cartographer时间相关知识点

#include <chrono>
#include <ostream>
#include <ratio>

#include "cartographer/common/port.h"

namespace cartographer {
namespace common {

constexpr int64 kUtsEpochOffsetFromUnixEpochInSeconds =
    (719162ll * 24ll * 60ll * 60ll);//719162ll/365=1970.3

struct UniversalTimeScaleClock {
  using rep = int64;
  using period = std::ratio<1, 10000000>;//单位10ns
  using duration = std::chrono::duration<rep, period>;
  using time_point = std::chrono::time_point<UniversalTimeScaleClock>;
  static constexpr bool is_steady = true;
};

// Represents Universal Time Scale durations and timestamps which are 64-bit
// integers representing the 100 nanosecond ticks since the Epoch which is
// January 1, 1 at the start of day in UTC.
using Duration = UniversalTimeScaleClock::duration;
using Time = UniversalTimeScaleClock::time_point;

// Convenience functions to create common::Durations.
Duration FromSeconds(double seconds);
Duration FromMilliseconds(int64 milliseconds);

// Returns the given duration in seconds.
double ToSeconds(Duration duration);
double ToSeconds(std::chrono::steady_clock::duration duration);

// Creates a time from a Universal Time Scale.
Time FromUniversal(int64 ticks);

// Outputs the Universal Time Scale timestamp for a given Time.
int64 ToUniversal(Time time);

// For logging and unit tests, outputs the timestamp integer.
std::ostream& operator<<(std::ostream& os, Time time);

// CPU time consumed by the thread so far, in seconds.
double GetThreadCpuTimeSeconds();

}  // namespace common
}  // namespace cartographer

下面对上面的代码进行解释：

1、第一行的头文件#include <chrono>

在C++11中，<chrono>是标准模板库中与时间有关的头文件。该头文件中所有函数与类模板均定义在std::chrono命名空间中。

std::chrono::duration：记录时间长度，表示一段时间，如1分钟、2小时、10毫秒等。表示为类模板duration的对象，用一个count representation与一个period precision表示。例如，10毫秒的10为count representation，毫秒为period precision。

第一个模板参数为表示时间计数的数据类型。成员函数count返回该计数。第二个模板参数表示计数的一个周期，一般是std::ratio类型，表示一个周期（即一个时间滴答tick）是秒钟的倍数或分数，在编译时应为一个有理常量。

std::chrono::time_point:记录时间点的，表示一个具体的时间。例如某人的生日、今天的日出时间等。表示为类模板time_point的对象。用相对于一个固定时间点epoch的duration来表示。

std::chrono::clocks:时间点相对于真实物理时间的框架。至少提供了3个clock：

1）system_clock:当前系统范围（即对各进程都一致）的一个实时的日历时钟(Wallclock)。

2）steady_clock:当前系统实现的一个维定时钟，该时钟的每个时间滴答单位是均匀的（即长度相等）。

3）high_resolution_clock:当前系统实现的一个高分辨率时钟。

#include <chrono>  
#include <iostream>  
#include <ratio>  
#include <ctime>  
#include <iomanip>  
  
///////////////////////////////////////////////////////////////  
// reference: http://www.cplusplus.com/reference/chrono/duration/  
int test_chrono_duration()  
{

{ // duration::duration: Constructs a duration object  
  // chrono::duration_cast: Converts the value of dtn into some other duration type,  
  // taking into account differences in their periods  
    typedef std::chrono::duration<int> seconds_type;  //template<typename _Rep, typename _Period> struct duration,前面是类型，后面是单位
    typedef std::chrono::duration<int, std::milli> milliseconds_type;//typedef ratio<1,                      1000> milli;
    typedef std::chrono::duration<int, std::ratio<60 * 60>> hours_type; //template<intmax_t _Num, intmax_t _Den = 1>,分母默认值为1
  
    hours_type h_oneday(24);                  // 24h  
    seconds_type s_oneday(60 * 60 * 24);          // 86400s  
    milliseconds_type ms_oneday(s_oneday);    // 86400000ms  
  
    seconds_type s_onehour(60 * 60);            // 3600s  
    //hours_type h_onehour (s_onehour);          // NOT VALID (type truncates), use:  
    hours_type h_onehour(std::chrono::duration_cast<hours_type>(s_onehour));  
    milliseconds_type ms_onehour(s_onehour);  // 3600000ms (ok, no type truncation)  
  
    std::cout << ms_onehour.count() << "ms in 1h" << std::endl;  
}  
  
{ // duration operators: +、-、*、/、>、<、!=、and so on  
    std::chrono::duration<int> foo;  //默认单位都是秒
    std::chrono::duration<int> bar(10);  
  
