chrono是一个time-library,要使用chrono这个库需要#include<chrono>.

通常要使用chrono这个库必定少不了ratio这个库，需包含#include<ratio>,如果已经#include<chrono>就不需要再#include<ratio>了.

我们首先来明确两个概念:

所谓duration(时间段): 指的是在某时间单位上的一个明确的tick(片刻数).举个例子，“3分钟”指的是“3个一分钟”。

所谓timepoint(时间点):的意思是一个duration和一个epoch(起始点）的组合. 典型的例子是“2000年新年午夜”时间点，它被描述为“自1970年1月1日开始的第1 262 300 400秒”。

ratio:

 template <intmax_t Numerator, intmax_t Denominator = 1> class ratio;

Numerator:  分子.

Denominator: 分母.

ratio有两个成员:

ratio::num; //获得ratio的分子.

ratio::den; //获得ratio的分母.

#include <iostream>
#include <ratio>

int main()
{std::ratio<1, 4> oneForth;std::cout<<"Numrator: "<<oneForth.num<<std::endl; //输出: 1.std::cout<<"Denominator: "<<oneForth.den<<std::endl; //输出: 4.return 0;
}

std::chrono::duration

用来表示一段时间.

 template <class Rep, class Period = ratio<1> >
class duration;

Rep: 是一个算术类型(int, double....),或者一类模拟算术类型.

Period: 是一个std::ratio类型.

比如:

 std::chrono::duration<int, std::ratio<1,1>> seconds(3);//3秒.

由于各种duration的表示是不同的(比如不同的std::duration,用的std::ratio是不同的)因此标准库还提供了:

template <class ToDuration, class Rep, class Period>constexpr ToDuration duration_cast (const duration<Rep,Period>& dtn);

ToDuration: 是我们想要转成duration的类型.

Rep: 是将要被我们转换的duration类型的算术类型.

Period: 是将要被我转换的duration类型的std::ratio类型.

duration的成员函数:

 constexpr rep count() const;

返回该duration中std::ratio的数量.

 static constexpr duration zero();

返回长度为0的duration.

 static constexpr duration min();

返回此类型之最小的可能.

 static constexpr duration zero();

返回此类型之最大的可能.

#include <iostream>
#include <ratio>
#include <chrono>

int main()
{using second_type = std::chrono::duration<int, std::ratio<1, 1>>;using minutes_type = std::chrono::duration<int, std::ratio<60, 1>>;using hour_type = std::chrono::duration<int, std::ratio<3600, 1>>; second_type oneDay1(24*3600); //一天：24*3600秒. minutes_type oneDay2(24*60); //一天: 24*60分钟. hour_type oneDay3(24);      //一天： 24小时. //hour_type oneDay4(oneDay2); //error.不能直接转换. hour_type oneDay5(oneDay3); // ok.hour_type oneDay6(std::chrono::duration_cast<hour_type>(oneDay2)); //ok.std::cout<< oneDay3.count() <<std::endl; //输出： 24.hour_type threeDay = oneDay3 * 3;        // 3天: 3*24 std::cout<< threeDay.count() <<std::endl;//输出: 3 * 24. std::chrono::steady_clock::time_point t1 = std::chrono::steady_clock::now(); //获取当前时间. std::cout<< "Print something..."<<std::endl;std::chrono::steady_clock::time_point t2 = std::chrono::steady_clock::now(); //获取当前时间. std::chrono::steady_clock::duration d = t2 - t1; //计算出std::cout<<"Print something..."<<std::endl;花费的时间. //注意上面使用的是steady_clock中的duration. if(d == std::chrono::steady_clock::duration::zero()){  //判断这段时间是不是为0. std::cout<<"The interal clock did not tick\n"<<std::endl;}else{std::cout<<"The interal clock advanced! "<< d.count() <<std::endl;}using namespace std::chrono;milliseconds ms(7255042);hours hh = duration_cast<hours>(ms);minutes mm = duration_cast<minutes>(ms % hours(1)); //注意这里ms是个std::chrono::milliseconds类型. seconds ss = duration_cast<seconds>(ms % minutes(1));std::cout<<"hours: "<<hh.count()<<" minutes: "<<mm.count()<<" seconds: "<<ss.count()<<std::endl;return 0;
}

Clock:

Clock定义出了一个epoch(起始点)和一个tick(周期，也可以称作period).例如某个Clock也许定义周期(tick)为毫秒（millionsecond），起始点是UNIX epoch(起始点): 1970年1月1日, 或者定义周期(tick)为纳秒(nanosecond),起始点为程序员的开始时间.

