Remove std::string histogram name from HistogramBase object.

base/metrics/histogram_base.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 BASE_METRICS_HISTOGRAM_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
@@ skipping 115 matching lines @@
   enum Inconsistency : uint32_t {
     NO_INCONSISTENCIES = 0x0,
     RANGE_CHECKSUM_ERROR = 0x1,
     BUCKET_ORDER_ERROR = 0x2,
     COUNT_HIGH_ERROR = 0x4,
     COUNT_LOW_ERROR = 0x8,
 
     NEVER_EXCEEDED_VALUE = 0x10,
   };
 
-  explicit HistogramBase(const std::string& name);
+  // Construct the base histogram. The name is not copied; it's up to the
+  // caller to ensure that it lives at least as long as this object.
+  explicit HistogramBase(const char* name);
   virtual ~HistogramBase();
 
-  const std::string& histogram_name() const { return histogram_name_; }
+  const char* histogram_name() const { return histogram_name_; }
 
   // Comapres |name| to the histogram name and triggers a DCHECK if they do not
   // match. This is a helper function used by histogram macros, which results in
   // in more compact machine code being generated by the macros.
   void CheckName(const StringPiece& name) const;
 
   // Get a unique ID for this histogram's samples.
   virtual uint64_t name_hash() const = 0;
 
   // Operations with Flags enum.
@@ skipping 104 matching lines @@
   // Write textual description of the bucket contents (relative to histogram).
   // Output is the count in the buckets, as well as the percentage.
   void WriteAsciiBucketValue(Count current,
                              double scaled_sum,
                              std::string* output) const;
 
   // Retrieves the callback for this histogram, if one exists, and runs it
   // passing |sample| as the parameter.
   void FindAndRunCallback(Sample sample) const;
 
+  // Gets a permanent string that can be used for histogram objects when the
+  // original is not a code constant or held in persistent memory.
+  static const char* GetPermanentName(const std::string& name);
+
   // Update report with an |activity| that occurred for |histogram|.
   static void ReportHistogramActivity(const HistogramBase& histogram,
                                       ReportActivity activicty);
 
   // Retrieves the global histogram reporting what histograms are created.
   static HistogramBase* report_histogram_;
 
  private:
   friend class HistogramBaseTest;
 
-  const std::string histogram_name_;
+  // A pointer to permanent storage where the histogram name is held. This can
+  // be code space or the output of GetPermanentName() or any other storage
+  // that is known to never change. This is not StringPiece because (a) char*
+  // is 1/2 the size and (b) StringPiece transparently casts from std::string
+  // which can easily lead to a pointer to non-permanent space.
+  const char* const histogram_name_;
+
+  // Additional information about the histogram.
   AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_