    // counts: foo bar  
    //         --- ---  
    foo = bar;                 // 10  10  
    foo = foo + bar;           // 20  10  
    ++foo;                     // 21  10  
    --bar;                     // 21   9  
    foo *= 2;                  // 42   9  
    foo /= 3;                  // 14   9  
    //bar +=  (foo % bar);      // 14  14  
  
    std::cout << std::boolalpha;  
    std::cout << "foo==bar: " << (foo == bar) << std::endl;  
    std::cout << "foo: " << foo.count() << std::endl;  
    std::cout << "bar: " << bar.count() << std::endl;  
}  
  
{ // duration::count: Returns the internal count (i.e., the representation value) of the duration object.  
    using namespace std::chrono;  
    // std::chrono::milliseconds is an instatiation of std::chrono::duration:  
    milliseconds foo(1000); // 1 second  
    foo *= 60;  
  
    std::cout << "duration (in periods): ";  
    std::cout << foo.count() << " milliseconds.
";  
  
    std::cout << "duration (in seconds): ";  
    std::cout << foo.count() * milliseconds::period::num / milliseconds::period::den;  
    std::cout << " seconds.
";  
}  
  
{ // duration::max: Returns the maximum value of duration  
  // duration::min: Returns the minimum value of duration  
    std::cout << "system_clock durations can represent:
";  
    std::cout << "min: " << std::chrono::system_clock::duration::min().count() << "
";  
    std::cout << "max: " << std::chrono::system_clock::duration::max().count() << "
";  
}  
  
{ // duration::zero: Returns a duration value of zero  
    using std::chrono::steady_clock;  
  
    steady_clock::time_point t1 = steady_clock::now();  
  
    std::cout << "Printing out something...
";  
  
    steady_clock::time_point t2 = steady_clock::now();  
  
    steady_clock::duration d = t2 - t1;  
  
    if (d == steady_clock::duration::zero())  
        std::cout << "The internal clock did not tick.
";  
    else  
        std::cout << "The internal clock advanced " << d.count() << " periods.
";  
}  
  
{ // chrono::time_point_cast: Converts the value of tp into a time_point type with a different duration internal object,  
  // taking into account differences in their durations's periods.  
    using namespace std::chrono;  
  
    typedef duration<int, std::ratio<60 * 60 * 24>> days_type;  
  
    time_point<system_clock, days_type> today = time_point_cast<days_type>(system_clock::now());  
  
    std::cout << today.time_since_epoch().count() << " days since epoch" << std::endl;  
}  
  
    return 0;  
}  
  
//////////////////////////////////////////////////////////////////  
// reference: http://www.cplusplus.com/reference/chrono/high_resolution_clock/  
int test_chrono_high_resolution_clock()  
{  
    // high_resolution_clock::now: Returns the current time_point in the frame of the high_resolution_clock  
    using namespace std::chrono;  
  
    high_resolution_clock::time_point t1 = high_resolution_clock::now();  
  
    std::cout << "printing out 1000 stars...
";  
    for (int i = 0; i<1000; ++i) std::cout << "*";  
    std::cout << std::endl;  
  
    high_resolution_clock::time_point t2 = high_resolution_clock::now();  
  
    duration<double> time_span = duration_cast<duration<double>>(t2 - t1);  
  
    std::cout << "It took me " << time_span.count() << " seconds.";  
    std::cout << std::endl;  
  
    return 0;  
}  
  
///////////////////////////////////////////////////////////////////////  
// reference: http://www.cplusplus.com/reference/chrono/steady_clock/  
int test_chrono_steady_clock()  
{  
    // steady_clock is specifically designed to calculate time intervals.  
    // steady_clock::now: Returns the current time_point in the frame of the steady_clock.  
    using namespace std::chrono;  
  
