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 <iostream>#include <boost/static_assert.hpp>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<false>'，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 <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./////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<Timestamp>                  //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<time_t>(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<int64_t>(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<double>(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<int64_t>(seconds * Timestamp::kMicroSecondsPerSecond);  return Timestamp(timestamp.microSecondsSinceEpoch() + delta);}}#endif  // MUDUO_BASE_TIMESTAMP_H

下面是cpp：

#include <muduo/base/Timestamp.h>#include <sys/time.h>#include <stdio.h>#ifndef __STDC_FORMAT_MACROS#define __STDC_FORMAT_MACROS#endif#include <inttypes.h>#include <boost/static_assert.hpp>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<time_t>(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<int>(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}