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是在C++11中引入的，是一个模板库，用来处理时间和日期的Time library。要使用chrono库，需要include<chrono>，其所有实现均在chrono命名空间下。
std::chrono::duration：记录时间长度的，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。
template<class Rep, class Period = ratio<1> > class duration;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(即一个时间嘀嗒tick)是秒钟的倍数或分数，在编译时应为一个有理数常量。
std::chrono::time_point：记录时间点的，表示一个具体时间。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，该时钟的每个时间嘀嗒单位是均匀的(即长度相等)。
(3)、high_resolution_clock：当前系统实现的一个高分辨率时钟。
chrono is the name of a header, but also of a sub-namespace: All the elements in this header(except for the common_type specializations) are not defined directly under the std namespace (like most of the standard library) but under the std::chrono namespace.
The elements in this header deal with time. This is done mainly by means of three concepts:
(1)、Durations: They measure time spans, like: one minute, two hours, or ten milliseconds. In this library, they are represented with objects of the duration class template, that couples a count representation and a period precision (e.g., ten milliseconds has ten as count representation and milliseconds as period precision).
(2)、Time points:A reference to a specific point in time, like one's birthday, today's dawn, or when the next train passes. In this library, objects of the time_point class template express this by using a duration relative to an epoch (which is a fixed point in time common to all time_point objects using the same clock).
(3)、Clocks: A framework that relates a time point to real physical time. The library provides at least three clocks that provide means to express the current time as a time_point: system_clock, steady_clock and high_resolution_clock.
std::chrono::duration:A duration object expresses a time span by means of a count and a period.
std::chrono::duration_values:This is a traits class to provide the limits and zero value of the type used to represent the count in a duration object.
std::chrono::high_resolution_clock:The members of clock classes provide access to the current time_point.high_resolution_clock is the clock with the shortest tick period. It may be a synonym for system_clock or steady_clock.
std::chrono::steady_clock/std::chrono::system_clock:Clock classes provide access to the current time_point. system_clock is a system-wide realtime clock. steady_clock is specifically designed to calculate time intervals.
std::chrono::time_point:A time_point object expresses a point in time relative to a clock's epoch.
下面是从其他文章中copy的<chrono>测试代码，详细内容介绍可以参考对应的reference：
#include "chrono.hpp"#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 periodstypedef 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); // 24hseconds_type s_oneday(60 * 60 * 24); // 86400smilliseconds_type ms_oneday(s_oneday); // 86400000msseconds_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 onstd::chrono::duration<int> foo;std::chrono::duration<int> bar(10);// counts: foo bar// --- ---foo = bar; // 10 10foo = foo + bar; // 20 10++foo; // 21 10--bar; // 21 9foo *= 2; // 42 9foo /= 3; // 14 9//bar += (foo % bar); // 14 14std::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 secondfoo *= 60;std::cout << "duration (in periods): ";std::cout << foo.count() << " milliseconds.\n";std::cout << "duration (in seconds): ";std::cout << foo.count() * milliseconds::period::num / milliseconds::period::den;std::cout << " seconds.\n";}{ // duration::max: Returns the maximum value of duration // duration::min: Returns the minimum value of durationstd::cout << "system_clock durations can represent:\n";std::cout << "min: " << std::chrono::system_clock::duration::min().count() << "\n";std::cout << "max: " << std::chrono::system_clock::duration::max().count() << "\n";}{ // duration::zero: Returns a duration value of zerousing std::chrono::steady_clock;steady_clock::time_point t1 = steady_clock::now();std::cout << "Printing out something...\n";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.\n";elsestd::cout << "The internal clock advanced " << d.count() << " periods.\n";}{ // 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_clockusing namespace std::chrono;high_resolution_clock::time_point t1 = high_resolution_clock::now();std::cout << "printing out 1000 stars...\n";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...\n";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: 2000timeinfo.tm_mon = 0; // month: januarytimeinfo.tm_mday = 1; // day: 1ststd::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_clockusing 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 valuesystem_clock::duration dtn(duration<int>(1)); // 1 second// tp tp2 dtn// --- --- ---tp += dtn; // e+1s e 1stp2 -= dtn; // e+1s e-1s 1stp2 = tp + dtn; // e+1s e+2s 1stp = dtn + tp2; // e+3s e+2s 1stp2 = tp2 - dtn; // e+3s e+1s 1sdtn = tp - tp2; // e+3s e+1s 2sstd::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 objectusing namespace std::chrono;system_clock::time_point tp_epoch;// epoch valuetime_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 pointusing 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/Chronostatic long fibonacci(unsigned n){if (n < 2) return n;return fibonacci(n - 1) + fibonacci(n - 2);}int test_chrono_1(){{ // std::chrono::time_pointstd::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 << '\n';std::chrono::steady_clock::time_point start = std::chrono::steady_clock::now();std::cout << "Hello World\n";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.\n";}{ // std::chrono::durationusing 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:\n";std::cout << std::chrono::duration_cast<shakes>(sec).count() << " shakes\n";std::cout << std::chrono::duration_cast<jiffies>(sec).count() << " jiffies\n";std::cout << microfortnights(sec).count() << " microfortnights\n";std::cout << nanocenturies(sec).count() << " nanocenturies\n";}{ // std::chrono::time_point<std::chrono::system_clock> start, end;start = std::chrono::system_clock::now();std::cout << "f(42) = " << fibonacci(42) << '\n';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\n";}return 0;}
GitHub： https://github.com/fengbingchun/Messy_Test