12 C++20’s <chrono> Calendars and Time Zones in MSVC Miya Natsuhara ("MEE-yuh Not-soo-HAR-uh") Miya.Natsuhara@microsoft.com Software Engineer, Visual C++ LibrariesWelcome to CppCon 2021! Join tracking the issues, PRs, discussions, etc. related to C++20 chrono: Extensions to <chrono> (github.com) • Feature branch (feature/chrono) for rapid development and collaboration • Code Review Videos class types • chrono::day • chrono::month • chrono::year • chrono::month_day • chrono::weekday • chrono::weekday_indexed • chrono::year_month_day • chrono::year_month_day_last • chrono::year_month_weekday_last

implement: std::chrono::nanoseconds elapsed_time(uint64_t num_cpu_ticks); 1 2 3 4 8Example: “CPU ticks” time units constexpr uint64_t CPU_CLOCK_HZ = 400'000'000; // API to implement: std::chrono::nanoseconds num_cpu_ticks); 1 2 3 4 std::chrono::nanoseconds elapsed_time(uint64_t num_cpu_ticks) { using NS_PER_TICK = std::ratio<1'000'000'000, CPU_CLOCK_HZ>; return std::chrono::nanoseconds{ num_cpu_ticks uint64_t CPU_CLOCK_HZ = 400'000'000; // API to implement: std::chrono::nanoseconds elapsed_time(uint64_t num_cpu_ticks); 1 2 3 4 std::chrono::nanoseconds elapsed_time(uint64_t num_cpu_ticks) { using

0_C ; // floating point UDL IPv4Addr loopback = "127.0.0.1"_IPv4; // string UDL std::chrono::hours half_day = 12h ; // integer UDL const std::string greeting = "hello"s ; 2i; // complex(5.0, -3.2)Chrono duration UDLs ■ In sub-namespace chrono_literals: ■ Namespace std::chrono::chrono_literals is an alias for std::literals::chrono_literals. 10/26/21 Pablo Halpern literal type h chrono::hours Appropriate floating-point instantiation of chrono::duration min chrono::minutes s chrono::seconds ms chrono::milliseconds us chrono::microseconds ns chrono::nanoseconds

} 49 . 1TESTING WITH MORE TYPES TESTING WITH MORE TYPES using namespace std::chrono_literals; std::vectorchrono::seconds> v = {0s, 1s, 2s, 3s, 5s}; assert(min_absent(std::begin(v), std::end(v)) 4s); 50 . 1TESTING WITH MORE TYPES TESTING WITH MORE TYPES using namespace std::chrono_literals; std::vectorchrono::seconds> v = {0s, 1s, 2s, 3s, 5s}; assert(min_absent(std::begin(v), std::end(v)) EVEN MORE TYPES using namespace std::chrono_literals; using T = std::chrono::time_pointchrono::system_clock, std::chrono::seconds>; std::vector v = {T{0s}, T{1s}

(4. * min + 40. * s) / km; const quantity ttg = time_to_goal(half_marathon_distance, pace); std::chrono::seconds seconds = value_cast(ttg); std::println("Expected race time {:%T}" (4. * min + 40. * s) / km; const quantity ttg = time_to_goal(half_marathon_distance, pace); std::chrono::seconds seconds = value_cast(ttg); std::println("Expected race time {:%T}" * min + 40. * s) / km; const quantity ttg = time_to_goal(half_marathon_distance, 1 / pace); std::chrono::seconds seconds = value_cast(ttg); std::println("Expected race time {:%T}"

Agenda  Compile-Time Rational Numbers  Durations  Clocks  Time Points  Dates  Time Zones <chrono> 4 Agenda  Compile-Time Rational Numbers  Durations  Clocks  Time Points  Dates rational numbers at compile time  Always normalized representation  Needed for durations in the <chrono> library6 Compile-Time Rational Numbers  Define a rational number: using r1 = ratio<1, 60>; Time Zones10 Durations  Interval between two points in time  Represented by std::duration from <chrono>, contains:  A tick  A tick period = seconds between 2 ticks = rational constant  template

温馨提示： 1. 会用到第二讲（ RAII 与智能指针）里的知识 2. 课件中一部分代码是基于 C++17 的 个人认为， C++11 中很多特性， 其实可以看做是为了支持多线程而 顺带引入的……如 chrono 、移动 、 lambda 、 RAII…… 第 0 章：时间 C 语言如何处理时间： time.h • long t0 = time(NULL); // 获取从 1970 年 1 月 std::chrono • 利用 C++ 强类型的特点，明确区分时间点与时间段，明确区分不同的时间单位。 • 时间点例子： 2022 年 1 月 8 日 13 点 07 分 10 秒 • 时间段例子： 1 分 30 秒 • 时间点类型： chrono::steady_clock::time_point 等 • 时间段类型： chrono::milliseconds ， chrono::seconds chrono::seconds ， chrono::minutes 等 • 方便的运算符重载：时间点 + 时间段 = 时间点，时间点 - 时间点 = 时间段 • auto t0 = chrono::steady_clock::now(); // 获取当前时间点 • auto t1 = t0 + chrono::seconds(30);

，KI::get_num_ joints()>::Random(); fk_interface::ForwardKinematicsInterface fk_interfacei start = std::chrono::high_resotution_ctock::now(); (int k = 9; k < muttiptiery kt+) { (int = 9; < iterations; i++r) { vatues[il); fk_interface.forward kinematics(); stop = std::chrono: :high_resotution_ ctocks:now(); duration = std::chrono::duration_castchrono::nanoseconds>(stop - start); cout << ts()>::Random(); fk_interface: :ForwardkinematicsInterface fk_interface; auto start = std::chrono::high_resotution_ ctock::now(); for (int k = 9; k < muttiptiery kt+) { for (int 1 = 69; 1< iterations;

// #include , <chrono>, template auto test(F f, Args &&... args) { using namespace std; using namespace std::chrono; auto pre = high_r attack(hero, *p); return victims.size(); // whatever }); using namespace std::chrono; std::print("Attacking {} victims {} times in {}", std::size(victims), M, durat attack(hero, *p); return victims.size(); // whatever }); using namespace std::chrono; std::print("Attacking {} victims {} times in {}", std::size(victims), M, durat

~INT() {} }; 46Libraries that wrap integers ● Smart pointers similarly wrap raw pointers ● std::chrono ● All other units libraries ● Safe integers ● Bindings for other languages 47Wrapper over int the popular libraries, right?std::chrono #include <chrono> int64_t int_seconds() { return 60; } std::chrono::seconds chrono_seconds() { return std::chrono::seconds{60}; } static_assert( static_assert(std::is_same_vchrono::seconds::rep>); 65 https://godbolt.org/z/E5e1nGY94armv8-a clang 18.1.0 x86-64 gcc 14.2 x64 msvc v19.40 VS17.10 mov w0, #60 ret mov eax, 60 ret mov eax, 60