    steady_clock::time_point t1 = steady_clock::now();  
  
    std::cout << "printing out 1000 stars...
";  
    for (int i = 0; i<1000; ++i) std::cout << "*";  
    std::cout << std::endl;  
  
    steady_clock::time_point t2 = steady_clock::now();  
  
    duration<double> time_span = duration_cast<duration<double>>(t2 - t1);  
  
    std::cout << "It took me " << time_span.count() << " seconds.";  
    std::cout << std::endl;  
  
    return 0;  
}  
  
//////////////////////////////////////////////////////////////  
// reference: http://www.cplusplus.com/reference/chrono/system_clock/  
int test_chrono_system_clock()  
{  
    // system_clock is a system-wide realtime clock.  
  
{ // system_clock::from_time_t: Converts t into its equivalent of member type time_point.  
    using namespace std::chrono;  
  
    // create tm with 1/1/2000:  
    std::tm timeinfo = std::tm();  
    timeinfo.tm_year = 100;   // year: 2000  
    timeinfo.tm_mon = 0;      // month: january  
    timeinfo.tm_mday = 1;     // day: 1st  
    std::time_t tt = std::mktime(&timeinfo);  
  
    system_clock::time_point tp = system_clock::from_time_t(tt);  
    system_clock::duration d = system_clock::now() - tp;  
  
    // convert to number of days:  
    typedef duration<int, std::ratio<60 * 60 * 24>> days_type;  
    days_type ndays = duration_cast<days_type> (d);  
  
    // display result:  
    std::cout << ndays.count() << " days have passed since 1/1/2000";  
    std::cout << std::endl;  
}  
  
{ // system_clock::now: Returns the current time_point in the frame of the system_clock  
    using namespace std::chrono;  
  
    duration<int, std::ratio<60 * 60 * 24> > one_day(1);  
  
    system_clock::time_point today = system_clock::now();  
    system_clock::time_point tomorrow = today + one_day;  
  
    time_t tt;  
  
    tt = system_clock::to_time_t(today);  
    std::cout << "today is: " << ctime(&tt);  
  
    tt = system_clock::to_time_t(tomorrow);  
    std::cout << "tomorrow will be: " << ctime(&tt);  
}  
  
{ // system_clock::to_time_t: Converts tp into its equivalent of type time_t.  
    using namespace std::chrono;  
  
    duration<int, std::ratio<60 * 60 * 24> > one_day(1);  
  
    system_clock::time_point today = system_clock::now();  
    system_clock::time_point tomorrow = today + one_day;  
  
    time_t tt;  
  
    tt = system_clock::to_time_t(today);  
    std::cout << "today is: " << ctime(&tt);  
  
    tt = system_clock::to_time_t(tomorrow);  
    std::cout << "tomorrow will be: " << ctime(&tt);  
}  
  
    return 0;  
}  
  
//////////////////////////////////////////////////////  
// reference: http://www.cplusplus.com/reference/chrono/time_point/  
int test_chrono_time_point()  
{  
{ // time_point operators: +、-、==、!=  
    using namespace std::chrono;  
  
    system_clock::time_point tp, tp2;                // epoch value  
    system_clock::duration dtn(duration<int>(1));  // 1 second  
  
    //  tp     tp2    dtn  
    //  ---    ---    ---  
    tp += dtn;          //  e+1s   e      1s  
    tp2 -= dtn;         //  e+1s   e-1s   1s  
    tp2 = tp + dtn;     //  e+1s   e+2s   1s  
    tp = dtn + tp2;     //  e+3s   e+2s   1s  
    tp2 = tp2 - dtn;    //  e+3s   e+1s   1s  
    dtn = tp - tp2;     //  e+3s   e+1s   2s  
  
    std::cout << std::boolalpha;  
    std::cout << "tp == tp2: " << (tp == tp2) << std::endl;  
    std::cout << "tp > tp2: " << (tp>tp2) << std::endl;  
    std::cout << "dtn: " << dtn.count() << std::endl;  
}  
  
