muduo库的Timestamp类剖析

分析之前我们先来看，Timestamp中用了一个BOOST_STATIC_ASSERT宏，这是编译时断言，而我们平时用的assert是运行时断言。

示例：

#include 
#include 

class Timestamp {
private:
    int64_t microSecondsSinceEpoch_;
};

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

int main()
{
    BOOST_STATIC_ASSERT(3 == 2); 

    return 0;
}

第一个通过，第二个3=2不会通过，输出test.cpp:13:2: error: invalid application of 'sizeof' to incomplete type 'boost::STATIC_ASSERTION_FAILURE'，编译器断言检查。

来看一下它的接口图：

下面来分析正文，里面都加上了注释：

#ifndef MUDUO_BASE_TIMESTAMP_H
#define MUDUO_BASE_TIMESTAMP_H

#include 
#include 

#include 

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.
///
//muduo::copyable空基类，标识类，值类型，凡是继承了它就可以拷贝  
//值语义，可以拷贝，拷贝之后与原对象脱离关系
//对象语义，要么不能拷贝，要么可以拷贝，拷贝之后与原对象仍存在一定关系，比如共享一定资源，取决于自己的拷贝构造函数
class Timestamp : public muduo::copyable,
                  public boost::less_than_comparable
                  //less_than_comparable<>是boost的一个模板类，继承该类要求实现<，然后它会自动实现>,<=,>=，有点牛逼。。。
{
 public:
  ///
  /// Constucts an invalid Timestamp.
  ///
  Timestamp()
    : microSecondsSinceEpoch_(0)  //从1970年到当前的时间，单位微秒
  {
  }

  ///
  /// Constucts a Timestamp at specific time
  ///
  /// @param microSecondsSinceEpoch
  explicit Timestamp(int64_t microSecondsSinceEpochArg)
    : microSecondsSinceEpoch_(microSecondsSinceEpochArg)
  {
  }

  //交换函数,形参是引用
  void swap(Timestamp& that)
  {
    std::swap(microSecondsSinceEpoch_, that.microSecondsSinceEpoch_);
  }

  // default copy/assignment/dtor are Okay

  string toString() const;
  string toFormattedString(bool showMicroseconds = true) const;

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

  // for internal usage.
  int64_t microSecondsSinceEpoch() const { return microSecondsSinceEpoch_; }
  //time_t不是结构体，定义为tydef long time_t，一般存放从unxi时间到当前的秒数
  time_t secondsSinceEpoch() const     //微秒转化为秒
  { return static_cast(microSecondsSinceEpoch_ / kMicroSecondsPerSecond); }

  ///
  /// Get time of now.
  ///
  static Timestamp now();
  static Timestamp invalid()
  {
    return Timestamp();  //获取一个无效时间，即时间等于0
  }

  static Timestamp fromUnixTime(time_t t)   //time_t类型时间转化为内部是unix时间的Timestamp结构
  {
    return fromUnixTime(t, 0);  //调用内部函数执行实际操作，微秒忽略
  }

  static Timestamp fromUnixTime(time_t t, int microseconds)
  {
    return Timestamp(static_cast(t) * kMicroSecondsPerSecond + microseconds); 
  }

  static const int kMicroSecondsPerSecond = 1000 * 1000;  //一百万，一微秒等于百万分之一秒

 private:
  int64_t microSecondsSinceEpoch_;
};

//只需重载<,> <= >= 这几种less_than_comparable()帮我们实现
inline bool operator<(Timestamp lhs, Timestamp rhs)
{
  return lhs.microSecondsSinceEpoch() < rhs.microSecondsSinceEpoch();
}

//判等
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-microsecond
/// resolution for next 100 years.
inline double timeDifference(Timestamp high, Timestamp low)  //实现两个事件的差
{
  int64_t diff = high.microSecondsSinceEpoch() - low.microSecondsSinceEpoch();
  return static_cast(diff) / Timestamp::kMicroSecondsPerSecond;   //将返回时间差的秒数，注意单位!
}

///
/// Add @c seconds to given timestamp.
///
/// @return timestamp+seconds as Timestamp
///
inline Timestamp addTime(Timestamp timestamp, double seconds)   //
{
  //把秒转化为微秒，构造一个对象，再把它们的时间加起来，构造一个无名临时对象返回
  int64_t delta = static_cast(seconds * Timestamp::kMicroSecondsPerSecond);
  return Timestamp(timestamp.microSecondsSinceEpoch() + delta);
}

}
#endif  // MUDUO_BASE_TIMESTAMP_H

下面是cpp：

#include 

#include 
#include 

#ifndef __STDC_FORMAT_MACROS
#define __STDC_FORMAT_MACROS
#endif

#include 

#include 

using namespace muduo;

//编译阶段断言
BOOST_STATIC_ASSERT(sizeof(Timestamp) == sizeof(int64_t));

string Timestamp::toString() const
{
  char buf[32] = {0};
  int64_t seconds = microSecondsSinceEpoch_ / kMicroSecondsPerSecond;
  int64_t microseconds = microSecondsSinceEpoch_ % kMicroSecondsPerSecond;
  //PRId64跨平台打印64位整数，因为int64_t用来表示64位整数，在32位系统中是long long int，64位系统中是long int
  //所以打印64位是%ld或%lld，可移植性较差，不如统一同PRID64来打印。
  snprintf(buf, sizeof(buf)-1, "%" PRId64 ".%06" PRId64 "", seconds, microseconds);
  return buf;
}

//把它转换成一个格式化字符串
string Timestamp::toFormattedString(bool showMicroseconds) const
{
  char buf[32] = {0};
  //计算1970到现在的秒数
  time_t seconds = static_cast(microSecondsSinceEpoch_ / kMicroSecondsPerSecond);
  struct tm tm_time;
  gmtime_r(&seconds, &tm_time);  //把秒数转换成tm结构体，加_r表示是一个线程安全的函数

  if (showMicroseconds)  //是否显示微秒标识 
  {
    //取余是微秒
    int microseconds = static_cast(microSecondsSinceEpoch_ % kMicroSecondsPerSecond);
    //年月日时分秒
    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);
  }
  else
  {
    snprintf(buf, sizeof(buf), "%4d%02d%02d %02d:%02d:%02d",
             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);
  }
  return buf;   //返回string，隐式构造!!!
}

Timestamp Timestamp::now()
{
  struct timeval tv;    
  gettimeofday(&tv, NULL);   //获得当前时间，第二个参数是一个时区，当前不需要返回时区，就填空指针
  int64_t seconds = tv.tv_sec;   //取出秒数
  return Timestamp(seconds * kMicroSecondsPerSecond + tv.tv_usec);  //秒*100万+微秒，就是从1970到现在的微秒数
}