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。
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 periods
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;
}
{ // duration operators: +、-、*、/、>、<、!=、and so on
std::chrono::duration<int> foo;
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.\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 duration
std::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 zero
using 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";
else
std::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_clock
using 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: 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 << '\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::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:\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;
}
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