{ // time_point::time_point: Constructs a time_point object  
    using namespace std::chrono;  
  
    system_clock::time_point tp_epoch;  // epoch value  
  
    time_point <system_clock, duration<int>> tp_seconds(duration<int>(1));  
  
    system_clock::time_point tp(tp_seconds);  
  
    std::cout << "1 second since system_clock epoch = ";  
    std::cout << tp.time_since_epoch().count();  
    std::cout << " system_clock periods." << std::endl;  
  
    // display time_point:  
    std::time_t tt = system_clock::to_time_t(tp);  
    std::cout << "time_point tp is: " << ctime(&tt);  
}  
  
{ // time_point::time_since_epoch: Returns a duration object with the time span value between the epoch and the time point  
    using namespace std::chrono;  
  
    system_clock::time_point tp = system_clock::now();  
    system_clock::duration dtn = tp.time_since_epoch();  
  
    std::cout << "current time since epoch, expressed in:" << std::endl;  
    std::cout << "periods: " << dtn.count() << std::endl;  
    std::cout << "seconds: " << dtn.count() * system_clock::period::num / system_clock::period::den;  
    std::cout << std::endl;  
}  
  
    return 0;  
}  
  
///////////////////////////////////////////////////////////////////  
// reference: https://zh.wikibooks.org/wiki/C%2B%2B/STL/Chrono  
static long fibonacci(unsigned n)  
{  
    if (n < 2) return n;  
    return fibonacci(n - 1) + fibonacci(n - 2);  
}  
  
int test_chrono_1()  
{  
{ // std::chrono::time_point  
    std::chrono::system_clock::time_point now = std::chrono::system_clock::now();  
    std::time_t now_c = std::chrono::system_clock::to_time_t(now - std::chrono::hours(24));  
    std::cout << "24 hours ago, the time was " << now_c << '
';  
  
    std::chrono::steady_clock::time_point start = std::chrono::steady_clock::now();  
    std::cout << "Hello World
";  
    std::chrono::steady_clock::time_point end = std::chrono::steady_clock::now();  
    std::cout << "Printing took "  
        << std::chrono::duration_cast<std::chrono::microseconds>(end - start).count() << "us.
";  
}  
  
{ // std::chrono::duration  
    using shakes = std::chrono::duration<int, std::ratio<1, 100000000>>;  
    using jiffies = std::chrono::duration<int, std::centi>;  
    using microfortnights = std::chrono::duration<float, std::ratio<12096, 10000>>;  
    using nanocenturies = std::chrono::duration<float, std::ratio<3155, 1000>>;  
  
    std::chrono::seconds sec(1);  
  
    std::cout << "1 second is:
";  
  
    std::cout << std::chrono::duration_cast<shakes>(sec).count() << " shakes
";  
    std::cout << std::chrono::duration_cast<jiffies>(sec).count() << " jiffies
";  
    std::cout << microfortnights(sec).count() << " microfortnights
";  
    std::cout << nanocenturies(sec).count() << " nanocenturies
";  
}  
  
{ //   
    std::chrono::time_point<std::chrono::system_clock> start, end;  
    start = std::chrono::system_clock::now();  
    std::cout << "f(42) = " << fibonacci(42) << '
';  
    end = std::chrono::system_clock::now();  
  
    std::chrono::duration<double> elapsed_seconds = end - start;  
    std::time_t end_time = std::chrono::system_clock::to_time_t(end);  
  
    std::cout << "finished computation at " << std::ctime(&end_time)  
        << "elapsed time: " << elapsed_seconds.count() << "s
";  
}  
    return 0;  
}  

int main(int argc, char** argv)
{

test_chrono_duration();
return 0;
}

看一下时间点，参考其他博客，再次写一下chrono相关的知识

chrono是一个time library, 源于boost，现在已经是C++标准。话说今年似乎又要出新标准了，好期待啊！ 

　　要使用chrono库，需要#include<chrono>，其所有实现均在std::chrono namespace下。注意标准库里面的每个命名空间代表了一个独立的概念。所以下文中的概念均以命名空间的名字表示！ chrono是一个模版库，使用简单，功能强大，只需要理解三个概念：duration、time_point、clock

