Skeleton code of SparseHistogram

base/metrics/sparse_histogram.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_SPARSE_HISTOGRAM_H_
+#define BASE_METRICS_SPARSE_HISTOGRAM_H_
+
+#include <map>
+#include <string>
+
+#include "base/base_export.h"
+#include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram_base.h"
+
+namespace base {
+
+class Lock;
+
+class BASE_EXPORT_PRIVATE SparseHistogram : public HistogramBase {

[Ilya Sherman, 2012/08/04 01:18:35]

Can you help satisfy my curiosity and explain how BASE_EXPORT_PRIVATE differs
from BASE_EXPORT?

[kaiwang, 2012/08/08 03:59:33]

Definitions are exactly the same... But I have to export the class or I can not
test.
Anyone writing code outside base knows he should not depend on this
EXPORT_PRIVATE, but there's no hard restrictions.

+ public:
+  typedef std::map<Sample, Count> SampleCounts;

[Ilya Sherman, 2012/08/04 01:18:35]

nit: I strongly prefer not to typedef this; you're saving very few characters,
at the cost of clarity.  However, if you do keep this typedef, please name it
something more like "SampleToCountMap"; by convention, "SampleCounts" should be
a list of SampleCount, which this is not.

[kaiwang, 2012/08/08 03:59:33]

ok
I don't like this kind of typedefs either. Just want to be consistency with
other code of histogram

+
+  // If there's one with same name, return the existing one. If not, create a
+  // new one.
+  static HistogramBase* FactoryGet(const std::string& name, Flags flags);
+
+  virtual ~SparseHistogram();
+
+  virtual void Add(Sample value) OVERRIDE;
+
+  virtual void SnapshotSample(SampleCounts* sample) const;

[Ilya Sherman, 2012/08/04 01:18:35]

nit: Who will be calling this method?  It seems odd to have any non-overridden
public methods defined for this class, given that there's no way for a client to
construct an object of this type.

[Ilya Sherman, 2012/08/04 01:18:35]

nit: This method's name is confusing, since elsewhere "Sample" refers to a
single int32, whereas here is seems to refer to a map.

[kaiwang, 2012/08/08 03:59:33]

This is a temporary method.. Keep it for testing before I can abstract the data
structure from Histogram::SampleSet

[Ilya Sherman, 2012/08/08 05:00:29]

Ok.  In that case, please add a comment indicating that this is a temporary
method, intended only for testing.  Maybe also name the method
SnapshotSampleForTesting(), so that it's hard for non-test dependencies on it to
accidentally creep in.

+  virtual void WriteHTMLGraph(std::string* output) const OVERRIDE;
+  virtual void WriteAscii(std::string* output) const OVERRIDE;
+
+ protected:
+  // Clients should always use FactoryGet to create SparseHistogram.
+  SparseHistogram(const std::string& name);
+
+ private:
+  friend class SparseHistogramTest;  // For constuctor calling.
+
+  SampleCounts sample_;
+
+  // Protects access to above map.
+  scoped_ptr<base::Lock> lock_;

[Ilya Sherman, 2012/08/04 01:18:35]

nit: Why is this heap-allocated?  It seems more natural to declare it as an
inline member.

[kaiwang, 2012/08/08 03:59:33]

Tried to limit header file dependencies. But I think you are right, it does not
worth

+};

[Ilya Sherman, 2012/08/04 01:18:35]

nit: DISALLOW_COPY_AND_ASSIGN?

[kaiwang, 2012/08/08 03:59:33]

Done.

+
+}  // namespace base
+
+#endif  // BASE_METRICS_SPARSE_HISTOGRAM_H_