Clock还提供了一个类型给“与此Clock关联”的任何timepoint使用，例如Clock的now()成员函数可以产出一个代表“现在时刻”的timepoint对象.

Clock(时钟）有三种：

std::system_clock;  //它所表现的timepoint(时间点)将关联到现行系统的即时时钟.这个clock提供便捷函数to_time_t()和from_time_t(),允许我们在timepoint和C语言的系统时间类型time_t之间转换。

std::steady_clock;  //它保证绝对不会被调整。因此当实际时间流逝，其timepoint值也绝对不会减少，而且这些timepoint对于真实时间都有稳定的前进速率.

std::high_resolution_clock;  //它表现的是当前系统中带有最短tick(周期）的clock.(我们常用的时间周期是1秒).也就是说它是更高精度的std::steady_clock

注意，标准库并不强制规定各种clock的精确度, epoch(起始点), "最大和最小的timepoint的范围"

以上3种Clock都具有的成员类型:

Clock::duration  //获得Clock的duration(时间段)类型.因为一般情况下我们是不知道Clock的计时单位的因此用这个比较保险.

Clock::rep  //获得上面得到的duration的算术类型比如是:double啊，int啊，等等,等价于:Clock::duration::rep

Clock::period //获得当前Clock的duration类型的，计时单位，是分？ 是秒？其实也就是一个std::ratio类型.

也相当于Clock::duration::period.

Clock::is_steady //如果该Clock是一个steady类型，该值为true.

Clock::now()  //一个static函数，获得调用该函数时候的当前时刻.

 #include <iostream>
#include <chrono>
#include <type_traits>

template<typename Clock, typename = typename std::enable_if< std::is_same<Clock, std::chrono::steady_clock>::value|| std::is_same<Clock, std::chrono::system_clock>::value|| std::is_same<Clock, std::chrono::high_resolution_clock>::value>::type>
void printClockData()
{std::cout<<"- precision: ";using P = typename Clock::period; //相当于Clock::duration::period;其实是一个std::ratio类型.if(std::ratio_less_equal<P, std::milli>::value){using TT = typename std::ratio_multiply<P, std::kilo>::type; //这里的type是std::ratio类型.std::cout<< std::fixed << double(TT::num)/TT::den << " milliseconds " << std::endl; }else{std::cout<< std::fixed << double(P::num)/P::den <<" seconds"<<std::endl;} std::cout<<"- is_steady: "<< std::boolalpha << Clock::is_steady << std::endl;
}

int main()
{std::cout<< "system_clock: " <<std::endl;printClockData<std::chrono::system_clock>();std::chrono::system_clock::time_point t1 = std::chrono::system_clock::now();std::cout<< "system_clock::now() "<<std::endl;std::chrono::system_clock::duration sd = std::chrono::system_clock::now() - t1;std::cout<< "cost: "<< sd.count() <<std::endl; //输出: 0 std::cout<< "\nhigh_resolution_clock: "<<std::endl;printClockData<std::chrono::high_resolution_clock>();std::chrono::high_resolution_clock::time_point t2 = std::chrono::high_resolution_clock::now();std::cout<< "high_resolution_clock::now() " <<std::endl;std::chrono::high_resolution_clock::duration hd = std::chrono::high_resolution_clock::now() - t2;std::cout<< "cost: "<< hd.count() <<std::endl; //输出: 0std::cout<< "\nsteady_clock: "<<std::endl;printClockData<std::chrono::steady_clock>();std::chrono::steady_clock::time_point t3 = std::chrono::steady_clock::now();std::cout<< "steady_clock::now() "<<std::endl;std::chrono::steady_clock::duration ssd = std::chrono::steady_clock::now() - t3;std::cout<<"cost: "<< ssd.count() <<std::endl;std::chrono::duration<std::chrono::steady_clock::rep, std::chrono::steady_clock::period> du = ssd;std::chrono::steady_clock::period ratioForSteady_clock;std::cout<< ratioForSteady_clock.num << " " << ratioForSteady_clock.den <<std::endl;return 0;
}

Timepoint(时间点):

夹带（搭配）那些clock，甚至用户自定义的clock，你将可以处理timepoint。Class time_point提供了相应接口，以一个clock为参数.

template <class Clock, class Duration = typename Clock::duration>class time_point;

下面4个特定的timepoint扮演了特殊的角色.