1）、

Durations

std::chrono::duration 表示一段时间，比如两个小时，12.88秒，半个时辰，一炷香的时间等等，只要能换算成秒即可。

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

其中

Rep表示一种数值类型，用来表示Period的数量，比如int float double

Period是ratio类型，用来表示【用秒表示的时间单位】比如second milisecond

常用的duration<Rep,Period>已经定义好了，在std::chrono::duration下：

ratio<3600, 1>                hours

ratio<60, 1>                    minutes

ratio<1, 1>                      seconds

ratio<1, 1000>               microseconds

ratio<1, 1000000>         microseconds

ratio<1, 1000000000>    nanosecons

 

这里需要说明一下ratio这个类模版的原型：

template <intmax_t N, intmax_t D = 1> class ratio;

N代表分子，D代表分母，所以ratio表示一个分数值。

注意，我们自己可以定义Period，比如ratio<1, -2>表示单位时间是-0.5秒。

由于各种duration表示不同，chrono库提供了duration_cast类型转换函数。

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

典型用法：

// duration constructor
#include <iostream>
#include <ratio>
#include <chrono>

int main ()
{
  typedef std::chrono::duration<int> seconds_type;
  typedef std::chrono::duration<int,std::milli> milliseconds_type;
  typedef std::chrono::duration<int,std::ratio<60*60>> hours_type;

  hours_type h_oneday (24);                  // 24h
  seconds_type s_oneday (60*60*24);          // 86400s
  milliseconds_type ms_oneday (s_oneday);    // 86400000ms

  seconds_type s_onehour (60*60);            // 3600s
//hours_type h_onehour (s_onehour);          // NOT VALID (type truncates), use:
  hours_type h_onehour (std::chrono::duration_cast<hours_type>(s_onehour));
  milliseconds_type ms_onehour (s_onehour);  // 3600000ms (ok, no type truncation)

  std::cout << ms_onehour.count() << "ms in 1h" << std::endl;

  return 0;
}

duration还有一个成员函数count()返回Rep类型的Period数量，看代码：

// duration::count
#include <iostream>     // std::cout
#include <chrono>       // std::chrono::seconds, std::chrono::milliseconds
                        // std::chrono::duration_cast

int main ()
{
  using namespace std::chrono;
  // std::chrono::milliseconds is an instatiation of std::chrono::duration:
  milliseconds foo (1000); // 1 second
  foo*=60;

  std::cout << "duration (in periods): ";
  std::cout << foo.count() << " milliseconds.
";

  std::cout << "duration (in seconds): ";
  std::cout << foo.count() * milliseconds::period::num / milliseconds::period::den;
  std::cout << " seconds.
";

  return 0;
}

2）Time points

std::chrono::time_point表示一个具体时间，如上个世纪80年代、你的生日、今天下午、火车出发时间等，只要它能用计算机时钟表示。鉴于我们使用时间的情景不同，这个time point具体到什么程度，由选用的单位决定。一个time point必须有一个clock计时。

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

下面是构造使用time_point的例子

// time_point constructors
#include <iostream>
#include <chrono>
#include <ctime>

int main ()
{
  using namespace std::chrono;

  system_clock::time_point tp_epoch;    // epoch value

  time_point <system_clock,duration<int>> tp_seconds (duration<int>(1));

  system_clock::time_point tp (tp_seconds);

  std::cout << "1 second since system_clock epoch = ";
  std::cout << tp.time_since_epoch().count();
  std::cout << " system_clock periods." << std::endl;

  // display time_point:
  std::time_t tt = system_clock::to_time_t(tp);
  std::cout << "time_point tp is: " << ctime(&tt);

  return 0;
}

time_point有一个函数time_from_eproch()用来获得1970年1月1日到time_point时间经过的duration。

举个例子，如果timepoint以天为单位，函数返回的duration就以天为单位。

 

由于各种time_point表示方式不同，chrono也提供了相应的转换函数 time_point_cast。

1 template <class ToDuration, class Clock, class Duration>
2   time_point<Clock,ToDuration> time_point_cast (const time_point<Clock,Duration>& tp);

