1 Timestamp類主要的檔案

muduo/base/Timestamp.cc  
muduo/base/Timestamp.h  
muduo/base/copyable.h
muduo/base/Types.h

其中Timestamp.cc和Timestamp.h 是類的檔案。
copyable.h檔案中包括的空類是一個標識類，表示繼承該類的所有類都是可複製的，是值語義。
Types.h檔案主要使用了該檔案中對string類的包含。

2 Timestamp.h

#ifndef MUDUO_BASE_TIMESTAMP_H
#define MUDUO_BASE_TIMESTAMP_H

#include <muduo/base/copyable.h>
#include <muduo/base/Types.h>

#include <boost/operators.hpp>

namespace muduo
{

///
/// Time stamp in UTC, in microseconds resolution.
///
/// This class is immutable.
/// It's recommended to pass it by value, since it's passed in register on x64.
///
class Timestamp : public muduo::copyable,
                  public boost::less_than_comparable<Timestamp>
{
 public:
  ///
  /// Constucts an invalid Timestamp.
  ///
  Timestamp()
    : microSecondsSinceEpoch_(0)
  {
  }

  ///
  /// Constucts a Timestamp at specific time
  ///
  /// @param microSecondsSinceEpoch
  explicit Timestamp(int64_t microSecondsSinceEpoch);

  void swap(Timestamp& that)
  {
    std::swap(microSecondsSinceEpoch_, that.microSecondsSinceEpoch_);
  }

  // default copy/assignment/dtor are Okay

  string toString() const;
  string toFormattedString() const;

  bool valid() const { return microSecondsSinceEpoch_ > 0; }

  // for internal usage.
  int64_t microSecondsSinceEpoch() const { return microSecondsSinceEpoch_; }
  time_t secondsSinceEpoch() const
  { return static_cast<time_t>(microSecondsSinceEpoch_ / kMicroSecondsPerSecond); }

  ///
  /// Get time of now.
  ///
  static Timestamp now();
  static Timestamp invalid();

  static const int kMicroSecondsPerSecond = 1000 * 1000;

 private:
  int64_t microSecondsSinceEpoch_;
};

inline bool operator<(Timestamp lhs, Timestamp rhs)
{
  return lhs.microSecondsSinceEpoch() < rhs.microSecondsSinceEpoch();
}

inline bool operator==(Timestamp lhs, Timestamp rhs)
{
  return lhs.microSecondsSinceEpoch() == rhs.microSecondsSinceEpoch();
}

///
/// Gets time difference of two timestamps, result in seconds.
///
/// @param high, low
/// @return (high-low) in seconds
/// @c double has 52-bit precision, enough for one-microseciond
/// resolution for next 100 years.
inline double timeDifference(Timestamp high, Timestamp low)
{
  int64_t diff = high.microSecondsSinceEpoch() - low.microSecondsSinceEpoch();
  return static_cast<double>(diff) / Timestamp::kMicroSecondsPerSecond;
}

///
/// Add @c seconds to given timestamp.
///
/// @return timestamp+seconds as Timestamp
///
inline Timestamp addTime(Timestamp timestamp, double seconds)
{
  int64_t delta = static_cast<int64_t>(seconds * Timestamp::kMicroSecondsPerSecond);
  return Timestamp(timestamp.microSecondsSinceEpoch() + delta);
}

}
#endif  // MUDUO_BASE_TIMESTAMP_H

3 繼承boost::less_than_comparable

boost::less_than_comparable<>是模版類，繼承這個類，表明要求實現<，然後就自動實現>,<=,>=這些比較運算子。這種程式設計是一種模版元的程式設計思想。

可以減少實現的程式碼。

4 Timestamp.cpp

#include <muduo/base/Timestamp.h>

#include <sys/time.h>
#include <stdio.h>
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#undef __STDC_FORMAT_MACROS

#include <boost/static_assert.hpp>

using namespace muduo;

BOOST_STATIC_ASSERT(sizeof(Timestamp) == sizeof(int64_t));

