 matching lines @@
   // safe.  It returns NULL if a matching histogram is not found.
   static HistogramBase* FindHistogram(const std::string& name);
 
   // GetSnapshot copies some of the pointers to registered histograms into the
   // caller supplied vector (Histograms). Only histograms which have |query| as
   // a substring are copied (an empty string will process all registered
   // histograms).
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
  private:
+  // HistogramNameRef holds a weak const ref to the name field of the associated
+  // Histogram object, allowing re-use of the underlying string storage for the
+  // map keys. The wrapper is required as using "const std::string&" as the key
+  // results in compile errors.
+  struct HistogramNameRef {
+    explicit HistogramNameRef(const std::string& name) : name_(name) {};
+
+    // Operator < is necessary to use this type as a std::map key.
+    bool operator<(const HistogramNameRef& other) const {
+      return name_ < other.name_;
+    }
+
+    // Weak, owned by the associated Histogram object.
+    const std::string& name_;
+  };
+
   // We keep all registered histograms in a map, from name to histogram.
-  typedef std::map<std::string, HistogramBase*> HistogramMap;
+  typedef std::map<HistogramNameRef, HistogramBase*> HistogramMap;
 
   // 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, std::list<const BucketRanges*>*> RangesMap;
 
   friend struct DefaultLazyInstanceTraits<StatisticsRecorder>;
   friend class HistogramBaseTest;
   friend class HistogramSnapshotManagerTest;
   friend class HistogramTest;
@@ skipping 16 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_