/ time_point_cast
#include <iostream>
#include <ratio>
#include <chrono>

int main ()
{
  using namespace std::chrono;

  typedef duration<int,std::ratio<60*60*24>> days_type;

  time_point<system_clock,days_type> today = time_point_cast<days_type>(system_clock::now());

  std::cout << today.time_since_epoch().count() << " days since epoch" << std::endl;

  return 0;
}

3）clocks

std::chrono::system_clock 它表示当前的系统时钟，系统中运行的所有进程使用now()得到的时间是一致的。

每一个clock类中都有确定的time_point, duration, Rep, Period类型。

操作有：

now() 当前时间time_point

to_time_t() time_point转换成time_t秒

from_time_t() 从time_t转换成time_point

典型的应用是计算时间日期：

// system_clock example
#include <iostream>
#include <ctime>
#include <ratio>
#include <chrono>

int main ()
{
  using std::chrono::system_clock;

  std::chrono::duration<int,std::ratio<60*60*24> > one_day (1);

  system_clock::time_point today = system_clock::now();
  system_clock::time_point tomorrow = today + one_day;

  std::time_t tt;

  tt = system_clock::to_time_t ( today );
  std::cout << "today is: " << ctime(&tt);

  tt = system_clock::to_time_t ( tomorrow );
  std::cout << "tomorrow will be: " << ctime(&tt);

  return 0;
}

std::chrono::steady_clock 为了表示稳定的时间间隔，后一次调用now()得到的时间总是比前一次的值大（这句话的意思其实是，如果中途修改了系统时间，也不影响now()的结果），每次tick都保证过了稳定的时间间隔。

操作有：

now() 获取当前时钟

典型的应用是给算法计时：

// steady_clock example
#include <iostream>
#include <ctime>
#include <ratio>
#include <chrono>

int main ()
{
  using namespace std::chrono;

  steady_clock::time_point t1 = steady_clock::now();

  std::cout << "printing out 1000 stars...
";
  for (int i=0; i<1000; ++i) std::cout << "*";
  std::cout << std::endl;

  steady_clock::time_point t2 = steady_clock::now();

  duration<double> time_span = duration_cast<duration<double>>(t2 - t1);

  std::cout << "It took me " << time_span.count() << " seconds.";
  std::cout << std::endl;

  return 0;
}

最后一个时钟，std::chrono::high_resolution_clock 顾名思义，这是系统可用的最高精度的时钟。实际上high_resolution_clock只不过是system_clock或者steady_clock的typedef。

 

2、第三行#include <ratio>

其是在c++11中引入的，模板类std::ratio以及相关的模板类（如std::ratio_add）提供编译时有理数算术支持。

此模板的每个实例化都准确表示任意有限有理数。他们都是用来表示比例关系的模板类。

声明

template<intmax_t N, intmax_t D = 1> class ratio;

其中N表示分子，D表示分母；intmax_t表示最大的有符号整数类型，N和D的绝对值都应该在intmax_t可表示的范围内，D不能为0.

std::ratio类一般不通过它的对象来表示，而是这个类型本身来表示的，但也可以通过它的对象来表示。std::ratio有两个成员常量：num表示的是分子，den表示的是分母。这里的num和den已经经过化简，因此输出值可能和定义时传入的不同，如typedef std::ratio<100, 10> ratio1, 输出值ratio1::num为10，ratio1::den为1，求其最大公约数。

在include<ratio>中，除std::ratio外，还有编译时有理数算术：std::ratio_add、std::ratio_subtract、std::ratio_multiply、std::ratio_divide，它们和std::ratio一样，也都有两个成员常量：num和den。编译时有理数比较：std::ratio_equal、std::ratio_not_equal、std::ratio_less、std::ratio_less_equal、std::ratio_greater、std::ratio_greater_equal，它们都有value成员常量。它们的计算公式如下：

template <typename R1, typename R2>
using ratio_add = ratio < R1::num*R2::den+R2::num*R1::den, R1::den*R2::den >
 
template <typename R1, typename R2>
using ratio_subtract = std::ratio < R1::num*R2::den-R2::num*R1::den, R1::den*R2::den >
 