Timestamp::Timestamp(int64_t microseconds)
  : microSecondsSinceEpoch_(microseconds)
{
}

string Timestamp::toString() const
{
  char buf[32] = {0};
  int64_t seconds = microSecondsSinceEpoch_ / kMicroSecondsPerSecond;
  int64_t microseconds = microSecondsSinceEpoch_ % kMicroSecondsPerSecond;
  snprintf(buf, sizeof(buf)-1, "%" PRId64 ".%06" PRId64 "", seconds, microseconds);
  return buf;
}

string Timestamp::toFormattedString() const
{
  char buf[32] = {0};
  time_t seconds = static_cast<time_t>(microSecondsSinceEpoch_ / kMicroSecondsPerSecond);
  int microseconds = static_cast<int>(microSecondsSinceEpoch_ % kMicroSecondsPerSecond);
  struct tm tm_time;
  gmtime_r(&seconds, &tm_time);//thread safe function

  snprintf(buf, sizeof(buf), "%4d%02d%02d %02d:%02d:%02d.%06d",
      tm_time.tm_year + 1900, tm_time.tm_mon + 1, tm_time.tm_mday,
      tm_time.tm_hour, tm_time.tm_min, tm_time.tm_sec,
      microseconds);
  return buf;
}

Timestamp Timestamp::now()
{
  struct timeval tv;
  gettimeofday(&tv, NULL);
  int64_t seconds = tv.tv_sec;
  return Timestamp(seconds * kMicroSecondsPerSecond + tv.tv_usec);
}

Timestamp Timestamp::invalid()
{
  return Timestamp();
}

5 PRId64 是什麼東東

是標頭檔案/usr/include/inttypes.h中定義的一個巨集。可以自動根據作業系統的版本，來表示是"ld"還是"lld"。

6 gmtime_r函式

struct tm *gmtime(const time_t *timep);
struct tm *gmtime_r(const time_t *timep, struct tm *result);

gmtime(執行緒不安全的)是把日期和時間轉換為格林威治(GMT)時間的函式。將引數timep 所指的time_t 結構中的資訊轉換成真實世界所使用的時間日期表示方法，然後將結果由結構tm返回。使用gmtime後要立即處理結果，否則返回的指標指向的內容可能會被覆蓋。
一個好的方法是使用gmtime_r（執行緒安全的），gmtime_r()函式功能與此相同，但是它可以將資料儲存到使用者提供的結構體中，由於使用了使用者分配的記憶體，是不會出錯的。

7 測試用例

muduo/base/tests/Timestamp_unittest.cc檔案是該類的測試檔案。

#include <muduo/base/Timestamp.h>
#include <vector>
#include <stdio.h>

using muduo::Timestamp;

void passByConstReference(const Timestamp& x)
{
  printf("%s\n", x.toString().c_str());
}

void passByValue(Timestamp x)
{
  printf("%s\n", x.toString().c_str());
}

void benchmark()
{
  const int kNumber = 1000*1000;

  std::vector<Timestamp> stamps;
  stamps.reserve(kNumber);
  for (int i = 0; i < kNumber; ++i)
  {
    stamps.push_back(Timestamp::now());
  }
  printf("front: %s\n", stamps.front().toString().c_str());
  printf("back: %s\n", stamps.back().toString().c_str());
  printf("back-front: %f\n", timeDifference(stamps.back(), stamps.front()));

  int increments[100] = { 0 };
  int64_t start = stamps.front().microSecondsSinceEpoch();
  for (int i = 1; i < kNumber; ++i)
  {
    int64_t next = stamps[i].microSecondsSinceEpoch();
    int64_t inc = next - start;
    start = next;
    if (inc < 0)
    {
      printf("reverse!\n");
    }
    else if (inc < 100)
    {
      ++increments[inc];
    }
    else
    {
      printf("big gap %d\n", static_cast<int>(inc));
    }
  }

  for (int i = 0; i < 100; ++i)
  {
    printf("%2d: %d\n", i, increments[i]);
  }
}

int main()
{
  Timestamp now(Timestamp::now());
  printf("%s\n", now.toString().c_str());
  passByValue(now);
  passByConstReference(now);
  benchmark();
}