1,Epoch, 由任何clock的time_point的default构造函数产出.

2,Current time, 由任何clock的static成员函数now()产出.

3,Minimnum timepoint, 由任何clock的timepoint的static成员函数min()产出.

4,Maximum timepoint, 由任何clocktimepoint的static成员函数max()产出.

#include <iostream>
#include <chrono>
#include <ctime>
#include <string>std::string asString(const std::chrono::system_clock::time_point& tp)
{std::time_t t = std::chrono::system_clock::to_time_t(tp);std::string timeString = std::asctime(std::gmtime(&t));//timeString.resize(timeString.size()-1);return timeString;
} int main()
{std::chrono::system_clock::time_point tp;std::cout<< "epoch: "<< asString(tp) <<std::endl;tp = std::chrono::system_clock::now();std::cout<< "now: "<< asString(tp) <<std::endl;/*tp = std::chrono::system_clock::time_point::min();std::cout<< "min: "<< asString(tp) <<std::endl;*/ //这种情况下是不行的不能得到最小时间点(timepoint).//使用 std::chrono::time_point_cast; using userDefined = std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<int, std::micro>>;userDefined tp1 = std::chrono::time_point_cast<std::chrono::duration<int, std::micro>>(std::chrono::system_clock::time_point::min());std::cout<< "min: "<< asString(tp1) <<std::endl;//使用 std::chrono::time_point的构造函数. std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<int>> t_p;std::chrono::system_clock::time_point tp2(t_p);tp2 = std::chrono::system_clock::time_point::max();std::cout<< "max: "<< asString(tp2)<<std::endl;return 0;
}

std::chrono::time_point的operator(s):

#include <iostream>
#include <ctime>
#include <chrono>
#include <string>std::string asString(const std::chrono::system_clock::time_point& tp)
{std::time_t t = std::chrono::system_clock::to_time_t(tp);std::string timeString(std::ctime(&t));return timeString;
}int main()
{using Days = std::chrono::duration<int, std::ratio<3600*24>>; using Hours = std::chrono::duration<int, std::ratio<3600>>;using Minutes = std::chrono::duration<int, std::ratio<60>>;using Seconds = std::chrono::duration<int, std::ratio<1,1>>;using Years = std::chrono::duration<unsigned long long, std::ratio<3600*24*365>>;std::chrono::time_point<std::chrono::system_clock> tp1;  std::cout<< "Epoch: "<< asString(tp1) <<std::endl;  //等同于: asString(std::chrono::system_clock::time_point()); //加1天，23小时，55分钟.tp1 += Days(1) + Hours(23) + Minutes(55);std::cout<<"Later: " << asString(tp1) << std::endl;std::chrono::duration<std::chrono::system_clock::rep, std::chrono::system_clock::period> dis = tp1 - std::chrono::system_clock::time_point();//注：上面减号后面的 std::chrono::system_clock::time_point();获得的是Epoch.std::cout<< "Diff:    "<< (std::chrono::duration_cast<std::chrono::minutes>(dis)).count()<< " minute(s)." <<std::endl;Days days = std::chrono::duration_cast<Days>(dis);std::cout<< "days:    "<< days.count() << " day(s)."<<std::endl;tp1 -= Days(365); //该时间点往前推移一年.std::cout<< "-1 year:   " << asString(tp1) <<std::endl; tp1 -= Years(50); //该时间点往前推移五十年.std::cout<< "-50 years:  " << asString(tp1) <<std::endl;tp1 -= Years(50); //该时间点往前推移五十年.std::cout<< "again -50 years:  " << asString(tp1) <<std::endl;return 0; }

16/6/21：

两种打印时间字符串的方法:

#include <iostream>
#include <chrono>
#include <ratio>int main()
{//case 1: 这种情况下是考虑时区的. std::chrono::time_point<std::chrono::system_clock> tp; //epochstd::time_t t = std::chrono::system_clock::to_time_t(tp);std::string strOne = std::ctime(&t);strOne.resize(strOne.size()-1);  //去除尾部的换行符. std::cout<< strOne << std::endl;//case 2: 不考虑时区. std::chrono::time_point<std::chrono::system_clock> tp_one; //epochstd::time_t tTwo = std::chrono::system_clock::to_time_t(tp_one);std::string strTwo = std::asctime(std::gmtime(&tTwo));std::cout<< strTwo <<std::endl;return 0;
}