template <typename R1, typename R2>
using ratio_multiply = std::ratio < R1::num * R2::num, R1::den * R2::den >;
 
template <typename R1, typename R2>
using ratio_divide = ratio < R1::num * R2::den, R2::num * R1::den >;
 
template <class R1, class R2>
struct ratio_equal : integral_constant<bool, R1::num==R2::num && R1::den==R2::den> {}
 
template <class R1, class R2>
struct ratio_less : integral_constant < bool, R1::num*R2::den < R2::num*R1::den > {};
 
template <class R1, class R2>
struct ratio_greater : integral_constant < bool, ratio_less<R2,R1>::value > {};
 
template <class R1, class R2>
struct ratio_greater_equal : integral_constant < bool, !ratio_less<R1,R2>::value > {};
 
template <class R1, class R2>
struct ratio_less_equal : integral_constant < bool, !ratio_less<R2,R1>::value > {}
 
template <class R1, class R2>
struct ratio_equal : integral_constant < bool, !ratio_equal<R1,R2>::value > {};

#include "ratio.hpp"
#include <iostream>
#include <ratio>
 
//
// reference: http://www.cplusplus.com/reference/ratio/
int test_ratio_1()
{
    typedef std::ratio<1, 3> one_third;
    typedef std::ratio<2, 4> two_fourths;
    typedef std::ratio<2, 3> two_thirds;
    typedef std::ratio<1, 2> one_half;
 
    std::cout << "one_third= " << one_third::num << "/" << one_third::den << std::endl;
    std::cout << "two_fourths= " << two_fourths::num << "/" << two_fourths::den << std::endl; // Note: 1/2
    std::cout << "two_thirds= " << two_thirds::num << "/" << two_thirds::den << std::endl;
    std::cout << "one_half= " << one_half::num << "/" << one_half::den << std::endl;
    std::cout << std::endl;
 
{ // std::ratio
    typedef std::ratio_add<one_third, two_fourths> sum;
 
    std::cout << "sum= " << sum::num << "/" << sum::den;
    std::cout << " (which is: " << (double(sum::num) / sum::den) << ")" << std::endl;
    std::cout << "1 kilogram has " << (std::kilo::num / std::kilo::den) << " grams";
    std::cout << std::endl;
}
 
{ // std::ratio_add
    typedef std::ratio_add<one_half, two_thirds> sum;
 
    std::cout << "sum = " << sum::num << "/" << sum::den;
    std::cout << " (which is: " << (double(sum::num) / sum::den) << ")" << std::endl;
}
 
{ // std::ratio_subtract
    typedef std::ratio_subtract<two_thirds, one_half> diff;
 
    std::cout << "diff = " << diff::num << "/" << diff::den;
    std::cout << " (which is: " << (double(diff::num) / diff::den) << ")" << std::endl;
}
 
{ // std::ratio_multiply
    typedef std::ratio_multiply<one_half, one_third> result;
 
    std::cout << "result = " << result::num << "/" << result::den;
    std::cout << " (which is: " << (double(result::num) / result::den) << ")" << std::endl;
}
 
{ // std::ratio_divide
    typedef std::ratio_divide<one_half, one_third> result;
 
    std::cout << "result = " << result::num << "/" << result::den;
    std::cout << " (which is: " << (double(result::num) / result::den) << ")" << std::endl;
    std::cout << std::endl;
}
 
{ // std::ratio_equal
    std::cout << "1/2 == 2/4 ? " << std::boolalpha;
    std::cout << std::ratio_equal<one_half, two_fourths>::value << std::endl;
}
 
{ // std::ratio_greater
    std::cout << "1/3 > 1/2 ? " << std::boolalpha;
    std::cout << std::ratio_greater<one_third, one_half>::value << std::endl;
}
 
{ // std::ratio_greater_equal
    std::cout << "1/3 >= 1/2 ? " << std::boolalpha;
    std::cout << std::ratio_greater_equal<one_third, one_half>::value << std::endl;
}
 
{ // std::ratio_less
    std::cout << "1/3 < 1/2 ? " << std::boolalpha;
    std::cout << std::ratio_less<one_third, one_half>::value << std::endl;
}
 
