C++ 學習筆記：chrono - Part 2 clock 與 timepoint

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#include <chrono> #include <iostream> #include <iomanip> #include <thread> #include <ctime> #include <string> #include <vector> #include <numeric> template <typename T> void printClockData() { using PeriodType = typename T::period; std::cout << " - precision: "; std::cout << static_cast<double>(PeriodType::num) / PeriodType::den << " seconds" << std::endl; std::cout << " - is_steady: " << std::boolalpha << T::is_steady << std::endl; } std::string asString(const std::chrono::system_clock::time_point& tp) { constexpr bool blUseLocalTime = false; std::time_t t = std::chrono::system_clock::to_time_t(tp); std::string ts = blUseLocalTime ? std::ctime(&t) : std::asctime(gmtime(&t)); ts.resize(ts.size() - 1); return ts; } volatile int sink; int main() { /* Clock */ std::cout << "system_clock: " << std::endl; printClockData<std::chrono::system_clock>(); std::cout << "\nsteady_clock: " << std::endl; printClockData<std::chrono::steady_clock>(); std::cout << "\nhigh_resolution_clock: " << std::endl; printClockData<std::chrono::high_resolution_clock>(); /* system_clock */ auto system_start = std::chrono::system_clock::now(); // do some heavy work here, more than five seconds using namespace std::chrono_literals; // std::this_thread::sleep_for(6s); if (std::chrono::system_clock::now() > system_start + std::chrono::seconds(5)) { // If someone adjust the clock, condition may produce "false" std::cout << "Checked!" << std::endl; } auto diffSec = std::chrono::system_clock::now() - system_start; auto sec = std::chrono::duration_cast<std::chrono::milliseconds>(diffSec); std::cout << "This program runs: " << sec.count() << " milliseconds" << std::endl; /* steady_clock */ for (auto size = 1ull; size < 1000000000ull; size *= 100) { auto start = std::chrono::steady_clock::now(); std::vector<int> Vec(size, 123); sink = std::accumulate(Vec.begin(), Vec.end(), 0u); // make sure it's a side effect auto end = std::chrono::steady_clock::now(); std::chrono::duration<double> diffSec = end - start; std::cout << "Time to fill and iterate a vector of " << size << " ints : " << diffSec.count() << " s\n"; } /* Time Point */ std::chrono::system_clock::time_point tp; auto sysClockNow = std::chrono::system_clock::now(); auto sysClockMin = std::chrono::system_clock::time_point::min(); auto sysClockMax = std::chrono::system_clock::time_point::max(); std::cout << "epoch: " << asString(tp) << std::endl; std::cout << "current time: " << asString(sysClockNow) << std::endl; std::cout << "minimum time: " << asString(sysClockMin) << std::endl; std::cout << "maximum time: " << asString(sysClockMax) << std::endl; /* Timepoint and Duration Arithmetic */ using TypeDays = std::chrono::duration<int, std::ratio<3600 * 24>>; std::chrono::time_point<std::chrono::system_clock> timePoint; std::cout << "epoch: " << asString(tp) << std::endl; tp += TypeDays(1) + std::chrono::hours(23) + std::chrono::minutes(55); auto diff = tp - std::chrono::system_clock::time_point(); auto minutesDiff = std::chrono::duration_cast<std::chrono::minutes>(diff); TypeDays daysDiff = std::chrono::duration_cast<TypeDays>(diff); std::cout << "later: " << asString(tp) << std::endl; std::cout << "diff: " << minutesDiff.count() << " minute(s)" << std::endl; std::cout << "diff: " << daysDiff.count() << " day(s)" << std::endl; tp -= std::chrono::hours(24 * 365); std::cout << "-1 year: " << asString(tp) << std::endl; tp -= std::chrono::duration<int, std::ratio<3600 * 24 * 365>>(50); std::cout << "-50 years: " << asString(tp) << std::endl; tp -= std::chrono::duration<int, std::ratio<3600 * 24 * 365>>(505000); std::cout << "-505000 years: " << asString(tp) << std::endl;// underflow }

chrono 的 clock 與 time point

• clock
• 用來定義一個 epoch (time_point) 與一個 tick period/rate (duration)，不同的 clock 有不同的 epoch，例如：
• epoch: std::chrono::system_clock::time_point
• tick period: std::chrono::system_clock::period
• C++ 提供三種 clock:
• system_clock
• 系統上真實時間的 clock (system-wide real time wall clock)
• 提供 function 可以與 C system time time_t 做轉換，且僅有 system_clock 支援此功能
• 可能不是單調遞增的，因為系統時間可以隨時被調整
• steady_clock
• 單調遞增的 clock (monotonic clock)
• 相較於 system_clock，絕對不會被調整，即隨著真實時間演進，此 clock 的 timepoint 絕對不會變小
• 不會對應到真實時間，可以是上次 reboot 時間
• 非常適合用來測量時間間隔
• high_resolution_clock
• 系統上具有最短 tick period 的 clock
• 它可能是 system_clock 或 steady_clock 或是另外一個獨立的 clock 的別名
• 關於 system_clock
• std::chrono::system_clock::now() 會根據系統上的時鐘時間調整，所以有一些運算或是條件判斷是不可信的
• 相較於 system_clock，steady_clock 時間是單調遞增的，運算或是條件式可信
• 根據使用的 clock，提供一個 time_point 型別給 timepoint
• 使用 clock 的 static member function now 可以取得當前的 time_point
• timepoint
• 給定一個 clock 下，根據一個正或負的 duration 來表示一個特定的時間點
• 透過提取 clock 的 time_point (通常是 epoch 或是 now())，再加減上一個 duration 得到另外一個 time_point
• time point 可以表示為(T 為上述三種 clock 任一種)
• std::chrono::T::time_point
• std::chrono::time_point<T>
• 一個 clock 的 time_point 型別具有以下三個重點
• epoch: default-construct 的 time_point 即是 epoch (講白話，起始時間)
• min: 可產生最小的 time_point
• max: 可產生最大的 time_point
• timepoint 在做算術運算時，不考慮閏年

Reference:

1. The C++ Standard Library: A Tutorial and Reference, Second Edition