{ // std::ratio_less_equal
    std::cout << "1/3 <= 1/2 ? " << std::boolalpha;
    std::cout << std::ratio_less_equal<one_third, one_half>::value << std::endl;
}
 
{ // std::ratio_not_equal
    std::cout << "1/2 != 2/4 ? " << std::boolalpha;
    std::cout << std::ratio_not_equal<one_half, two_fourths>::value << std::endl;
}
    std::cout << std::endl;
 
    return 0;
}
 
/
// reference: https://stackoverflow.com/questions/25005205/why-can-i-have-a-ratio-object-in-c
int test_ratio_2()
{
    // If you don't use a typedef you're creating an instance of std::ratio<1, 3> named one_third,
    // which is not suitable for passing as a type argument.In that case you'll need to use decltype
    // to get to the appropriate type that can be passed to ratio_add
    std::ratio<1, 3> one_third;
    std::ratio<2, 4> two_fourths;
    std::ratio_add<decltype(one_third), decltype(two_fourths)> sum;
    std::cout << decltype(sum)::den << std::endl;
 
    return 0;
}

std::ratio<num, den> 定义分式

    std::ratio<60, 1> minutes;//<分子，分母>一分钟60秒
    std::ratio<60 * 60> hource;//1小时3600秒
    std::ratio<1, 1000> milliseconds; //1ms是1/1000秒

chrono命名空间定义好的时间单位

typedef duration <Rep, ratio<3600,1>> hours;
   typedef duration <Rep, ratio<60,1>> minutes;
   typedef duration <Rep, ratio<1,1>> seconds;
   typedef duration <Rep, ratio<1,1000>> milliseconds;
   typedef duration <Rep, ratio<1,1000000>> microseconds;
   typedef duration <Rep, ratio<1,1000000000>> nanoseconds;

举例

chrono::minutes mintu{2};//2分钟
    chrono::seconds sec{3};//3秒钟
    chrono::milliseconds mills{500};//500毫秒
    auto dul = sec - mills;//两者差值，单位默认转到更小的 2500ms
    dul.count(); //值为2500
    std::this_thread::sleep_for(std::chrono::milliseconds(100)); //当前线程休眠100毫秒

time.cc

#include "cartographer/common/time.h"

#include <time.h>

#include <cerrno>
#include <cstring>
#include <string>

#include "glog/logging.h"

namespace cartographer {
namespace common {

Duration FromSeconds(const double seconds) {
  return std::chrono::duration_cast<Duration>(
      std::chrono::duration<double>(seconds));
}

double ToSeconds(const Duration duration) {
  return std::chrono::duration_cast<std::chrono::duration<double>>(duration)
      .count();
}

double ToSeconds(const std::chrono::steady_clock::duration duration) {
  return std::chrono::duration_cast<std::chrono::duration<double>>(duration)
      .count();
}

Time FromUniversal(const int64 ticks) {
    //using duration = std::chrono::duration<rep, period>;
    //  using time_point = std::chrono::time_point<UniversalTimeScaleClock>;
    printf("ticks is %ld ", ticks);//add by gary
    printf("Duration'count is %lf ",Duration(ticks).count());
    printf("Time is %lf
",Time(Duration(ticks)).time_since_epoch().count());
    return Time(Duration(ticks));
}

int64 ToUniversal(const Time time) { return time.time_since_epoch().count(); }

std::ostream& operator<<(std::ostream& os, const Time time) {
  os << std::to_string(ToUniversal(time));
  return os;
}

common::Duration FromMilliseconds(const int64 milliseconds) {
  return std::chrono::duration_cast<Duration>(
      std::chrono::milliseconds(milliseconds));
}

double GetThreadCpuTimeSeconds() {
#ifndef WIN32
  struct timespec thread_cpu_time;
  CHECK(clock_gettime(CLOCK_THREAD_CPUTIME_ID, &thread_cpu_time) == 0)
      << std::strerror(errno);
  return thread_cpu_time.tv_sec + 1e-9 * thread_cpu_time.tv_nsec;
#else
  return 0.;
#endif
}

}  // namespace common
}  // namespace cartographer